Organization of motor output in slow finger movements in man.

Abstract

SUMMARY1. Slow finger movements were analysed in normal human subjects with regard to kinematics and EMG activity of the long finger muscles. Surface EMG from the finger extensor and flexor muscles on the forearm was recorded along with angular position and angular velocity during voluntary ramp movements at single metacarpophalangeal joints. Angular acceleration was computed from the velocity record.2. It was found that movements were not smooth but characterized by steps or discontinuities, often recurring at intervals of 100-125 ms, yielding velocity and acceleration profiles dominated by 8-10 Hz cycles. The discontinuities were manifest from the very first trial and thus not dependent on training. Their amplitude and amount varied between subjects but were relatively stable for the individual subject.3. The 8-10 Hz cycles were seen with voluntary ramp movements of widely varying velocities, higher velocities being associated with larger steps recurring with the same repetition rate as the small steps of slow voluntary ramps. Maximal step amplitude observed was more than one order of magnitude larger than physiological tremor.4. The individual 8-10 Hz cycle was asymmetrical in that decelerations usually reached higher peaks than the preceding acceleration, suggesting that the antagonist contributed with a braking action. Moreover, in very slow voluntary ramps, the movement cycles were often interspaced by periods of zero velocity, providing a highly non-sinusoidal velocity profile.5. The EMG of the agonist and the antagonist muscles was modulated in close relation to the accelerations and decelerations respectively of the individual movement cycle. These modulations were present in both extensor and flexor muscles, although they were more consistent and usually more prominent in the former.6. The findings indicate that a feature of slow finger movements was an 8-10 Hz periodic output to the muscular system, suggesting that slow finger movements are implemented by a series of biphasic force pulses, involving not only the shortening agonist muscle propelling the movement, but the antagonist muscle as well whose activity increased shortly after the agonist and contributed to a sharp deceleration of the individual step of movement.7. It is proposed, as a hypothesis, that this biphasic motor output may reflect a similar organization of the descending motor command for slow finger movements.}S 1294 6 A. B. VALLBO AND J WESSBERG Hence, this command would include a series of biphasic pulses, concatenated at a rate of 8-10 per second and a pulse-height regulator capable of setting the size of the pulse and thus the overall speed of the movement.

Fluctuations in acceleration during voluntary contractions lead to greater impairment of movement accuracy in old adults

“…The greatest difference in the standard deviation of acceleration between concentric and eccentric contractions occurred at the fastest movement because of an enhanced contribution to the fluctuations in index finger acceleration in the 0-to 15-Hz bandwidth (peak 5-10 Hz) (27,39,41). This effect increased with movement speed (41) for both types of contractions.…”

Two-phase model of the basal ganglia: implications for discontinuous control of the motor system

“…(1) experiments have shown that single reaching movements have pulses of acceleration at approximately 10 Hz, indicating that single movements contain submovements [16]. Submovements can also be observed by direct recording from muscle; Brown & Cooke [17] found that electromyogram bursts do not have a continuous distribution of durations; instead, they have durations that occur in increments of 70 -80 ms.…”

Movement Smoothness Changes during Stroke Recovery

“…Strong direct evidence for discrete movement primitives in frog wiping reflexes has been shown Kargo and Giszter, 2000) in both force profiles and EMG measurements. Physiological evi-dence for discrete submovements has been reported in healthy human subjects as well; in slow finger movements, Vallbo and Wessberg (1993) showed both discrete kinematic jumps in finger position as well as synchronized pulses of EMG activity in the finger flexors and extensors. If it is shown to be feasible, locating and measuring this internal coding of motor commands could lead to insight into the nature of human motor behavior and motor system pathologies.…”

“…Evidence of discrete submovements has also been found in the movements of healthy subjects (Milner, 1992;Vallbo and Wessberg, 1993). Complex movements have been decomposed into submovements as an analysis tool (Morasso and Mussa-Ivaldi, 1982;Flash and Henis, 1991;Berthier, 1996;Burdet and Milner, 1998) with apparent success.…”