1. Sudden dorsiflexions and plantarflexions of the foot were imposed on normal human subjects under various states of voluntary activity. 2. Under conditions of constant muscle contraction, the myotatic reflex in soleus and lateral gastrocnemius muscles is linearly and highly correlated with the rate of muscle stretch. The slope of this curve characterizes part of the reflex arc "gain." 3. The gain is linearly proportional to the level of tonic voluntary activation. 4. The gain is reduced by tonic contraction of antagonists. 5. The above statements can be summarized by the following equation (formula: see text), where d theta/dt is the rate of joint rotation. Ts and Tat are measures of voluntary contraction (tension) of all the extensor and flexor muscles acting at the ankle. The term S represents the level of preexisting spinal excitability that can be altered by prior instruction to the subject. 6. A phasic voluntary contraction of the soleus muscle, which leads to muscle shortening, will alter the reflex gain. The gain initially increases with increasing rates of shortening, but at higher rates the gain is reduced. This is in contradiction to the observation for tonic activation as stated above and may be due to an inability of the coactivated fusimotor system to produce sufficiently rapid cocontraction of the spindle fibers. 7. During lengthening of a muscle caused by voluntary contraction of its antagonists, the myotatic reflex gain is reduced. 8. The above facts are interpreted to imply that a functional role for the myotatic reflex in the leg extensors is limited to conditions of postural maintenance or slow, precise movement. During rapid movement, the myotatic reflex is ineffective and load-compensating reactions are mediated by longer latency loops. 9. The duration of the myotatic reflex EMG is from 10 to 40 ms, too brief to be a simple response to a velocity-sensing receptor organ. Either the response is in large measure due to the initial burst of spindle activity that occurs at the start of a ramp stretch, or motoneuron pool dynamics act as a high-pass filter on afferent inputs. 10. In the anterior tibial muscle, the relationships between stretch velocity and reflex amplitude and tonic voluntary contraction and reflex gain are qualitatively similar to those found in the ankle extensors.

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1. Torques were applied to dorsiflex or plantarflex the ankle joint of normal human subjects who were instructed to react to them in various ways. Myoelectric and mechanical responses were recorded and analyzed. 2. Myoelectric responses in triceps surae or tibialis anterior with times of onset between 100 and 200 ms are conditional on the instruction to the subject and not the direction of the applied torque. These responses are termed postmyotatic responses. 3. The latency of the postmyotatic response is the same in both triceps surae and tibialis anterior. It is not dependent on the direction of the applied torque and consequently is not a "stretch reflex," but a triggered reaction. 4. When the subject is sitting relaxed and instructed to resist the applied torque, the magnitude of the postmyotatic response is highly and linearly correlated with the rate of ankle rotation produced by the disturbing torque. 5. Tonic contraction of the muscles prior to the onset of a stretching torque has only a slight effect on the magnitude of the postmyotatic response and usually reduces it. Prior tonic contraction of the antagonist also has a modest effect in reducing the response. 6. Instructing the subject to resist a torque as quickly and strongly as possible increases the magnitude of the postmyotatic response but has little effect on its linear functional relationship to the magnitude of the mechanical stimulus. 7. The postmyotatic response, although voluntary and kinesthetically triggered, is not completely equivalent to a visually triggered voluntary reaction because under experimental situations designed to measure simple reaction times and choice reaction times of the postmyotatic response, no significant difference is found between the two conditions. 8. It is suggested that the peripheral afferent pathways of the stretch reflex play a dual role in determining postmyotatic responses. First, they provide to supraspinal centers one early proprioceptive signal about limb perturbation that can trigger a previously selected mode of response. Second, they modulate that response by convergence, at the segmental level, of peripheral afferent signals with the descending command signals.

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