Figure 5. Modulation of discharge rate during anisometric contractions.

The discharge and recruitment patterns of two motor units (MU1 and MU2) in tibialis anterior during an initial isometric contraction and subsequent shortening (A) and lengthening (B) contractions. The vertical dotted lines indicate the beginning and the end of each movement. The traces indicate angular ankle displacement (a), rectified surface (b) and intramuscular (c) EMG of the tibialis anterior, and instantaneous discharge rate of MU1 (d) and MU2 (e). MU2 was recruited during the shortening contraction at an ankle angle of 1 deg dorsiflexion and derecruited during the lengthening contraction at a more extended ankle joint angle (8 deg plantarflexion). At the transition from the initial isometric contraction to the anisometric contraction, there was either a transient decrease (shortening contraction) or increase (lengthening contraction) in discharge rate due to an unloading reflex or stretch reflex, respectively (d and e). Subsequently, there was greater modulation of discharge rate for both motor units during the shortening contraction than during the lengthening contraction. Reproduced from Pasquet et al. (2006) with permission.