The flexor digitorum superficialis is a diagastric muscle with a proximal muscle belly from which the tendons to the index, ring and little fingers arise. These tendons are not independent and are not good motors for non-synergistic transfers, such as for finger extension. The muscle and tendon to the middle finger arise separately and are therefore more suitable for non-synergistic transfers.
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Agee et al. (1991) studied this question.