Muscle spindle activity in man during shortening and lengthening contractions.

1. The responses of forty‐one muscle spindle endings, mostly in tibialis anterior, were studied in human subjects during voluntary movements of the ankle joint performed at various speeds against different external loads. 2. During slow shortening contractions, the discharge rates of spindle endings in the contracting muscle accelerated after the appearance of the first e.m.g. potentials but before sufficient force had been generated to move the limb. With some endings, the discharge rate decreased during the shortening movement while the e.m.g. activity was increasing, but it always remained higher than before the onset of contraction. If the speed of the movement was increased fewer spindle discharges were seen during muscle shortening. If the shortening contraction was opposed by an external load, so that greater effort was required to perform the same movement, more discharges were seen and the discharge pattern became less modulated by the change in muscle length. 3. These findings indicate that during shortening contractions the fusimotor system is activated together with the skeletomotor system. However, the fusimotor drive is generally insufficient to maintain a significant spindle discharge unless movement is slow or the muscle is shortening against an external load. 4. During lengthening contractions the spindle responses were greater than to passive stretch of similar amplitude and velocity, suggesting heightened fusimotor outflow. 5. During shortening and lengthening contractions small iregularities in the speed of movement occurred commonly. Unintended acceleration of a shortening movement caused a pause in spindle firing, and unintended acceleration of a lengthening movement caused an increased discharge from spindle endings. These spindle responses were associated with corresponding alterations in the discharge pattern of the voluntarily activated motor units at latencies consistent with the operation of spinal reflex mechanisms. 6. It is suggested that a functional role for the fusimotor activation during slow shortening contractions is to provide spindle endings with a background discharge so that they can detect irregularities in the movement and initiate the appropriate reflex correction.

[1]  A B Vallbo,et al.  Discharge characteristics of human muscle afferents during muscle stretch and contraction. , 1968, Experimental neurology.

[2]  P. Matthews,et al.  The sensitivity of muscle spindle afferents to small sinusoidal changes of length , 1969, The Journal of physiology.

[3]  P. Matthews.  Evidence that the secondary as well as the primary endings of the muscle spindles may be responsible for the tonic stretch reflex of the decerebrate cat , 1969, The Journal of physiology.

[4]  K. Hagbarth,et al.  Afferent impulses in median nerve fascicles evoked by tactile stimuli of the human hand. , 1970, Brain research.

[5]  A. Vallbo,et al.  Slowly adapting muscle receptors in man. , 1970, Acta physiologica Scandinavica.

[6]  A. Vallbo,et al.  Discharge patterns in human muscle spindle afferents during isometric voluntary contractions. , 1970, Acta physiologica Scandinavica.

[7]  A. Vallbo,et al.  Muscle spindle response at the onset of isometric voluntary contractions in man. Time difference between fusimotor and skeletomotor effects , 1971, The Journal of physiology.

[8]  A. Vallbo,et al.  Muscle Spindle Afferent Discharge from Resting and Contracting Muscles in Normal Human Subjects , 1973 .

[9]  A. Struppler,et al.  Afferent and efferent activation of human muscle receptors involved in reflex and voluntary contraction. , 1973, Experimental neurology.

[10]  Recordings from Muscle Afferents in Parkinsonian Rigidity1 , 1973 .

[11]  P. Matthews,et al.  Mammalian muscle receptors and their central actions , 1974 .

[12]  A. Struppler,et al.  Activity of muscle spindles during muscle twitch and clonus in normal and spastic human subjects. , 1974, Electroencephalography and clinical neurophysiology.

[13]  A. Vallbo,et al.  Human muscle spindle discharge during isometric voluntary contractions. Amplitude relations between spindle frequency and torque. , 1974, Acta physiologica Scandinavica.

[14]  L. Harrison,et al.  Analysis of activity of muscle spindles of the jaw‐closing muscles during normal movements in the cat. , 1975, The Journal of physiology.

[15]  E. Luschei,et al.  Discharge of spindle afferents from jaw-closing muscles during chewing in alert monkeys. , 1975, Journal of neurophysiology.

[16]  K. Hagbarth,et al.  Muscle spindle activity in man during voluntary fast alternating movements. , 1975, Journal of neurology, neurosurgery, and psychiatry.

[17]  A. Prochazka,et al.  Discharges of single hindlimb afferents in the freely moving cat. , 1976, Journal of neurophysiology.

[18]  D. Burke,et al.  The responses of human muscle spindle endings to vibration of non‐contracting muscles. , 1976, The Journal of physiology.

[19]  D. Burke,et al.  The responses of human muscle spindle endings to vibration during isometric contraction. , 1976, The Journal of physiology.

[20]  A. Prochazka,et al.  Ia afferent activity during a variety of voluntary movements in the cat , 1977, The Journal of physiology.

[21]  Lars Lofstedt.  An Apparatus for Generating Controlled Ramp Movements During Studies of Muscle Spindle Afferent Activity and Muscle Tone in Man , 1978, IEEE Transactions on Biomedical Engineering.