FIG. 1.

Conceptual basis for force covariance mapping. A: muscles have biomechanical actions in endpoint force space, represented hypothetically as action vectors in the endpoint force space of isometric abduction/adduction (F₁) and flexion/extension (F₂) forces exerted by the index finger. B: activation of any single muscle alone, as a prime mover, will generate an average force vector in endpoint force space, with force covariance due to signal-dependent noise. Four such covariances are illustrated, each largely aligned with one muscle's action direction. We refer to this type of covariance as target-directed. C: ≥2 muscles may be coactivated, for example, to generate a force (arrow) with direction between the individual muscle action directions. Coactivation produces a much broader covariance, representing the combined effect of the individual muscles (see appendix). We refer to this type of covariance as non-target-directed. D: if the coactivation of multiple muscles is fixed by a synergy, the synergy produces a similar net action, with a similar covariance to C. E: the force covariance map summarizes the covariances for a variety of target directions, at fixed levels of endpoint force magnitude. If muscles are not in fixed synergies, both target-directed and non-target-directed covariance may be observed. F: if all muscles are in fixed synergies, only non-target-directed covariance will be observed.