The role of joint biomechanics in determining stretch reflex latency at the normal human ankle

In order to study the influence of biomechanical factors on the timing of stretch reflex activity in the ankle extensor musculature, well defined, small amplitude and relatively rapid dorsiflexing stretch was applied to the ankle of seated normal human subjects at a series of angles within the range of physiological movement. If the ankle musculature was relaxed, a single reflex component appeared in the Triceps surae (TS) EMG with a latency compatible with a predominantly monosynaptic pathway. The latency of this response could be prolonged by applying stretch from an initially plantarflexed position and, similarly, decreased by applying stretch from a dorsiflexed position. A decrease in latency of 5-30 ms could be achieved by altering the pre-displacement ankle angle from 105 to 75 degrees. Intermediate changes in the start angle led to intermediate changes in latency. This trend was highly linear. If stretch was applied while the subject maintained a low level contraction in the TS, however, this shift in latency was abolished, with the earliest reflex components appearing with a latency obtained in the relaxed state at or close to maximum dorsiflexion. It is suggested that this shift in latency results from the properties of the long, compliant tendon through which joint movements are transmitted to the TS muscle. This shift in latency caused by passive alteration in the ankle angle at which a reflex was evoked should be taken into account when classifying reflexes arising from a mechanical input, or when using latency determinations as evidence for the involvement of particular pathways in their genesis.