Fig. 4. Cumulative work done during 10 contractions in the protocols described in the Materials and methods and in Fig. 3. `I', shortening contractions preceded by an isometric phase; `S', shortening contractions preceded by a stretch; `shorten', the work done during the shortening portion of the contraction; `net', the net work done during the entire contraction (note: for contractions preceded by an isometric phase, net work equals shortening work); `stretch', the work required to stretch the muscle. Work is expressed relative to muscle mass. Values are means ± s.e.m. P values are comparisons with I-shorten using work values uncorrected for muscle mass.