INJURY PREVENTION

Joint stability is a function of congruency, ligament strength, and muscle function. Regardless of the degree of disassociation of the joint surfaces, from subluxation through to dislocation, pain with a multitude of distribution scenarios can develop. The goal of this article is to examine applied musculo-neural physiology in relation to joint function and movement control. This relationship will start with the timing associated with muscle contraction and the differences in planning and reaction to forces, and the variables that influence muscle function. Current research will provide the basis for functional control of joints leading to improved joint stabilisation.

JOINT STABILISATION THROUGH DYNAMIC MUSCLE ACTIVATION

By Richard DeMont, PhD, CAT(C), ATC, and Deanna Errico, PT, DPT, MSEd, ATC.

JOINT STABILITY Given that within any movement activity there is little control over the influence of the vestibular (the balance system controlled in the ear) and visual input to the motor control system, to improve control, the somatosensory input must be enhanced. The most important factor in this somatosensory input is joint and muscle position sense.

Generally, in thinking about the gait pattern and balance, one’s automatic response typically derives from changes in the somatosensory component and is reflexive in nature. In many circumstances, a reflexive movement may be enough to control movement of a joint, and even prevent injury. However, in the occurrence of non-contact joint injury, the reflexive motor response is not sufficient. This means therefore that to prevent injury, one would need to improve the stability of a joint prior to the offending position. It could be said that in the case of subluxed joints, a similar action from muscles may be of benefit. In addition, with the appropriate functioning of muscle contraction (muscle order, coordination, and force ability), a person would be able to distribute evenly the force going through the surrounding muscles to avoid overuse syndromes.

The muscular system can maintain joint congruency when the static restraints (such as ligaments) are jeopardised (1,2) and this is accomplished through compensation for the injury through changes in muscle activation (3). From this paradigm, the question arises about the ability of

the muscular system to provide preventative joint stability through pre-activation.

Role of preactivation in joint stability Timing of the muscle activation is paramount to explaining the

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In a recent study (9) using a dual-task paradigm, subjects exhibited changes in muscle activation during walking while under the pressures of a cognitive load. While less direct influence on the

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stability efforts of muscle. Lephart and colleagues (4) failed to confirm the prevailing belief that the restraint mechanism was mediated by strength. While strength is an important component to function, timing of the contraction may prevail in importance to joint stability. Studies related to reaction time in the lower extremity suggest that following perturbations, reaction time of the hamstring are beyond 200ms (5), yet joint positions representing injury are typically thought to occur in less than 150ms. A measure to evaluate muscle activation in the 150ms prior to accepting force in weight bearing activities has been labelled 'pre-activation'.

One of the first papers (6) to describe pre-activation was done in arm catching, where the load of objects caught changed, and subjects were unprepared for these changes based on their experience. In other studies of pre-activation, differences have been found in different activities, which may reflect both the specific activity (gravity and/or position) and the efforts of the neural control mechanism to prepare the joint for the predicted (experience based) load acceptance. Evidence to support this is available in studies of upper and lower extremity movements, and in studies comparing healthy, injured, and rehabilitated subjects. In one study by DeMont (3), examining these three groups in downhill walking, the post-rehabilitation subjects returned to the pre-activation levels of the healthy subjects and the ligament deficient individuals exhibited less pre-activation. In some of his other studies (7,8) on position and activity, subjects’ pre-activation was altered in gravity dependent positions, but also appeared to be dependent on items such as speed of activity and difficulty level. However, more research in this area is required. Another influence on pre-activation could be cognitive in nature.