Response to sudden torques about ankle in man: myotatic reflex

Abstract

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.

Modulation of reflex activity of motor units in response to stretch of a human finger muscle.

“…In contrast to the situation of a tonic contraction, the magnitude of the stretch reflex response induced during phasic movement changes according to phase of the movement (DUFRESNE et al, 1980) and velocity of the movement (GOTTLIEB and AGARWAL, 1979). During tonic contraction, the stretch reflex response is modulated by various factors, such as magnitude and velocity of length perturbation (GOTTLIEB and AGARWAL, 1979), instruction of how to react to the disturbance (see the introduction), level of tonic contraction, predictability of the disturbance, and co-contraction of antagonist muscles.…”

“…During tonic contraction, the stretch reflex response is modulated by various factors, such as magnitude and velocity of length perturbation (GOTTLIEB and AGARWAL, 1979), instruction of how to react to the disturbance (see the introduction), level of tonic contraction, predictability of the disturbance, and co-contraction of antagonist muscles. In the present study, we attempted to keep these factors constant.…”

“…From the first study of HAMMOND (1956), it has been extensively investigated whether or not stretch reflex responses are modulated according to types of motor task (HAGBARTH, 1967;EVARTS and TANJI, 1974;CRAGO et al, 1976;EVARTS and GRANIT, 1976;ILES,1977;COLEBATCH et a1.,1979;GOTTLIEB and AGARWAL, 1979;ROTHWELL et a1.,1980). In these studies, subjects were instructed to react in one of two contrasting ways to the occurrence of a mechanical disturbance, e.g., either to resist or to ignore the disturbance.…”

“…Changes in the long latency stretch reflex response by virtue of the instruction were observed, but the gain of the short latency reflex (myotatic reflex) could hardly be affected by such a difference in instructions. The operating force of the muscle, however, could change the gain of the myotatic reflex to a great extent (GOTTLIEB and AGARWAL, 1979).…”

Reflex responsiveness of a human hand muscle when controlling isometric force and joint position

“…The magnitude of the reflex response has been shown to increase in proportion to the level of preexisting tonic muscle activation (Gottlieb and Agarwal, 1979;Al-Falahe and Vallbo, 1988), which is often greater for novel tasks involving unstable loads due to the coactivation of agonist and antagonist muscles (De Serres and Milner, 1991;Osu et al, 2002). In an attempt to minimize the effects of coactivation on tonic activity of the first dorsal interosseus muscle, all subjects practiced the MVC, force, and position tasks during a familiarization session 1-3 days prior to the experiment.…”