Unilateral training of both lateral limbs increases unilateral muscle strength, whereas bilateral training increases bilateral muscle strength, a phenomenon known as lateral specificity in resistance training. Although motor imagery (MI) combined with action observation (AO) (i.e., MI + AO) training increases muscle strength, it is not completely understood whether such training increases the lateral specificity of muscle strength in a way resistance training does. To investigate whether MI + AO induces lateral specificity of muscle strength increase, 18 healthy subjects were divided into groups: MI + AO and the control groups. The control group watched a movie of natural sceneries for ten minutes per day five times a week for three weeks, whereas the MI + AO group imagined bilateral shoulder flexion while watching a movie of athletes performing bilateral shoulder flexion with barbells or dumbbells, with the same time schedule. The MI + AO group alone showed a significant increase in bilateral shoulder strength at three weeks after the intervention compared with the baseline. Unilateral shoulder strength was not significantly altered. These results suggest that MI + AO training increases muscle strength, providing evidence that similar to resistance training, lateral specificity also exists in MI + AO training.

[Purpose] This study aimed to develop a simple, inexpensive, and accurate method for measuring the strength of shoulder flexion (Experiment 1) and evaluate the bilateral force deficit in shoulder flexion (Experiment 2) in healthy subjects. [Subjects and Methods] In Experiment 1, maximal voluntary contractions (MVCs) in isometric shoulder flexion were measured on both sides using an isometric dynamometer (ID) and a hand-grip dynamometer (HGD), as an alternative dynamometer, in six subjects. In Experiment 2, bilateral force deficit was evaluated using HGD in 21 subjects who performed unilateral and bilateral MVCs in isometric flexions of the shoulder. The peak value of electromyography (PVE) in the lateral head of the biceps brachii and anterior deltoid was measured during MVCs. [Results] In Experiment 1, ID and HGD showed almost similar coefficients of variation. A strong positive correlation was found between the values obtained using the two methods. In Experiment 2, the bilateral force deficit in shoulder flexion associated with a reduced PVE (‒10.9%) was found in 85.7% of subjects. [Conclusion] The reproducibility of measurements was similar between ID and HGD. HGD could evaluate the bilateral force deficit in shoulder flexion and is a practical tool for measuring shoulder strength.