Proprioception: A Structural Overview and
The Pilates Method as a Form of Proprioceptive Rehabilitation
by
Laura Curry
25 November 2004
BASI Training Course, London
Abstract:
Proprioception is the process by which the body can vary muscle contraction in immediate response to incoming information regarding external forces. This information is gathered by specialized sensory receptors (proprioceptors) found in muscles, tendons, ligaments, joint capsules and skin, and is processed neurologically as both feedback and feedforward information. It defines where and how the body will move at any one moment, and works on both conscious and unconscious levels. Injury to structures containing proprioceptors will lead to a loss of proprioception in the affected area. Special care must be taken to rehabilitate these structures with specific proprioceptive training or reinjury is likely to occur. With its emphasis on the mind-body connection and repertoire of controlled functional movements, the Pilates method serves as an ideal system of proprioceptive rehabilitation. Current research indicates such exercises increase stimulation of proprioceptors and thus improve joint stability, and encourage optimal neuromuscular programming.
Contents:
I PROPRIOCEPTORS ANATOMICAL DESCRIPTION
II PROPRIOCEPTION EXPLAINED
Proprioception is one of the most widely used and misused terms in exercise physiology. It is often used interchangeably with ‘kinesthesia’, the sense of joint acceleration, or is erroneously described as ‘joint position sense’. In fact, proprioception is a broader term that encompasses both kinesthesia and joint position sense. According to the most up to date research, proprioception describes afferent (incoming) information arising from internal peripheral areas of the body that contribute to postural control, joint stability and several conscious sensations (Riemann & Lephart 2002). Thus the term refers to entire process by which information is received and perceived by the body, and how it responds to such information with muscle contraction.
Proprioception was first defined a century ago by Sir Charles Scott Sherrington, an English scientist who won a Nobel Prize for his studies in neurology. In his ‘proprioceptive system’, proprioception is the afferent information arising from special sensory receptors for stimuli that "are traceable to actions of the organism itself, and since the stimuli to the receptors are delivered by the organism itself, the deep receptors may be termed proprioceptors..." (Sherrington 1906). His model described proprioception as being used for the regulation of total posture and segmental posture, as well as initiating several conscious peripheral sensations or ‘muscle senses’. This is still the basis of our current understanding of proprioception although some aspects of his original system have evolved over the years.
Contemporary models of proprioception continue to include the concept of proprioceptors although they are somewhat different to those described by Sherrington. His system made a distinction between mechanoreceptors located within the body, and superficial mechanoreceptors. Today proprioceptors include mechanoreceptors within muscles, tendons, joints and ligaments, cutaneous mechanoreceptors (within the skin), and portions of the vestibular (inner ear) apparatus responsible for conveying information regarding the orientation of the head with respect to gravity (Riemann & Lephart 2002). Mechanoreceptors are special sensory receptors responsible for translating mechanical events occurring in their host tissues into neural signals. These signals travel to various areas of the central nervous system (portions of the brain and spinal cord) for processing. This information is interpreted and cross-checked against information stored from previous experiences. This then determines the body’s response to a particular mechanical stimulus.
Homeostasis is the dynamic process through which an organism maintains its internal environment despite perturbations from external forces (Clayman 1989). Proprioception is one system responsible for maintaining homeostasis in the body. In particular, proprioception controls the body’s response to joint movement resulting from gravity or external loads. To maintain homeostasis, information from proprioceptors is processed as either feedback or feedforward information. Feedback controls stimulate a corrective response within the corresponding system after sensory detection. Feedforward controls are anticipatory actions occurring before the sensory detection of a homeostatic disruption. Together feedback and feedforward systems work to maintain stability. In terms of proprioception, this is specifically to maintain functional stability in joints -- that is to say the ability of a joint to remain unchanged or promptly return to proper alignment through an equalization of forces.
III. STRUCTURAL AND FUNCTIONAL OVERVIEW OF MECHANORECEPTORS
Although proprioceptors include receptors within the skin and mechanisms of the inner ear, this paper will focus on sensory receptor organs located within the structures that are directly involved in movement. These are the mechanoreceptors contained in muscles, tendons, ligaments and joints. Muscle spindle organs are the mechanoreceptors found in muscles, and golgi tendon organs are the mechanoreceptors that exist in tendons. These two structures have a special complimentary relationship, and this will be a primary focal point of this essay. It is important however to view all the mechanisms of proprioception as integral to the seamless function of proprioception as a whole. Contemporary studies indicate that the type of proprioceptor activity that predominates in the body varies from area to area. In some parts of the anatomy, joint mechanoreceptor activity may be more prevalent while in others it can be spindle activity. It appears to be the function of the area of the body that dictates the amount and source of proprioceptive activity. Although there is a clearer picture emerging of the hierarchy of proprioceptive function throughout the body, experiments that decisively determine such things in humans are difficult, often invasive, and bring with them a host of ethical implications.
All mechanoreceptors are specialized sense organs that respond to specific stimulus and transmit this information to the central nervous system as a neurological signal. Just like receptors in the eyes that respond to light, mechanoreceptors in muscle, tendon, ligament, and joint tissues respond to stimuli such as movement or mechanical deformation. These receptors thus indicate body part location, their relation to one another and the outside world, and also how fast and to where they are moving. Each type of receptor can be classified as fast or slow adapting, and has either a low or high threshold for activation. The speed at which the receptor adapts refers to the rate of the response to stimulus. The threshold is determined by the strength of the stimulus necessary for the receptor to begin signaling.
Mechanoreceptors found in and around joints are divided into four categories. Type I articular mechanoreceptors are found in ligaments, and are called Ruffini Corpusles. They are slow adapting, have a low threshold, and respond to changing mechanical stress in the joint. They are active in every position of the joint, even when it is immobile. These receptors continuously transmit impulses as long as stimulus is present. This activity keeps the brain aware of the body and its surroundings. It also allows the central nervous system to ascertain the status of muscle contraction, and amount of load present both in static and dynamic situations.
Type II articular mechanoreceptors are called Pascinian Corpuscles. These exist in joint capsules and intraaricular and extraarticular fat pads. They have a low threshold for activation and are rapidly adapting. They are entirely inactive in immobile joints, but react strongly when change takes place. They are active for brief periods at the onset of movement and thus signal joint acceleration to the central nervous system.
The Type III receptors include golgi tendon organs and also golgi type organs found in ligaments. They have a high threshold for activation and are slowly adapting. Golgi ligament organs are completely inactive in immobile joints and activate only at the extremes of joint motion. In this way, they signal the amount of tension present within a joint. Golgi tendon organs are mechanoreceptors located at the junction between muscle fibers and tendons. They lie in series with the muscle and are sensitive to the contraction of muscle fibers. They convey information regarding load or tension in the muscle relatively independent of muscle length.
The last type of receptor, Type IV, is a collection of free nerve endings found in the ligaments, joint capsules and articular fat pads. They are inactive in normal circumstances and respond to extreme mechanical deformation or chemical irritation. Most detect pain but some detect crude touch, pressure and tactile sensations.
Muscle Spindle Organs are elongated structures that lie in the fleshy portions of nearly all striated muscles. They range from 4 – 10 mm in length and form a spindle or fusiform shape. Specialized fibers within the spindle are thus referred to as intrafusal fibers and the rest of the muscle is referred to as the extrafusal fibers. They are slow adapting and respond to muscle stretch. Not all muscle fibers contain spindles, moreover postural muscles are found to contain more spindles.
IV. THE NEUROLOGY OF PROPROCEPTION
All mechanoreceptors contain afferent neurons that relay incoming information to the central nervous system. Efferent neurons on the other hand, are nerves that send outgoing information from the central nervous system to structures in the body and thus elicit a particular response from those structures. There is only one type of mechanoreceptor, muscle spindles, that contains both afferent and efferent nerves. Nevertheless, the process of proprioception involves the participation of both afferent and efferent neurological activity regardless of the source of proprioceptive information.
Afferent neurons, those that carry messages to the central nervous system, are classified as either Type I or Type II. Type I is a faster acting neuron compared to the Type II. Most mechanoreceptors contain one type of afferent neuron, although muscle spindles contain both the fact acting Type I afferents and the slower Type II receptors.
Alpha motor neurons are efferent motor neurons that enervate the contractile fibers of muscles (extrafusal fibers). They take outgoing messages from the motor cortex of the brain and transfer them to muscle fibers in order to produce movement. All the muscle fibers controlled by one alpha motor neuron are referred to as a motor unit. A motor unit can be from 3-100 fibers depending on the strength or accuracy of control needed. Most muscles contain motor units of various sizes, and the motor units are always recruited from smallest to largest. Smaller units have smaller alpha motor neurons and are excited more easily. Motor units are also divided into slow acting (Type I) and fast acting (Type II) categories. Slow motor units react slowly but produce a long sustained contraction. They are associated with endurance type activity. Fast motor units are usually recruited when a sudden high degree of force is required, but they fatigue quite quickly. It is interesting to note that muscle spindles are almost entirely absent in regions composed mainly of fast motor units (Johansson et al 1991).
Gamma motor neurons are efferent motor neurons that activate contractile regions of muscle spindles (intrafusal fibers). Since the contractile portions of spindles are quite small, gamma motor neuron activation does not significantly contribute to the strength of the muscle contraction. The exact mechanism or process that causes the gamma motor neurons to fire is not fully understood. It is known however, that afferent activity and gamma motor neuron activity have a direct relationship.
Muscle Spindles have the most complex neurology of the mechanoreceptors. They contain efferent gamma motor neurons, and also contain two types of afferent sensors (type II and type Ia). The type Ia sensory endings are more sensitive to the velocity of stretch, while the type II endings are length sensitive. Muscle spindle organs respond to the stimulus of muscle stretch, and since the intrafusal fibers are parallel to the extrafusal fibers they are quite sensitive to an increase in muscle length.
When stretch is detected, the muscle spindle organ records a change in length and conveys this information to the spine via its afferent nerve endings. This elicits an immediate efferent response back to the muscle causing it to contract. This is called the Myotatic Reflex or Stretch Reflex. The Type Ia and II afferent fibers in muscle spindles make connections with the alpha motor neurons of its parent muscle forming what is called a Monosynaptic Reflex Circuit. This can be thought of as a short circuit running directly between the muscle and spinal cord that eliminates the need for the impulse to travel all the way up to the brain and back again. The velocity of the muscle’s reflex contraction is proportional to the amount of stretch in the receptor. So if one taps the tendon of the quadriceps with a hammer, the reaction will be a quick strong kick. If lengthening in a muscle is gradual, the motor neurons will produce a proportionally gradual signal to maintain homeostasis. This forms the basis for automatic length control system in muscles. This allows for constant readjustment to outside forces without the necessity of conscious thought.
When gamma motor neuron co-activation is added to this length control system, the result is an even more efficient and accurate system that functions seamlessly during all stages of movement. Studies show that gamma motor neuron activation during alpha motor neuron activation of the surrounding serves to increase the dynamic sensitivity of the muscle spindles. This process is referred to alpha-gamma co-activation. When a muscle contracts, the intrafusal fibers within the spindles go slack. Thus the muscle spindle organ becomes insensitive to the stretch stimulus. In order to maintain sensitivity to stretch even during muscle contraction, gamma motor neurons fire causing the intrafusal fibers to contract and therefore restore the spindle’s sensitivity.
Golgi tendon organs contain Type Ib afferent fibers. Only one of these large fibers enters a capsule around part of the musculocutaneous junction. The fibers then branch out and interweave with bundles of collagen. Since collagen fibers are stiffer than muscle, stretching causes far less mechanical deformation of the golgi tendon organ than it does to muscle spindles. However, when a muscle contracts it pulls directly on the collagen fibers which causes increased firing in the tendon organ’s Ib afferents. This gives the golgi tendon mechanoreceptor a functionally inverse action to that of the spindles. Golgi tendon organs react to muscle contraction, while muscle spindles react to muscle stretch. The Ib afferent fibers of the golgi tendon organ have an inhibitory effect on its parent motor neuron and cause the muscle to relax in response to contraction stimulus. This is known as the Inverse Myotatic Reflex or Clasped Knife Reflex, which is again a functionally inverse action to that of the muscle spindles.
Nerve fibers of muscle spindles and the golgi tendon organ also make reciprocal connections with one another via interneurons. As motor neurons of the agonist muscle are inhibited by the signals from the golgi tendon organ in the agonist, the motor neurons of the antagonist are excited by the same signals. This forms the basis of reciprocal inhibition, also known as reciprocal innervation. Consequently, the function of the golgi tendon organ is not merely to protect the muscle against overloads. The tendon organ and muscle spindle work synergistically to form a length and tension control system that provides continuous feedback during all stages of muscle contraction. The tension in a muscle will stay constant despite disturbances such as fatigue, velocity of shortening or changes in muscle length. At the same time, the system also functions as a brake for muscle activity allowing for termination of a contraction at a precise moment. This serves to optimize muscle function during activities that require precise control.
Muscle spindles and golgi tendon organs respond to external forces in this synergistic manner as long of the source of stimulus is present. As this process occurs, intrafusal fibers of muscle spindles reset to a higher gain and the golgi tendon organ pathway undergoes a brief desensitization. This short-term activation increases the excitability of the motor neuron pool, thereby providing a non-voluntary increase in the excitation of subsequent contractions (Hutton & Atwater 1992). This response is often referred to as proprioceptive trace. In this situation, the body begins to produce a predictable amount of force based on immediate proprioceptive stimulation. For example, in the first few days of a backpacking trip where an individual has to carry a heavy load, they will experience a feeling of lightness when the pack is initially removed at the end of a long day’s trek. This is an after-effect of the body’s immediate proprioceptive experience. Such anticipatory contraction in associated with an increase in gamma motor neuron activity, and what is referred to as a gamma efferent loop. Efferent gamma neuron activity increases intrafusal (spindle) contraction, and therefore increases Type Ia afferent activity. This increase in Ia afferent firing increases activity of alpha motor neurons which in turn signals the extrafusal muscle fibers to contract.
It is unclear exactly what synapses on the gamma motor neurons themselves. It is largely considered that gamma motor neuron impulses originate in local interneurons from the spine. Other studies attribute them to brain stem projections or cortex projections. Johannson et al. (1993) presents an additional integrative mechanism whereby the gamma system may be stimulated by cutaneous and joint mechanoreceptors. Nevertheless evidence indicates gamma motor neuron stimulation is largely influenced by peripheral afferent input. Therefore proprioceptor activation in movement helps increases excitation in spindles. This in turn causes greater recruitment of motor units and a stronger contraction (Simpson 2003). Furthermore gamma motor neuron activity affects the sensitivity of the spindle increasing its dynamic response. This therefore causes reflexes to occur faster.
In addition, increased gamma motor neuron activity in muscle spindles improves the pre-activation of muscles in anticipation of movement and joint loads. Pre-activated muscles can provide quick compensation for external loads and are crucial for dynamic joint stability (Swanik et al. 1997). It is also interesting to note that it is gamma motor neuron activity which contributes to the resting tone of a muscle. The higher the gamma motor neuron system fires, the firmer the resting tone of a muscle becomes. In some instances, gamma motor neurons can become overactive. Such hyperactivity in gamma motor neurons causes muscle spasticity.
Although muscle spindle and golgi tendon organ activity is mediated at the spinal cord level, this information is also dispersed throughout the central nervous system and is feed into the cerebral cortex of the brain. This is the area responsible for cognitive or motor programming. According to Fitts (Simpson 2003), there are 3 stages of motor learning: cognitive, associative and autonomous. The cognitive stage is when initial understanding of the movement takes place. The associative stage is then where the pattern is fine-tuned. Improvements at the associative stage are achieved through repetition; gains in this phase are relatively slow. Ultimately the autonomous stage is attained and the movement is effectively hard wired into the brain.
In order for these patterns to develop into automatic behavior or subconscious activity, progress has to be mediated by conscious evolvement and positive reinforcement. For every successful execution of the pattern that results in the intended objective, the conscious mind experiences positive reinforcement. This conscious positive is also experienced by the subconscious mind which solidifies muscle memory patterns. Once the precise movement pattern becomes fairly automatic, it forms an N-gram (engram) on the cerebral cortex of the brain. This is a basic cognitive programming unit of automatic behavior. These N-grams are motor memories, and like all memories they can be forgotten over time. This is the body economizing in response to the constant demands placed on motor memory by the environment. So it is necessary to constantly recall these N-grams to maintain their efficacy.
Motor learning can be illustrated once again with the example of backpacking. After the initial few days of the backpacking trip, the feeling of lightness experienced upon taking off the pack begins to diminish. By the end of the trip, the sensation may disappear completely. At this point, proprioceptive trace is kept to a minimum as the establishment of a more efficient muscle recruitment pattern takes place. If the backpacker waits a year before taking another trip, the sensation of lightness may occur again in the same way, thus signifying the N-gram in the motor cortex has been expunged. However if the backpacker takes many such trips with relative frequency, and becomes a ‘seasoned’ backpacker, the lightness sensation will most likely never return. So through consistent repetition, motor programs based on recurrent input of feedback information will develop into a more efficient and more permanent feedforward system.
V. CONCEPTUAL MODEL OF STABILITY AND THE ROLE OF PROPRIOCEPTORS
The neuromuscular activity associated with proprioceptors directly influences muscle tone, movement patterns, the accuracy and speed of joint reflexes, and therefore joint stability. This holds true for all joint systems, from the axial articulations of the spine to the appendicular articulations of the extremities. Manohar Panjabi (1992) devised a conceptual model used to describe spinal stability which is a useful paradigm to illustrate dynamic stability in all areas of the body. His system incorporates three subsystems: the Active Subsystem, the Passive Subsystem, and the Neurocontrol Subsystem. The Active Subsystem refers to the force generating capacity of muscles. The Passive Subsystem is composed of the bones, joints and ligaments which contribute to the control of movement and stability. Lastly, the Neurocontrol Subsystem is what coordinates muscle activity in advance of predictable challenges to stability -- and coordinates responses to afferent feedback from unpredictable challenges (Richardson et al. 1999).
Panjabi contends the three subsystems are interdependent components of the stabilization system with one system capable of compensating for deficits in another. Dysfunction in the body occurs when deficits in any of the subsystems can not compensated for by the other subsystems. So for example if there is a defect in the ligaments of the spine, the strength in the surrounding musculature may be unable to compensate. This in turn will lead to faulty alignment in the spine. If this progresses to a point where nerves become compressed, this spells dysfunction within in the Neurocontrol Subsystem.
Proprioception in therefore an important component of Panjabi’s model as it has structural and functional relevance in each of his three subsystems. It could be thought of as the link between all three. The Active Subsystem consists of muscle contraction, and this is directly regulated by activity in the muscle spindles and golgi tendon organs. Similarly the joints and ligaments of the Passive Subsystem are saturated with mechanoreceptors that pass on proprioceptive information to the central nervous system. Finally the Neurocontrol System is composed largely of impulses conducted by proprioceptors, and contains information shaped by the activity of proprioceptors in past experience. Thus proprioceptive information can be used to maximize function in all subsystems. In the same way, a lack of proprioceptive function in any of the three subsystems could lead to structural dysfunction.
Dysfunction in the Active Subsystem of muscle contraction can manifest itself in many ways. For optional joint stability, there needs to be a co-contraction of adequate force balanced equally between agonist and antagonist muscles. There also needs to be adequate contraction of stabilizers. With poor proprioception, the mis-sequencing and disproportionate force of recruitment of these muscle groups can be detrimental to the joint. In the knee for example, an overly developed quadriceps muscle can lead to pain in the patellar area due to an imbalance of pull between it and the hamstring muscles. In the spine, dysfunction can occur due to a lack of recruitment of stabilizers before the contraction of the main movers. So it is important to consider proprioception in terms of relative strength gains, muscle balance, and muscle recruitment sequences.
The Passive Subsystem is also subject to dysfunction which disrupts functional stability. Many of the essential structures of proprioception exist in the articular and ligamentous tissues of the Passive System. When these tissues are damaged, de-afferentation of the mechanoreceptors adversely alters the spinal reflex pathways to the motor nerves and muscle spindles, as well as to the cortical pathways for conscious and unconscious appreciation of proprioception (Swanik 1997). Studies show that this disruption of the mechanoreceptors produces a distorted joint position sense as well an increased tendency to develop compensatory movement patterns (Laskowski et al. 1997). The altered position sense is caused by previously non-existent forms of pressure stimuli. These can be in the form of swelling, scarring or displaced structures. This new stimulus exerts pressure on the mechanoreceptors and thus signals incorrect information. The compensatory patterns then established put the individual at risk of reinjury of the original structure or new trauma to surrounding areas. This would apply in the case of all joint or ligamentous injuries -- from a broken ankle to a slipped disk. So specific proprioceptive training to encourage proper alignment of these areas during movement is essential.
Proper functioning of the Neurocontrol Subsystem has the most obvious link to proprioception. Since the entire proprioceptive system relies on the constant flow of afferent and efferent nerve impulses, any disruption to the nervous system would be profoundly detrimental. Proprioception can be altered by a variety of nervous system disturbances including chemical irritation, mechanical injury to nerve structures, psychological disorders, strokes, and many other diseases and infections. One of the more frequently encountered dysfunctions in the Neurocontrol Subsystem occurs from the compression of nerves by surrounding structures. These are usually located in proximity to the shoulder, pelvis or spine. Not only is there a diminished sense of proprioception in this case, but it there is often a decrease in sensation all together. In these cases it is essential to use various forms of feedback to reinforce proper movement patterns and retrain proprioception.
As with any interdependent system there exists constant compensation in all subsystems. When dysfunction occurs, often it is impossible to categorically determine from which system it originates. For instance, it may seem the underlying cause of a particular problem is weakness in a certain muscle, but on closer examination it may seem instead to be an irregularity in connective tissue. Then upon even further examination it may seem the absolute cause of the problem lies in faulty neuromuscular programming which led to detrimental mismechanics in movement. However in reality it is impossible to say which of these things may have occurred first, and in the long run it will not make much difference to the method of treatment. In other cases, there is obvious disruption of a single subsystem as in the case of a broken ankle. But because of the interdependent nature of the stability system as a whole, significant impact will occur to all subsystems. So regardless of the source of dysfunction, rehabilitation must address the body globally across all three of Panjabi’s subsystems in order to eliminate any potential for future dysfunction.
Rather than waste time trying to determine which structure deteriorated first, or exactly which aspect of the problem is worst relative to the others, a practitioner must immediately address all aspects of stability. Goals are to strengthen surrounding musculature, ensure proper alignment in static and dynamic movements, and positively reinforce these proper patterns until they become automatic. It is also necessary to restore lost proprioception by addressing kinesthesia, joint position and the sense of resistance in the affected area. Again because of the interdependent nature of the subsystems, improvement in one subsystem will have a cyclical effect encouraging improvement in the other subsystems, and eventually restoring dynamic stability of the area.
VI. THE PILATES METHOD AND PROPRIOCEPTIVE REHABILITATION
According to Swanik et al. (1997), there are four elements crucial for reestablishing neuromuscular control and functional stability: proprioceptive & kinesthesia awareness, dynamic stability, preparatory and reactive muscle characteristics, and conscious and unconscious functional movement patterns. The Pilates method with its repertoire of functional movements against resistance, and emphasis on control, concentration, awareness, balance, centering and precision is an ideal system for regaining functional stability.
Awareness is one of the ten principles of the Pilates method. It refers to both a local awareness and a global awareness, and is therefore in essence much like the proprioceptive system. In the proprioceptive system, there is information gathered externally in response to outside forces, and there are also messages internally generated as nerve impulses. Moreover, the movement and resistance information is processed locally at the spinal level, then is dispersed globally throughout the body. To maintain homeostasis the system is constantly feeding back and feeding forward this information. This in turn shapes the body’s response to outside forces.
To maintain joint stability there must be an accurate conscious awareness of proprioception both locally and globally. Joint position sense, joint acceleration, velocity and the sense of heaviness or resistance all must be properly accounted for during movement, or faulty motor programs may develop. When performing exercises from the Pilates repertoire all of these objectives can be achieved. With this method there is emphasis locally on action within a single joint, and there is equal emphasis globally of the relationship of a particular joint to all others in the body.
Global and local awareness can be exemplified by Single Leg Footwork on the Reformer. Although this movement is supine it simulates on a basic level, a functional weight bearing movement pattern from everyday life. When performing this exercise, the client is asked to place one hip and knee in flexion at 90 degrees. The leg that remains on the footbar is required to move against resistance with proper alignment. Attention is also paid to the balanced co-contraction of the hamstring and quadriceps, preceded by the activation of lumbo-pelvic stabilizers. If the velocity of the contraction exceeds a limit where all these elements can remain correct and accounted for, the client is asked to slow down. In the beginning stages of practicing this exercise, the clients have difficulty perceiving the any alteration in the velocity of the movement. But after substantial practice their sense of joint velocity and acceleration improves, and they are able to keep a constant speed. Also, the leg that remains up often leaves its 90 degree position, moreover clients are then unable to control the leg on the footbar in correct alignment, or they are unable to recruit the correct muscles. Usually, the client cannot sense these aberrations. However this too improves with consistent repetition. With this and most other exercises from the Pilates repertoire, awareness and control is emphasized locally and globally. In Single Leg Footwork, the position of the hip and knee is a focus on a local level. At the same time the entire lower limb is globally significant in its relationship to the neutral position of the pelvis, spine and even the shoulder girdle. Feedback is provided by the instructor to positively reinforce these relationships. Then over time the appreciation of proprioception and kinesthesia improves significantly.
Addressing the sensation of effort or resistance is not something incorporated into most exercise regimes, but the Pilates method integrates this element into its system in many instances. It is common for an individual to report a low load as being perceived as a high load, but it also feels easier to sustain a higher load when there is a deficit of proprioception (Gibbons & Comerford 2001). Often the load on the legs in an exercise like Footwork on the Reformer, is a fraction of the load put through ones legs while walking. If clients perceive such low load resistance as high load, it is important to have them work within a range of resistance they can tolerate. At first it may be necessary to lower the resistance below what is considered normal so that the exercise can be performed properly. Over time, they will acclimate to increased load as proprioception improves with consistent practice.
In many cases, clients prefer to push higher spring loads when performing Footwork on the Reformer. The lack of resistance makes it harder for the movement to feel controlled. Rather than adding springs, the instructor encourages the client to focus on the muscle contraction and “create resistance from within”. This recruits slower motor units. Since slow motor units are more tightly bound into the muscle spindle loop and thus most easily recruited by spindle afferent input, this type of low load exercise will enhance proprioception. (Gibbons & Comerford 2001) (Johansson et al. 1991). Furthermore Gibbons & Comerford go on to state that movements incorporating high loads are actually a hindrance to the development of proprioception. Even an appropriate exercise [using high loads] may not be beneficial because this will encourage the recruitment of fast motor units. This is deleterious for a rehabilitation program because it is only a small portion of the day that our body actually uses a high load (Jones et al. 1989). For this reason, it is important to use low load exercises to stimulate the development of proprioception.
Stability in a joint refers to the state of a joint remaining or promptly returning to proper alignment through an equalization of forces (Riemann & Lephart 2002). Dynamic stability then is maintaining this homeostasis across a whole system of joints during bodily movements. This is dependent on the integrity of the joint structures themselves, as well as the appropriate feedback or feedforward information sent to the skeletal muscles that cross the joint.
As a Pilates instructor, one is unable to alter the actual structures of joints as would an orthopedic surgeon or an osteopath. It is possible however to enforce correct joint recruitment and alignment. This may help reduce or even eliminate chronic damage to articular structures. It is also possible to retrain stabilizer muscles to aid in supporting the skeleton during movement and thereby eliminate any friction or compression in joints. For example, the neutral position of the pelvis is emphasized in many exercises. This encourages potentially less injurious movement patterns in many instances. Such patterning emphasizes balanced contraction between abdominals and back extensors, as well as a balanced contraction between movers and stabilizers. This serves to return the pelvis to its balanced state in response to displacement or added load. Gibbons and Comerford further stress the importance of a neutral pelvis position in reference to mechanoreceptor input. The neutral position is where there is minimal support from the passive osteo-ligamentous system and where the active muscle system is essential for dynamic stability. There will be minimal feedback from other proprioceptive sources such as ligaments, and the muscle spindles will be responsible for providing this feedback. So to maximize the relative flexibility and stability of the pelvis, it is necessary to establish a system of balanced muscle contraction and discourage resting the spine or pelvis into a hammock of ligaments or other connective tissue.
When performing the Chest Lift, the client is encouraged to flex the trunk as high as possible without displacing the pelvis from neutral. This encourages co-contraction in the back muscles during forward flexion in order to protect structures of the lumbar spine. In addition, strong contraction of the transversus abdominus is emphasized equally if not more so than the contraction of the rectus abdominus. The contraction of the transversus is coupled with pelvic floor engagement to encourage the contraction of the multifidus muscles. These contractions together stabilize each segment of the spine and thus create an environment where articular compression can be kept to an absolute minimum.
All Pilates exercises are repeated with relative frequency to maintain proper neuromuscular programming. They are also done with the client concentrating on each element of the movement further integrating proprioceptive signals into all aspects of the central nervous system. In this way, movement patterns that encourage dynamic stability are constantly reinforced into the motor cortex areas of the brain. This promotes automatic reflexes favoring a neutral joint position and balanced muscular sequencing, thus minimizing potential damage to the system.
Favorable preparatory and reactive muscle characteristics depend on the strength and flexibility of the muscle as well as the increased activity of mechanoreceptors. This means it is necessary to possess adequate strength in global movers as well as stabilizers, as well as a balance of force during the recruitment of these muscles. Although strength gains are important in any exercise program, it is detrimental to promote the development of short overactive global movers. Such muscles are unable to move a joint through its full range of motion, and therefore prevent the stimulation of many articular and muscular mechanoreceptors.
Overactive mobilizers also discourage the activation of smaller stabilizers. Thus the stabilizers in a system dominated by overactive global mobilizers will be underdeveloped and seldom recruited properly. In most strength training regimes, there is little emphasis on the co-contraction of stabilizers, and high loads added to these programs only exacerbate this problem. According to Gibbons & Comerford (2001), when a mobilizer is dominant over its stabilizer synergist, it will continue to dominate it during traditional forms of strength training. Both muscle groups will increase in strength and there will be no change in the relative balance contribution in favor of the stabilizer.
The Pilates method encourages the engagement of stabilizers as a prerequisite for all movements. In a movement like the Knee Stretch Flat on the Reformer, the main movement is hip flexion and extension. However, equal if not more attention is paid to the stabilization of the shoulder girdle, pelvis and spine, than it is to hip movement. The dynamics of the exercise encourage the development of these stabilizers as they are cued to remain unmoved during the hip flexion. Due to the relatively low load of the spring resistance, the action of hip extension does not lead to over-development of hip extensors. Emphasis is also put on the hip flexion phase of this movement where the springs are returning the carriage to its start position. The client is encouraged to create internal resistance through voluntary contraction of hip flexors and deep abdominals. In this instance, there is constant co-contraction between agonists, antagonists, movers and stabilizers occurring across a variety of joint systems. Proprioceptive acuity will therefore develop locally as well as globally.
Muscle flexibility is essential to dynamic stability because it allows for full range of motion of a joint and therefore a maximum stimulation of proprioceptors. It is also important because allows a muscle to be subjected to a significant amount of mechanical deformation without injuring surrounding structures. Stretching a short global mover can encourage it to respond to low loads encouraging slow motor unit recruitment (Gibbons & Comerford 2001). Shortened dominant global movers are also associated with weak stabilizers as is the case with overactive hamstrings and weak gluteus medius muscles. In this example, a lack of stability is manifested in an exercise like the Standing Leg Press on the Wunda Chair. When the client performs the movement, lack of stability can be seen as postural sway. With consistent repetition, the gluteus medius will strengthen and a pattern integrating its activation during hip extension will begin to develop. This will eventually reduce the dominance of the overactive hamstring. This exercise in conjunction with a hamstring stretch like the Standing Lunge on the Reformer can restore stability and flexibility to the area. Furthermore when doing exercises like Footwork on the Reformer with such clients, it is necessary to cue them to take the limb to full extension. This will minimize any movement pattern that might contribute to the shortening of the hamstring muscle.
Relative flexibility between different body segments is also a prerequisite for dynamic stability. When joint systems can not maintain or return to a neutral position during global bodily movements, this can contribute to faulty and potentially damaging movement patterns. The reasons for lack of relative flexibility may be physiological, as is the case of a fused vertebra or other bone deformity that prevents full range of motion. More commonly though, there is a simply lack of dissociation in movement. In this case the joints may be fully functional, but either consciously or unconsciously the movement of one joint system is inextricably linked to that of another. This can be manifested locally for example in the lack of ability to separate inversion and eversion from plantarflexion and dorsiflexion in the foot. This can also be seen more globally in an exercise like the Roll-Up, where clients are often unable to dissociate spinal flexion and extension with scapular elevation and protraction. This type of movement pattern usually occurs throughout their daily lives, and may eventually lead to serious dysfunction in the upper limb. Such movement patterns are compounded by a lack of kinesthesia and joint position sense in the area, and individuals do not even realize when they are elevating the shoulders. So even when the joints are physically able to dissociate, the body lacks the awareness to do so. Therefore both on a global and local level, proprioceptive training is crucial to the restoration of relative flexibility.
For optimal muscle responsiveness, there also needs to be a smooth eccentric or lengthening contraction. There is often a ‘shaking’ motion present in muscles which lack the ability to eccentrically control a movement. Gibbons and Comerford (2001) hypothesize that in an attempt to complete the movement, the central nervous system alternatively holds and releases global muscles (shaking motion) because they lack the ability to smoothly eccentrically control this movement. Since the lengthening contraction usually occurs when there is little or no resistance, it is imperative that slow motor units are trained in the eccentric phase to increase proprioception in these areas. Pilates exercises emphasize the eccentric contraction in all movements. This muscle shaking is often seen in the eccentric phase of quadriceps contraction during Footwork on the Cadillac. Because the quadriceps is usually a dominant global mover, there is little slow motor unit recruitment in this area. The nature of the exercise forces a controlled lengthening contraction in this area. With consistent repetition, this exercise improves eccentric contraction, and the development of a more balanced co-contraction of hamstring and quadriceps in knee flexion/extension begins to take place; eventually the shaking disappears.
One of the most important elements in the development of muscle reflexes is the stimulation of the body’s mechanoreceptors. Sensory receptors in all tissues from all categories must be stimulated to promote optimum stability. The Pilates repertoire has the capacity to do this very effectively. The slow adapting Type I articular mechanoreceptors respond to the elastic component of spring resistance. The fast adapting Type II mechanoreceptors are triggered as tension of the springs changes with specific movement patterns. The Type III receptors fire at the ends of movement patterns further educating the sensory system as to exact positions. In addition, high quantities of muscle spindles and therefore tendon organs are recruited due to the relatively low resistance utilized in many Pilates exercises. These mechanoreceptors are further stimulated by the eccentric contractions emphasized in the repertoire. All of these factors will increase proprioceptive activity and contribute to improved accuracy and speed of preparatory and reactive muscle reflexes.
Another significant component of proprioceptive information comes from cutaneous mechanoreceptors. There are particularly high concentrations of these on the palms of the hands and plantar surfaces of the feet. Those on the feet have been shown to supply the central nervous system with information regarding weight distribution upon each foot as well as between the feet. Not only are they an important source of input on static position and body sway, they also play a significant role during dynamic and functional movement as (Riemann 2000). Any activity exerting pressure on the soles of the feet or palms of the hands will produce a strong reflex extension of the limbs concerned. This is referred to as the Positive Supportive Reaction, and functions to stabilize the associated limb. In the Pilates method, there is a great deal of stimulation to these areas when executing closed kinetic chain exercises. This can be in the form of contact with footbars, pedals, straps or even the floor. Emphasis is always put on grounding ones support base whether it is in the hands or feet. For example, people with proprioceptive deficits in the lower limb, and defects in gait will often respond well to doing their Reformer Footwork on the Jumpboard. This allows the client to “feel the feet”, and since the movement is done across gravity with the torso supported substantial gains in lower limb proprioception can be achieved.
To maintain gains in stability, components of the tension and balance must be continuously challenged (Simpson 2003). The Pilates method offers almost unlimited ways in which to do this. Variations to simple exercises allow for additional planes of movement to be incorporated. Also, instability can be introduced using equipment like the wobble board or rotator disks. This encourages the development of reflexes to respond to sudden variations in forces with fine tuned weight shifts brought about by quick precise contraction. Stability can also be challenged in many exercises by removing resistance. This is the case with exercises like the Up-Stretch Series, where a lower spring makes it more difficult to control the movement and support one’s own body weight.
Optimizing conscious and unconscious movement patterns occurs through cognitive motor programming. This can only happen if training is done with relative frequency, and with positive reinforcement feedback provided to the central nervous system. The Pilates method with its emphasis on the mind-body connection is ideally suited to stimulate the central nervous system on all levels. Because proprioception is merely one aspect of the body’s entire afferent intake system, there needs to be stimulation across a variety of receptive systems in order to drive concepts of motor programming into the cerebral cortex. Implementation of visual and tactile feedback is an extremely effective method for this, and is always incorporated into the Pilates practice. Aspects of consciousness should also be addressed. For this, Pilates instructors use imagery and pre-visualization as tool to enhance performance. They also ensure that the logical mind is stimulated, and that the client understands the physiological purpose and design of the exercises. Much of this incoming information will be redundant, however each message contains a unique element that contributes to a complex system of checks and balances that serves to refine the body’s response. This in turn maximizes the accuracy and efficiency of the body’s reflexes. Thus when trying to address proprioceptive issues in clients, it is important to consider them in context of all the global systems that control balance and movement. It is equally important to reinforce proprioceptive training by integrating conscious ideas into the subconscious. Training the physiological structures of the body to move with balance and stability, also promotes a conscious understanding and awareness in the individual. Thus by providing thorough stimulation to all aspects of the nervous system, the Pilates method can in fact improve proprioception, and therefore improve dynamic stability on both a local a global level.
VII. CASE STUDY
Client History:
Siobhan is a 35-year old internet project manager who suffered a three inch spiral fracture to the left fibula in September 2001. The fracture was located just proximal to the lateral malleolus. Bone grafts were recommended but not performed. She was immobilized in plaster until January 2002. Once the plaster was removed, she received physiotherapy only once weekly for the following ten weeks. In these sessions, the practitioners manually mobilized tissues in the ankle. There was no movement based therapeutic treatment given during these sessions, and the only homework given was simple plantarflexion to dorsiflexion movement. Consequently, restoration of ankle function was minimal, and progress if any was achieved very slowly. By September 2002, the left patella had completely displaced proximally and laterally, and Siobhan could barely walk. The NHS refused to treat her any further, and told her, “That’s what happens when you break your leg, it’s just never the same again.”
Prior to the accident, Siobhan was extremely active, doing ashtanga yoga, kickboxing and cycling on a regular weekly basis. It is also worth noting that she has an extremely stressful and largely sedentary job. She is also affected by stress physically, and tends to worry about things a great deal. Fortunately, she was eventually able to sue for damages, and by January 2003 she began seeing private specialists and therapists. Siobhan was referred to me by her physiotherapist whom I work with at Complete Health Care Centre. At that point she had been undergoing treatment for approximately eight months. Her first Pilates session was in November 2003.
Physical Assessment:
generally weak in all areas, including core stabilizers
good body awareness, processes instruction well
tends toward a fatigue posture
significant “chin poking”, and protracted shoulders
naturally quite flexible but not hypermobile
ankle weak and noticeably smaller than the unaffected side
range of motion in all directions was decreased in the left ankle
lower limb tracking is quite poor, especially on left side
compensatory patterns in whole left side, particularly in the hip
poor proprioception and control in left hip, knee & ankle
eft hip flexor tight, left quadratus lumborum tight
Objectives:
To restore ankle mobility and strength
To retrain compensatory patterns globally, paying particular attention to left side
To improve proprioception in extremities, particularly in left ankle
Treatment:
In the first few sessions, care was taken to take the joints through their full range of motion with proper alignment. Footwork on the Cadillac was done on one spring – a higher load could not be tolerated. Visual feedback provided by this exercise helped to reinforce proper alignment and muscle recruitment. Simple Legs in Straps exercises on the Reformer were given to improve pelvic stability. The early attempts at this exercise were shaky and manifested imbalances originating from the left side musculature. Core strength exercises such as Hundreds and stability exercises on the foam roller were given to promote proper recruitment of stabilizers. Siobhan purchased a foam roller and was able to practice these exercises at home. Breathing with the Roll-Up Bar was given to work spinal articulation, but also to dissociate scapular activity from spine movement. Knee Stretch Series exercises were given to promote dissociation of the hip joint. A Standing Lunge was given primarily to stretch the hip flexor. There were also Standing Lunge type exercises that emphasized contraction of the VMO to improve knee tracking.
After the first eight weeks, stability improved, as did core strength. The wobble board was introduced, the exercises became more unsupported and slightly higher loads were given. Flexibility began to improve in the left ankle as well. Imbalances throughout her left side still remained a problem however.
After sixteen weeks, the ankle problem was barely noticeable. Siobhan returned to activities like cycling and she also joined a gym. She did gym workouts two hours, twice a week on average. Imbalances in the left side remained to a small extent, usually manifesting as a hitching hip with slight forward torsion during Reformer Footwork. Some days were better than others. Bad days corresponded with periods of high stress in her job and personal life.
For the ten weeks or so that followed, both her physiotherapist and I would see her on a weekly basis. At the end of this period she was signed off by the physiotherapist, but continued to see me once a week for an hour private session.
To date, Siobhan has been coming to Pilates for a year. Physically she is a different person. There is almost no trace of ankle trauma. Although if she overdoes any physical activity when she is in a bad mental state she will experience moderate soreness in the ankle. Her fatigue posture still returns when she is physically or emotionally exhausted. However the strength of her prime movers and extremities continues to improve. She continues to lead a more active lifestyle, maintaining regular activities such as cycling, working out at the gym and taking long weekend walks. In her Pilates practice, she does many intermediate and advanced level exercises. These include Side-Overs on the ladder barrel (1st rung), the Up-Stretch series, full standing lunges on the reformer, and teasers. However, her regular time slot is 7:30pm on a Wednesday, so she often is exhausted and lacks concentration. If this is the case the session is focused on shoulder openers, gentle core strength, low load work focusing on alignment, and we move the spine in all directions (flexion, extension, lateral flexion and rotation).
Analysis:
We focused immediately on strengthening and mobilizing the affected ankle. But particular attention was paid to faulty movement patterns in the entire left side. We used visual and tactile feedback to reinforce these movement patterns. Low load exercises with functional movement patterns incorporating joints’ full range of motion were given wherever possible. The low loads recruited smaller motor units, which therefore stimulated proprioceptive activity. Repetition of these functional movements, accompanied by positive feedback regarding alignment and muscle recruitment reinforced this proprioceptive training. Furthermore, the Wobble Board introduced an unstable surface that challenged the ankle to adjust to timed weight shifts on a subconscious level.
Over time the movement patterns became more complex to challenge and improve neuromuscular programming. The strengthening and mobilizing of the musculature surrounding the ankle were important aspects to Siobhan’s rehabilitation. But without reprogramming her joint recruitment sequences and movement patterns, Siobhan’s recovery may not have been as complete. The exercises she performed emphasized pre-activation of core stabilizers and co-contraction of agonist and antagonist muscles. These patterns eventually became automatic and allowed her move in a more balanced manner. This not only allowed for an almost complete recovery, but also seems to prevent her from reinjury now that she has returned to more vigorous physical activity.
Bibliography:
Clayman, CB (1989). The American Medical Association Encyclopedia of Medicine (Random House, New York, NY)
Delcomyn, F (1998). Foundations of Neurobiology. New York: W.H. Freeman and Company.
Dover GC, Kaminski TW, Meister K, Powers ME, Horodyski M (2003). Assessment of Shoulder Proprioception in the Female Softball Athlete. American Journal of Sports Medicine, May/June
Felicetti G, Contardi A & Rossato S (2004). Proprioceptive Evaluation of a Group of Runners Before and After Training. The Rehabilitation of Sports Muscle and Tendon Injuries, International Congress 2004
Gibbons, Sean G.T. & Comerford, Mark J. (2001). Strength Versus Stability: Part 2: Limitations and Benefits. Orthopaedic Division Review. March/April: 28-33
Hodges, PW, Richardson, CA, Jull, G (1997). Contraction of the Abdominal Muscles Associated with Movement of the Lower Limb. Phys Ther, 77:132-14
Hutton RS, Atwater SW (1992). Acute and Chronic Adaptations of Muscle Proprioceptors in Response to Increased Use. Sports Medicine, Dec; 14(6): 406-21
Jeka J, Kiemel T, Creath R, Horak F & Peterka R (2004). Controlling Human Upright Posture: Velocity Information Is More Accurate Than Position or Acceleration. Journal of Neurophysiology 92: 2368-2379
Jones DA, Rutherford OM & Parker DF (1989). Physiological Mechanisms Involved in Genesis and Spread of Muscular Tension in Occupational Muscle Pain and Chronic Musculoskeletal Pain Syndromes. Medical Hypothesis, 35:196-203
Kaminski TW, Buckley BD, Powers ME, Hubbard TJ, Ortiz (2003). Effect of Strength and Proprioception Training on Eversion to Inversion Strength Ratios in Subjects with Unilateral Functional Ankle Instability. British Journal of Sports Medicine, Vol 37: 410-415
Kawaguchi, Jeffrey (1999). Ankle: Proprioceptive Exercises Balance Rehabilitation. BioMechanics Rehabilitation Supplement, November 1999.
Kravitz L & Heyward V (1995). Flexibility Training. Fitness Management, 11(2), 32-33, 36-38
Laskowski, ER, Newcomer-Aney K, Smith J (1997). Refining Rehabilitation With Proprioception Training: Expediting Return to Play. The Physician and Sportsmedicene. Vol 25, No. 10
Lephart SM, Pincivero DM, Giraldo JL & Fu FH (1997). The Role of Proprioception in the Management and Rehabilitation of Athletic Injuries. The American Journal of Sports Medicine. Vol 25, No. 1: 130-137
Mattacola, CG, Dwyer MK (2002). Rehabilitation of the Ankle After Acute Sprain or Chronic Instability. Journal of Athletic Training, Vol 37, No. 4:413
Newton CA (1982). Joint Receptor to Reflexive-Kinetic Responses. Physical Therapy 62(1):20-22
Nottingham, Suzanne (2003). Training for Proprioception & Function. Fitness Management Magazine
Pearson KG, Misiaszek JE & Fouad K (1998). Enhancement and Resetting of Locomotor Activity by Muscle Afferents. Annals of the New York Academy of Sciences, 860: 203-215
Panjabi, MM (1992). The Stabilising System of the Spine. Part 1. Neutral Zone and Stability Hypothesis. Journal of Spinal Disorders 5:390-397
Richardson C, Jull G, Hodges P, Hides J (1999). Therapeutic Exercise for Spinal Segmental Stabilization in Low Back Pain: Scientific Basis and Clinical Approach. London, UK: Churchill Livingstone.
Riemann BL, Guskiew KM, Contribution of Peripheral Somatosensory System to Balance and Postural Equilibrium. In Proprioception and Neuromuscular Control in Joint Stability, Lephart, S & Fu F (Eds.) p. 39-40. Champaign, IL: Human Kinetics, 2000.
Riemann BL, Lephart SM (2002). The Sensorimotor System Part I: The Physiologic Basis of Functional Joint Stability. Journal of Athletic Training. March; 37(1); 71-79
Riemann BL, Lephart SM (2002). The Sensorimotor System Part II: The Role of Proprioception in Motor Control and Functional Joint Stability. Journal of Athletic Training, March; 37(1); 80-84
Sargant, Darryn (2000). Proprioception: How Does It Work? A Simplified Overview of Anatomical Structures and Neurophysiological Actions Involved in Joint Stability. Australasian Journal of Podiatric Medicine. Vol 34, No. 3: 86-92
Sherrington CS (1906). On The Proprio-Ceptive System, Especially In Its Reflex Aspect. Brain 29:467-482
Simpson, Ralph (2003). Motor Control, Muscle Function and the ‘Instant Replay’. CDM Sport Online. http://www.cdmsport.com/studies/studies_nav.html
Swanik CB, Lephart SM, Giannan Tonio FP, Fu FH (1997). Reestablishing Proprioception in the ACL Injured Athlete. Journal of Sport Rehabilitation 6(2): 182-206
Tyler M, Danilov Y & Bach-Y-Rita P (2003). Closing an Open-Loop Control System: Vestibular Substitution Through the Tongue. Journal of Integrative Neuroscience, Vol 2, No. 2: 159-164. Imperial College Press
Van Boven RW, Johnson KO (1994) A psychophysical study of the mechanisms of sensory recovery following nerve injury in humans. Brain 117:149-167