Fig. 4.

Slopes of the firing rate-force relationship of individual motor units (Hz/N · m), calculated using linear regression. A: most motor units in healthy volunteers have a positive slope during contraction alone, indicating that firing rates increase with stronger contractions. The slope remains positive and often increases further when vibration of the muscle is added during contraction. B: the slopes of most motor units in PLS patients are lower than controls with contractions of increasing force and some are even negative. When the muscle is vibrated during contraction, most motor-unit slopes fail to increase and some decline.