Dissociation of Recovery of Muscle Activation and Force Following a Sustained Maximal Isometiric Contraction

Philadelphia Campus

To investigate time-dependency of nervous system recovery following muscular contractions, subjects (n=10) performed two bouts (B1, B2) of sustained maximal isometric handgrip contractions separated by 10- min recovery. Force and surface EMG were collected continuously throughout contraction bouts but were analyzed at 15 sec intervals (1 sec segments). iEMG and mean power frequency (MPF) were calculated for the brachioradialis (BR), flexor carpi radialis (FCR) flexor carpi ulnaris (FCU), and flexor digitorum profundus (FDP) muscles. Muscle activation (iEMG-MPF: BR:68-20%; FCR:72-16%; FCU: 65-20%; FDP:48-50%, respectively) and isometric force (88%) decreased following B1(120-sec contraction). Following recovery, initial force of B2 was significantly less than B1 (23%); however muscle activation (iEMG) was similar to B1 for BR, FCR and FDP. Initial B2 FCU iEMG activity was lower but increased to B1 levels by 15 sec; remaining similar throughout. MPF was similar and decreased similarly over time in both bouts but tended to be higher at initiation of B2. The mechanical response was similar in both bouts despite differences in initial force generation (B1: y = -0.172x+22.7; R2=0.98; B2: y = -0.182x+17.9 R2=0.97). Incomplete recovery of force observed in B2 suggests interference in excitation-contraction coupling while fatigue within each bout appears specifically related to changes in muscle activation.