Fig. 6. Efficiency of muscle work measured over 10 contractions, derived from work and oxygen consumption as described in Materials and methods. `I', shortening contractions preceded by an isometric phase; `S', shortening contractions preceded by a stretch; `net', calculations using the net work done during the entire contraction; `shorten', calculations using the work done during only the shortening portion of the contraction, but still the energy used during the entire contraction. Values are means ± s.e.m. P values compare the two measurements that they straddle.