Motor balance in formation and release of the extension grip. An experimental study.

This study has shown that the lumbrical system served as the prime mover in formation of an extension grip counterbalanced by the action of the extensor digitorum. The interosseal system was not activated until the final stage of grip formation. The balance of the fingers in the release phase was based on interplay between the action of the extrinsics, the extensor digitorum in particular, and the passive tension of the intrinsics. There was probably some difference in influence of the passive tension of the lumbricals and the interossei respectively. The passive tension of the lumbricals was probably exerted right from the very start of grip-release, whereas the passive tension of the interossei exerted its effect at a later stage of finger retroflexion, then preventing hyperextension of the MP joint and hyperflexion of the IP joints. A useful method is outlined for exercising both the extrinsic and intrinsic motor systems.