Role of Central and Proprioceptive Inputs in Muscle Activation

Presumably because of the relatively large number of sensor types and their complex behavior, the precise role of muscle receptors and their contribution to movement control are unknown. It is generally accepted that the initial accelerating part of fast goal-directed movements is made without the use of afferent feedback. Presumably the first effect of proprioceptive information on the motor program coming from the muscles themselves or the surrounding skin becomes evident about 115 msec after a perturbation or a deviation from the intended movement trajectory.

Reflex mechanisms may affect the muscle activation pattern at shorter delay times, and they give rise to the same relative activation of muscles as observed in isometric contractions. Because it has been demonstrated that variability in the accelerating phase is initially high and that it decreases during the first 100 msec of the movement (i.e., before sensory information can become effective), this has been interpreted as evidence for the use of an internal feedback loop.

After the initial accelerating phase, sensory information may affect the activation pattern of muscles. On the basis of all available evidence, virtually everybody now agrees that muscle sensors contribute to position sense and to the control of movements. Recent experiments have demonstrated that the type of information used depends on the motor task. For example, it has been shown that an observer who is instructed to maintain his arm at a given target position while his biceps is vibrated by pulses with a repetition rate of about 100 Hz flexes the arm. (The subject cannot see the arm!) When he is instructed to move the arm fast to the target position, he will be surprised by the instruction because he thinks he is already at the target.

However, when he is pushed to obey the instruction, he makes a movement and brings the arm accurately to the target. This suggests that certain motor acts employ a quite different map of limb position from that perceived. Electromyographic recordings ruled out a simple mass-spring strategy and demonstrated that subjects behaved as if the motor system really knew the correct position of the arm even though it was not perceived.

With regard to other types of sensors, the role of joint receptors in motor control is still unknown. Most experimental observations indicate that joint receptors are effectively excited only when the limb is moved to one of its extremes. In the middle of the range, most, if not all, receptors are normally silent. However, there is evidence that joint receptors may modulate the reflex gain not only in the extremes of the physiological range, but throughout the full range of joint action.

Recently, more and more information suggests that cutaneous receptors do contribute to normal movements and to reflex activity. However, there is no consensus yet about their precise role. The same is true for the Golgi-tendon organs, although several hypotheses have been put forward as to their role, for example, for stiffness regulation.

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