Is muscle structure influenced by genetical or functional factors? A study of three forearm muscles.

Relative fibre type composition and weights of 3 forearm muscles; the m. extensor carpi radialis longus (ECRL), m. extensor carpi radialis brevis (ECRB) and the m. brachioradialis (BR) from both the left and right arm were determined. Fibre type 1 muscle weight of the ECRB, which co-contracts posturally during gripping, was significantly greater in the right than the left forearm. Weights of type 2 fibres in this muscle were, however, similar in the left and right arm. Fibre type weights of both ECRL and BR were also similar in both arms. For all these muscles, intra-individual co-variation in relative fibre type occurrence was found. It is proposed that genetic factors may play a role in determining the relative fibre type composition at least as far as the upper extremities are concerned. However, functional demands are also important determining factors for development of muscle structure.