Effect of joint angle on mechanomyographic amplitude during unfused and fused tetani in the human biceps brachii muscle.

The influence of muscle length-dependent changes in contractile properties on the mechanomyogram (MMG) was investigated during evoked contractions of the biceps brachii muscle. The biceps of nine healthy subjects was stimulated by single twitches, unfused (10 Hz), and fused (30 Hz) frequencies at elbow joint angles of 75, 90, 105, 120, 135, and 150 degrees. During evoked contractions, the longitudinal movement was estimated by the fluctuation of torque signal, and the lateral movement was detected by MMG. Contraction time and half relaxation time were calculated from the single twitches. For repetitive stimulations, followed by eliminating DC component from torque signals, the root mean square values calculated from the torque and MMG signals, which were torque fluctuation and RMS-MMG, respectively. For the 10 Hz stimulation condition, reductions in the torque fluctuation and RMS-MMG were concomitantly observed with increasing elbow angle, and there was a significant correlation between the torque fluctuation and RMS-MMG. On the other hand, for 30 Hz stimulation, there were no significant differences in the torque fluctuation and RMS-MMG over all elbow angles, and no significant correlation between the two parameters. Moreover, the torque fluctuation and RMS-MMG for 10 Hz stimulation were correlated with the contraction time and half relaxation time obtained at each elbow angle, while there were no correlations for the 30 Hz condition. These data suggest that MMG could be a reliable tool to study the development of fusion and the changes in muscle contractile properties during repetitive unfused contractions.