An important role of the biarticular hamstrings is to exert internal/external rotation moments on the tibia during vertical jumping

Most research considering biarticular muscle function has tended to focus on the sagittal plane. Instead, the purpose of this study was to evaluate the internal/external rotation moment arms of the biarticular muscles of the knee, and then to explore their function. The FreeBody musculoskeletal model of the lower limb was used to calculate the moment arms and moments that each of the muscles of the knee exerted on the proximal tibia of 12 athletic males during vertical jumping. Biceps femoris and tensor fascia latae were external rotators of the tibia, whereas semimembranosus, semitendinosus, sartorius, gracilis, popliteus and the patellar tendon were internal rotators. The magnitudes of the internal/external rotation and flexion moments exerted on the tibia by the biarticular hamstrings were similar, suggesting that the creation of internal/external rotation is a key aspect of their role. One potential reason is to stabilise the tibia during femoral extension (and it is argued that it may be helpful to characterise the creation of active joint stability as the stabilisation of one segment during the rotation of an adjacent segment). A second explanation may be to mechanically couple hip abduction when the hip is flexed with internal rotation of the tibia.