Differential activation of motor units in the wrist extensor muscles during the tonic vibration reflex in man.

Abstract

1. Single motor unit activity was recorded in the extensor carpi radialis longus and extensor carpi radialis brevis muscles of five healthy human subjects, using metal microelectrodes. 2. Motor units were characterized on the basis of their twitch contraction times and their force recruitment thresholds during voluntary imposed-ramp contractions. 3. The discharge patterns of forty-three motor units were studied during tonic vibration reflex elicited by prolonged (150 s) trains of vibration (30 Hz) applied to the distal tendons of the muscles. The temporal relationships between the individual small tendon taps of the vibratory stimulus and the motor unit impulses were analysed on dot raster displays and post-stimulus time histograms. 4. After tendon taps, the impulses of motor units with long twitch contraction times (mean +/- S.D., 47.2 +/- 10.7 ms) and low recruitment thresholds (0.88 +/- 0.6 N) formed a single narrow peak (P1) with a latency (22.7 +/- 1.4 ms) which was comparable to that of the tendon jerk in the extensor carpi radialis muscles. These motor units were named 'P1 units'. On the other hand, the response of motor units with shorter twitch contraction times (31.1 +/- 3.3 ms) and higher recruitment thresholds (3.21 +/- 1.3 N) showed two peaks: a short latency (23.4 +/- 1.3 ms) P1 peak similar to the previous one and a P2 peak occurring 9.4 +/- 1.2 ms later. These motor units were named 'P1-P2 units'. 5. When the reflex contraction increased slowly, the P1 peaks of 'P1-P2 units' were clearly predominant at the beginning of the contraction, during the rising phase of the motor unit discharge frequency, while the P2 peaks became predominant when the units had reached their maximal discharge frequency. 6. Increasing the tendon vibration frequency (35, 55, 75, 95 Hz) did not modify the 'P1 unit' discharge pattern. Due to interference between vibration period and peak latencies, increasing the vibration frequency caused the P1 and P2 peaks of 'P1-P2 units' to overlap. 7. Superficial cutaneous stimulation of the dorsal side of the forearm during tendon vibration noticeably decreased the P1 peaks in both types of motor units. In the P2 peaks it could result in either a decrease or an increase but the average effect was a slight increase. 8. When applied 10 s before tendon vibration, cutaneous stimulation considerably suppressed the tonic vibration reflex.(ABSTRACT TRUNCATED AT 400 WORDS)

On the nature of the electromyographic signals recorded during vibration exercise

“…However, TVR seems also to be modulated by alterations in spindle sensitivity through γ control (Matthews 1966;Ribot-Ciscar et al 2000). By applying a vibration directly to the tendon, previous studies have suggested TVR to be composed of motor unit activity synchronized and unsynchronized with the vibration cycle (Hori et al 1989;Romaiguère et al 1991). Studies have also indicated that TVR may contribute to muscle fatigue and/or increase the risk of cumulative trauma disorders observed after repetitive exposure of the hand to vibration (Park and Martin 1993).…”

Analysis of Vibration Exercise at Varying Frequencies by Different Fatigue Estimators

“…TVR is derived from the stimulation of Ia-afferents when a sinusoidal vibration is directly applied to a muscle or a tendon [13], [15], [16] and composed of motor unit activity synchronized and unsynchronized with the vibration cycle [17], [18]. Through TVR, muscles try to achieve vibration dampening and joint stiffening by preferential recruitment of faster motor units [13], [19].…”

Facilitation of triceps brachii muscle contraction by tendon vibration after chronic cervical spinal cord injury

“…These reflex contractions may be of two types, depending on the context in which the vibratory stimulation is applied. Vibration may evoke a tonic vibratory reflex (TVR) in the parent muscle when the subject looks at his or her arm, a response that can involve both mono-and polysynaptic spinal pathways, as well as supraspinal processes (13,14,19,28). When the subjects close their eyes, tendon vibration induces a sensation of illusory movement, which is accompanied, in 70% of healthy subjects, by a contraction of the muscle(s) antagonistic to the vibrated muscle [antagonist vibratory response (AVR)] (4,5,10,26).…”