Fig. 4. (A) Work loop from a basalar muscle contracting at 37 Hz and an illustration of the variables measured for work-loop analyses. (B,C) Work loops at various contraction frequencies for two basalar muscles that represent the range of variation in specific work and power output. Force is plotted as a function of changes in muscle length. Arrowheads indicate the direction of progression around the loop. The muscle in B was larger (16.7 mg), yet generated less tension and power (maximum power at any frequency was 66 W kg^–1) than the muscle in C (14.8 mg; 156 W kg^–1). Both series were obtained at a tissue temperature of 32°C and a constant phase between electrical stimulation and muscle lengthening. Net work is as follows: B, 0.9 J kg^–1 for 20 Hz, 2.1 J kg^–1 for 30 Hz and 1.3 J kg^–1 for 40 Hz; C, 3.7 J kg^–1 for 20 Hz, 5.0 J kg^–1 for 20 Hz and 2.3 J kg^–1 for 20 Hz.