Bivalves protect themselves from other marine organisms and environmental change by closing their valves. Although prior studies have investigated the mechanical aspects of bivalves, such as their fatigue resistance, none have measured the adductor muscle force in live bivalves in seawater due to the difficulty in directly measuring the adductor muscle force. In this study, the deformation of the valve surface caused by valve closing was investigated by focusing on the evaluation of muscle activity in bivalves. Changes in valve strain when Mytilus galloprovincialis samples with different shell lengths were stimulated in artificial seawater were measured using strain gauges. The adductor muscle forces estimated from the valve strain using loading tests for force calibration were used to quantitatively evaluate the magnitude of the maximum adductor muscle forces and duration characteristics. Contraction duration was not significantly correlated with either shell length or muscle cross-sectional area (shell length;​ r = 0.423, p = 0.404, muscle cross-sectional area;​ r = 0.140, p = 0.791). On the other hand, there was a highly significant correlation between adductor muscle force and cross-sectional area (r = 0.890, p = 0.017). The stress estimated in this study was 0.253 ± 0.026 MPa, which was smaller than the result of loading tests for shell opening in Mytilus galloprovincialis living samples obtained in previous studies. These results confirmed that this method can effectively evaluated muscle force changes, such as duration characteristics, during valve closing.