Fig. 3

The influence of coactivity of other MUs on the transitory force decrease. a and d, the recording of two different fast resistant motor units tetanic contractions with transitory force decrease (stimulation pattern 40–90–40 Hz). b and e, recording of the same units as above evoked during parallel tetanic activity of several other motor units in the same muscle. The single motor unit unfused contraction was delayed by 250 ms in relation to the beginning of a conditioning tetanic contraction, whereas the end of this tetanic contraction was also delayed by 250 ms in relation to a single motor unit activity. c and f, the superimposed last phase of the low–high–low contractions from (a) and (b) or (d) and (e), respectively. Note that the transitory force decrease in a tetanus recorded in coactivity was increased (c) or reduced (f) depending on the force of the parallel contractions (119.0 mN in (b) and 353.0 mN in (e)). g, the amplitude of transitory force decrease calculated from independent recordings (black circles) or during the coactivity of other motor units (white circles), connected for the same motor unit (interrupted line with an arrow) and presented as a function of coactive tetanic force. When the force of coactive motor units exceeded 550 mN (interrupted line), the studied phenomenon disappeared, although this effect was not systematical when parallel contractions were weaker