Many muscles of the upper limb cross more than one joint. For example, each muscle that crosses the wrist also crosses at least one other joint. These muscles often have significant excursions at the other joints they cross [1]. The complex musculoskeletal anatomy of the upper limb results in passive mechanical couplings between joints; the passive moment produced about a given joint can be highly dependent on the positions of other joints [2]. This passive coupling can be clearly observed in the wrist and hand: when the elbow is flexed 90° and the forearm and hand are oriented so that gravity aids wrist flexion, the wrist rests in flexion and the fingers are extended. In contrast, supinating the forearm in this same elbow position (so that gravity aids wrist extension instead of wrist flexion) results in an extended wrist and flexed fingers.

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