Ia afferent input alters the recruitment thresholds and firing rates of single human motor units

Single motor units are recruited voluntarily in response to synaptic input from central and peripheral sources but they can also be recruited involuntarily through reflex pathways. In the present experiment, vibration applied to the patellar tendon was used to excite muscle spindles in the knee extensors and elicit a gradual increase in force via Ia afferent reflex pathways. With the aid of visual feedback, subjects (n = 10) voluntarily reproduced the force profile elicited by vibration and with the use of tungsten microelectrodes, the behaviour of the same motor unit (n = 135) in the vastus lateralis was examined under both conditions. Both the recruitment force thresholds (22.54 ± 1.73 N vs 27.03 ± 1.96 N) and firing rates (6.40 ± 0.1 Hz vs 8.76 ± 0.14 Hz) of the same motor units were lower during the involuntary condition compared to the voluntary condition (p < 0.001). To examine the effect of Ia input on the firing rates of already contracting muscle, 3 brief bursts of vibration were applied to the patellar tendon during a voluntary knee extension maintained at 30 N for approximately 45 seconds. In 35 single motor units from 7 subjects, the firing rate decreased at each vibration that was superimposed during the voluntary contraction (p < 0.01). Paradoxically, the reduced firing rates were accompanied by small increases in both the force and surface EMG recordings during vibration. These data suggest that Ia input driven by tendon vibration has both an excitatory and simultaneously inhibitory influence on motor units. Recruitment and inhibition occurring simultaneously are likely induced at the level of the spinal cord but we are unable to determine a definitive role of how supraspinal centres influence this phenomenon.