Sensori-motor control of the uninjured and injured human ankle
Introduction
Research of ankle sensori-motor control in humans has been of interest in neurophysiology, where the main focus has been on the reaction to changes in body equilibrium, and in sports traumatology concerning chronic functional ankle instability.
In the former line of research, the reaction of lower limb muscles to sudden but small perturbations around a dorsal/plantar flexion axis has been studied under various conditions. This has led to understanding of the different elements of the reflex EMG response, of the role of peripheral and higher center strategies on different response patterns, and of the effect of repetition and learning on the modulation of perturbation responses.
In the latter line of research, the sensori-motor control of the ankle/foot complex in the inversion/eversion plane has been studied with special reference to the maintenance and loss of functional ankle stability.
As the symposium, over which this theme issue is build up, was concerned primarily with sensori-motor control and ligament reflexes before and after peri-articular injury, this article deals with findings from research in the latter field.
The notion that damage to ankle sensori-motor control could have an impact on the functional stability of the ankle evolved from the observations that subjects with complaints of repeated ankle inversion injuries also showed an increased degree of sway during single limb stance [4], [26]. Research in this field has since then primarily concentrated on proving or disproving coexistence between functional ankle instability and deficits in different measurements of ankle sensori-motor control.
Section snippets
Functional ankle instability
The disability designated functional ankle instability has no universally agreed upon definition. The term is used to describe the repeated ankle inversion injuries and ‘giving way’ feelings that certain subjects experience after an ankle sprain. There are no requirements as to how often distortions need to be sustained, how long the disability has had to be present, at what level of activity injuries are sustained or at what degree of external action. The inversion injuries and the giving way
Maintaining and loosing functional ankle stability
Inversion injuries are by far the most common ankle distortion injury and their mechanism of injury will be considered below.
The axially loaded ankle/foot complex is usually very stable especially in the neutral flexion/extension position. As shown by Stormont et al. [27] the axially loaded talo-crural joint resists inversion due to the bony restrictions of the joint. The axis of the subtalar joint passes lateral in relation to the axially compressive forces, which are generated during
The source of peripheral afferent information in sensori-motor tests
Type II and III mechanoreceptors are present in the lateral ligaments of the ankle [22] but also in many other structures about the lateral aspect of the ankle (capsule, retinaculum, and tendon tissue). So afferent information from the area is not based on input from only one anatomical structure, like for example the lateral ligaments. In agreement with this, studies of proprioceptive ankle function in healthy subjects have not resulted in any deterioration in single limb stability or joint
Choosing methods for measuring sensori-motor control of the ankle
A number of tests have been used for measuring changes in ankle sensori-motor control. Tests measuring sway during single limb stance have been the most commonly used and they provide easy tools for testing patients for their overall neuromuscular function and can be very effective in helping patients realize the need for rehabilitation. The pathogenetic connection between a balance disability and the problem of sustaining repeated ankle sprains (characteristic for functional ankle instability)
Sensori-motor preparedness to avoid inversion prone situations
In the normal stride during level surface walking, the latter part of the swing phase seems to require very accurate proprioception. In this part of the stride the lateral border of the foot passes just 5 mm above the ground surface [30]. In a cadaver study, where the swing phase of the lower limb was simulated, we have found that an impact occurring between the lateral border of a 20° inverted foot and the ground surface resulted in rotation of the foot into at least 40° of inversion, 40° of
Sensori-motor reaction to sudden ankle inversion
When an inversion torque threatens to cause damage to the lateral structures of the ankle, subjects seem to react in a consistent way [17]. The reaction is a combination of a peripheral muscle response and higher center movement strategies.
The first muscular reaction measured after sudden inversion of the ankle is an EMG response in the peroneal muscles [17], [24] occurring from 49 to 90 ms after the inversion [1], [7], [9], [10], [14], [18], [19], [23], [28]. The former delay can suggest a
Conclusion
Since the mid-1960s a sensori-motor deficit has been considered an important pathogenetic factor in functional ankle instability. This conception was primarily based on observations of poor single limb balance among functionally unstable subjects. Subsequent research has primarily focused on proving or rejecting this connection using a variety of sensori-motor tests, while discussing the clinical relevance of the often very small deficits that could be detected. The pathogenetic models
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