Electromyographic analysis of muscle recruitment in primates: Limb retractor use in the slow loris (Nycticebus coucang) and scapulohumeral muscle activity patterns in prosimians.

The quadrupedal walking gait displayed by most primates differs from that of other mammals in several characteristics. To understand the functional implications of these characteristics, it is useful to explore known exceptions to these generalizations and to determine how widespread any such characteristic actually is. Lorises are one such exception in not exhibiting the typical primate pattern of higher peak vertical forces on their hind limbs than forelimbs. A distinctive pattern of scapulohumeral muscle recruitment is an example of a primate characteristic based only on anthropoids. This study uses electromyography (EMG) to 1) explore the activity patterns of limb retractors in the slow loris (Nycticebus coucang) to address questions related to their unusual weight support distribution and 2) determine if scapulohumeral muscle recruitment in prosimians is like that observed in anthropoids. The EMG results for the hind limb retractors in the slow loris were like those of other primates and did not match study predictions. While the forelimb retractor results did agree with predictions, their inferred contributions to locomotor mechanics were not what was expected. The recruitment patterns for the scapulohumeral muscles in the slow loris as well as four lemur species displayed a mix of similarities and differences to the EMG patterns observed in anthropoids and may relate to a shift in shoulder mechanics between prosimians and anthropoids. However, rather than explaining the higher forelimb than hind limb peak vertical forces, the limb retractor EMG results only raised additional questions about how this distribution of forces is brought about.