Response of the normal human ankle joint to imposed sinusoidal movements.

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Abstract
Ankle joints were subjected to sinusoidal movements at a range of amplitudes and frequencies. Records were made of electromyograms (emg) in calf muscles, and of the forces at the joints. When the leg is relaxed, the ankle joint resists an imposed sinusoidal movement with a small, approximately sinusoidal force. It is stiffer in its resistance to small movements than to large ones, and this resistance is greater when the joint is dorsiflexed than when it is plantarflexed. If the subject exerts a steady mean flexing force, the imposed sinusoidal movement generates reflex activity which may be recorded as a modulation of the gastrocnemius and soleus emg. The emg response to the sinusoidal movement occurs later in cycles of movement at high than at low frequencies, as could be expected of a reflex pathway that involves a delay. This delay apparently is between 50 and 60 ms, and under these circumstances spinal stretch reflexes are playing the important part. The relation of the resisting force to the movement was displayed as a vector. As the frequency changes, this vector describes the circular path that is characteristic of a system which includes delays or lags; this path enables one to draw conclusions about the amplitude and timing of the reflex resistance to the movement. When a subject exerts a moderate flexing force against the sinusoidal movement for some minutes, the reflex response becomes progressively potentiated. A subject whose reflex responses are normally slight may then exhibit a vigorous reflex response to the movement of that ankle. This enhancement of spinal reflex activity was accompanied by an increase in the myotatic reflex response at the ankle. Reflex responses to sinusoidal movement were most clearly seen when the subject exerted a mean flexing force that amounted to about 1/5 of his maximum. Very small movements (.+-. 0.5.degree.) generated little or no reflex response. With large amplitudes of movement there was more reflex activity, but at some amplitude (which varied from subject to subject and from time to time) the reflex mechanism appeared to saturate, and further increases in amplitude were not accompanied by comparable increases in the reflex response. With movements of 10-15 Hz the emg response of became large in alternate cycles, with less activity in the intervening cycles. In extreme cases emg activity was confined to alternate cycles, with electrical silence in the intervening ones. This alternation of the emg signal was usually accompanied by similar changes in the force records, so that each cycle with vigorous emg activity was followed by one in which the force rose to a high level.