Relationship between wire EMG activity, muscle length, and torque of the hamstrings

Objective. To determine the effect of change of muscle length on the torque and wire electromyographic activity of six knee flexor muscles. Design. Maximum isometric knee flexion torque and wire EMG data were collected at nine different positions. Background. In vivo EMG–length–tension relationship is difficult to determine because of the interaction between muscle length and moment arm. The study of two-joint muscles allows the change of muscle length at one joint while preserving stable mechanical relationships at the other. This model facilitates understanding of length–tension and EMG–length relationship in vivo. Methods. Nineteen subjects performed maximum isometric knee flexion contraction at nine positions of varying hip and knee angles. Wire EMG activity was recorded from semitendionsus, semimembranosus, long and short head of the biceps femoris, gracilis and sartorious muscles. Results. As the two-joint hamstrings were lengthened, torque was significantly increased. Maximum isometric torque ranged from 257 to 716 kg cm. The ratio of the torque values to EMG activity of all muscles was increased at longer muscle lengths. A change in the muscle length of the two-joint hamstrings did not produce a consistent change of EMG activity. The short head of the biceps femoris and sartorius muscles increased their activity as the angle of knee flexion increased. Conclusions. Maximum torque of knee flexion occurs at the most lengthened position of the hamstrings. EMG activity did not consistently change with the change in muscle length. Relevance Understanding in vivo length–tension relationship and associated EMG activity is important for designing rehabilitation protocols, tendon lengthening and transfer and interpretation of EMG data.