Figure 4. Force-time and angle-time histories and EMG during voluntary contractions.

For shortening contractions, the minimum forces during the shortening phase (F_Shortening) and the steady-state isometric forces (F_ISO) were measured for comparisons with the corresponding values obtained from isometric reference contractions. During the contractions, muscle activation was continuously monitored using EMG and superimposed twitch technique. During the steady-state isometric phase following muscle shortening, the RMS of EMG values and interpolated twitch forces (ITF) were measured and compared with the corresponding values obtained from the isometric reference contractions. The effects of shortening conditions were tested by systematically changing amplitudes (A) and speeds (B) of shortening.