Joint angle-dependence of elbow flexor activation levels during isometric and isokinetic maximum voluntary contractions.

OBJECTIVE: The influences of elbow joint angle and the type of contraction on the activation levels of biceps brachii and brachioradialis during maximum voluntary isometric and isokinetic contractions were investigated.
DESIGN: A within-session repeated measures design.
BACKGROUND: Activation of synergistic elbow flexor muscles has been reported to be affected disparately by elbow joint angle and contraction type.
METHODS: Ten subjects performed concentric isokinetic, eccentric isokinetic, and isometric maximum voluntary contractions of the elbow flexor muscles. For the isokinetic contractions the activation levels of two ranges of motion were compared. For the isometric contractions the activation levels at two joint angles were compared. The activation levels of the biceps brachii and brachioradialis acquired simultaneously using bipolar surface electrodes and a surface electrode array were compared.
RESULTS: Results from the electrode array were similar to those acquired using conventional bipolar electrodes. The activation of biceps brachii was significantly affected by joint angle during concentric isokinetic and isometric maximum voluntary contractions. The activation of brachioradialis was significantly affected by joint angle only during eccentric isokinetic maximum voluntary contractions.
CONCLUSIONS: The results confirm that joint angle and contraction type contribute to the distinction between the activation of synergistic elbow flexor muscles during isometric and isokinetic contractions. Relevance The results point to the complexity of control of elbow joint synergists and raise questions about the plasticity of this dependency of elbow flexor activation on joint angle. Solutions to these questions are of importance in the areas of upper extremity rehabilitation and modeling the upper extremity neuromechanics.