Journal of Neurophysiology, Vol 42, Issue 1 91-106, Copyright © 1979 by APS

ARTICLES

Response to sudden torques about ankle in man: myotatic reflex

G. L. Gottlieb and G. C. Agarwal

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.

This article has been cited by other articles:

				J. A. Pruszynski, I. Kurtzer, and S. H. Scott Rapid Motor Responses Are Appropriately Tuned to the Metrics of a Visuospatial Task J Neurophysiol, July 1, 2008; 100(1): 224 - 238. [Abstract] [Full Text] [PDF]

				N. J. Cronin, J. Peltonen, M. Ishikawa, P. V. Komi, J. Avela, T. Sinkjaer, and M. Voigt Effects of contraction intensity on muscle fascicle and stretch reflex behavior in the human triceps surae J Appl Physiol, July 1, 2008; 105(1): 226 - 232. [Abstract] [Full Text] [PDF]

				P.H.J.A. Nieuwenhuijzen and J. Duysens Proactive and Reactive Mechanisms Play a Role in Stepping on Inverting Surfaces During Gait J Neurophysiol, October 1, 2007; 98(4): 2266 - 2273. [Abstract] [Full Text] [PDF]

				C. Gruneberg, J. Duysens, F. Honegger, and J.H.J. Allum Spatio-Temporal Separation of Roll and Pitch Balance-Correcting Commands in Humans J Neurophysiol, November 1, 2005; 94(5): 3143 - 3158. [Abstract] [Full Text] [PDF]

				V. Dietz and S. J. Harkema Locomotor activity in spinal cord-injured persons J Appl Physiol, May 1, 2004; 96(5): 1954 - 1960. [Abstract] [Full Text] [PDF]

				V. Dietz, R. Muller, and G. Colombo Locomotor activity in spinal man: significance of afferent input from joint and load receptors Brain, December 1, 2002; 125(12): 2626 - 2634. [Abstract] [Full Text] [PDF]

				J. J. Eng and J. A. Hoffer Regional Variability of Stretch Reflex Amplitude in the Cat Medial Gastrocnemius Muscle During a Postural Task J Neurophysiol, August 1, 1997; 78(2): 1150 - 1154. [Abstract] [Full Text] [PDF]