Between-muscle differences in coactivation assessed using elastography

Purpose: This study aimed to assess muscle coactivation in quadriceps and hamstring muscles using ultrasound shear wave elastography. Methods: During maximal voluntary isometric contractions (MVC), both myoelectrical activity and shear modulus of antagonist muscles were measured (i.e., rectus femoris, vastus lateralis and vastus medialis during knee flexions and semitendinosus, semimembranosus and biceps femoris long head during knee extensions). To account for changes induced by inevitable joint rotation during MVC, the shear modulus values were compared to those measured at the same knee angle during a passive cycle. The difference between these values was considered as coactivation. Results: Myoelectrical activity was detected in all antagonist muscles (8.0 +/- 4.9% of maximal EMG RMS). Significant differences were observed between shear modulus values measured during MVC and those measured at the matched knee angle for all muscles (range: 2.7-4.8 kPa; all p < 0.011) except for semitendinosus (+1.7 +/- 5.0 kPa; p = 0.16) and semimembranosus (+1.2 +/- 5.6 kPa; p = 0.39). The magnitude of coactivation varied greatly among individuals. Conclusions: Although non-negligible myoelectrical activity was observed in all muscles, coactivation assessed using elastography was considered as negligible in both the semitendinosus and semimembranosus. Between-muscle and between-participants differences warrant further investigation.