The activity of 36 pairs of single motor units were recorded with intramuscular wire electrodes from m. extensor carpi radialis while subjects performed slow wrist extension and flexion movements. Periods of steady position holding were interposed between movements. The discharge trains from pairs of motor units were analysed statistically in the time and frequency domains. During extension movements, when the muscle recorded from was the agonist, coherence between motor units was significant below 12 Hz, with a peak at 6–12 Hz in 30 of 36 pairs (83 %). The magnitude of coherence decreased during position holding compared to movements in 26 pairs, while the difference in average firing rate was small. During movements, but not during position holding, coherence estimates between single motor units and acceleration showed a significant peak at 6–12 Hz in 56 out of 62 motor units, suggesting that a modulation of motor unit discharge contributed to angular acceleration at these frequencies. Common motor unit modulation was present at 3 Hz as well, although the coupling between motor unit activity was weaker than at 6–12 Hz. It is concluded that a 6–12 Hz common modulation of agonist motor units is a distinguishing feature of slow voluntary wrist movements, extending the previously established notion of an 8–10 Hz rhythmic organization of slow finger movements to more proximal limb segments. It is suggested that the 6–12 Hz input is specific for movements and is normally absent or much weaker during steady maintenance of position or force.

Twenty-five human muscle afferents from the extensor digitorum muscles of the forearm were studied with the microneurographic method. Single unit impulses were recorded while the subjects performed alternating movements of moderate speed at the appropriate metacarpophalangeal joint. For comparison, responses to imposed movements of similar amplitudes and velocities were also studied. Most spindle afferents (n = 17) provided a stretch response with both kinds of movement. However, the impulse rate was slightly higher and the interspike interval variability much larger during active movement. Two units provided deviating response profiles: a flat profile and a converse stretch response. Small and constant torque loads usually failed to modify the response profile but gave rise to a moderate increase of impulse rate in 50% of the spindle afferents. In one single unit, a converse stretch response appeared with opposing loads. Tendon organ afferents (n = 8) were totally unmodulated by imposed stretch in the relaxed muscle. In contrast, their impulse rate was highly modulated during active movements, often following the rectified EMG which resulted in a converse relationship to muscle length and velocity. The findings support the view that, in general, human muscle spindles monitor muscle length and velocity in routine movements of moderate speed as long as opposing loads are small, whereas Golgi tendon organs monitor the amount of muscle recruitment. The significance of the deviating response profiles from spindle afferents remains obscure.

1. The response of twenty-eight human muscle spindle afferents from m. extensor carpi radialis brevis to large amplitude ramp stretch and release at the wrist joint was recorded. The dynamic index was calculated as the difference in firing rate between that just before the end of stretch and that during the subsequent static phase of stretch. The value during steady voluntary contraction was compared with that during relaxation. 2. In twenty-three primary afferents, the dynamic index increased in eleven and decreased in twelve afferents with a range of −8 to +25 impulses s¢. In five secondary afferents the change was less than 2 impulses s¢. 3. The primary afferents abruptly stopped firing when the stretch was released in the relaxed muscle. This cessation was prevented during contraction in seventeen primary afferents. 4. The results suggest the presence of dynamic and static fusimotor actions on the human muscle spindles during voluntary contraction.

In visually-guided slow ramp elbow tracking, patients with cerebellar ataxia show irregular undulations of pursuit velocity which result in a position tracking pattern unlike the smooth constant velocity or rate tracking pattern of normal controls (Beppu et al., 1984). This task provides ample opportunity to use both visual and proprioceptive feedback information for correcting errors. The present study investigated the role of visual information for generation of this saccadic pursuit pattern in the patients. A television screen was divided into upper and lower halves in each of which a vertical strip was displayed. The upper strip (T, target) was moved horizontally from the centre of the screen to the left or right by ramp voltage. The lower strip (D, displacement of the handle) was moved in proportion to angular displacement of the handle by a potentiometer coupled to the handle axis. The subject, while sitting in front of the screen, had to make D match the movement of T by controlling the handle with his arm. The range of T movement was 30 deg in terms of the angular movement of the handle. T velocity was 6.0 or 7.5 deg/s. After a training session, the test was performed in which D or T was suddenly erased from the screen during pursuit, depriving the subject of visual information about the moving limb and/or performance. The procedure gave only minor effects on the performance of the control subjects, but it reduced significantly the velocity undulation of the patients with cerebellar ataxia, producing a smoother continuous pursuit. There were no significant differences in performance between D or T erase tasks. The result supports the hypothesis that the marked undulation pattern during pursuit movement in cerebellar ataxia is due to repeated visually-guided error correction responses. The relative importance of the visual pathway for conveying position information and the proprioceptive pathway for movement velocity information in this visual slow ramp tracking task is also discussed.

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