Figure 3. Behaviour of single motor units during ballistic contractions with and without pre-activation.

The discharge of the same motor unit during ballistic contractions performed from a resting state (A) or superimposed on a sustained muscle contraction (B) to similar torque levels (13.5 versus 15 N m, respectively). Traces correspond to the torque (a) and the intramuscular EMG of the tibialis anterior (b–d). The horizontal dotted line under the ballistic phase in B denotes zero torque for the pre-activation condition. The first four discharges of a motor unit at the onset of the ballistic contraction are shown with an extended time scale (c) and superimposed on one another (d). C, average (mean ± s.e.m.) discharge rates of single motor units during ballistic contractions without (○; n = 171) or with pre-activation (•; n = 222). Discharge rate was measured during the last 10 interspike intervals during the pre-activation phase (Sustained) and during the first three interspike intervals for ballistic contraction. The discharge of high threshold units (n = 51) that were activated during the ballistic contractions, but not during the preceding sustained activation, are also illustrated (▴). *P < 0.05 and ***P < 0.001 indicate a significant difference compared with the condition that did not involve pre-activation.