The mechanical power output was measured from scaphognathite (SG = gill bailer) muscle L2B of the crab Carcinus maenas (L.). The work was determined from the area of the loop formed by plotting muscle length against force when the muscle was subjected to sinusoidal length change (strain) and phasic stimulation in the length cycle. The stimulation pattern (10 stimuli per burst, burst length = 20% of cycle length) mimicked that which has been recorded from muscle L2B in intact animals. Work output was measured at cycle frequencies ranging from 0.5 to 5 Hz. The work output at optimum strain and stimulus phase increased with increasing cycle frequency to a maximum at 2–3 Hz and declined thereafter. The maximum work per cycle was 2.7 J kg−1 (15 °C). The power output reached a maximum (8.8 W kg−1) at 4 Hz. Both optimum strain and optimum stimulus phase were relatively constant over the range of burst frequencies examined.
Based on the fraction of the total SG musculature represented by muscle L2B (18%) and literature values for the oxygen consumption associated with ventilation in C. maenas and for the hydraulic power output from an SG, we estimate that at a beat frequency of 2 Hz the SG muscle is about 10% efficient in converting metabolic energy to muscle power, and about 19% efficient in converting muscle power to hydraulic power.