The lever systems, and structure and function of the adductor muscles, of the mandibles of large male Carcinus tnaenas (L.) were investigated. The functional mechanical advantage (FMA) of the external adductor muscles, measured at the middle of the biting edge of the mandible, was 1.53 ± 0.01; the ideal mechanical advantage (IMA), calculated from measured lever lengths, was 1.78±0.03 (mean±S.E.). The FMA of the internal adductor muscles was 0.53±0.01 and the IMA was 0.62±0.02.

When slowly loaded, the external adductor system broke at 3.09±0.16 N; when rapidly loaded, the system broke at 7.16±0.79 N. Short duration loading at 2.0 N or more was found to result in histologically detectable damage at the pivot. When slowly loaded, the internal adductor system broke at 0.68±0.05 N. When a load of 0.39 N was applied to the internal adductor system, the failure point on slow loading of the external adductor system was not significantly increased.

The mean maximum force of contraction of the external adductor muscles on electrical stimulation was 0.98±0.03 N, with a stress of 627kNm−2 in the fibres. The internal adductors contracted with a mean maximum force of 0.33±0.01 N. The total maximum bite force is therefore about 3.3 N, only 10% being contributed by the internal adductors. The safety factor of both internal and external systems is about 2.

The mean sarcomere length of half-contracted fibres of the external adductor muscles was 11.62±0.11 μm.

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