Fig. 2. MCP and interphalangeal joint shape contributes to suspensory performance.
(A) Free-body diagram depicting the third ray of a siamang positioned in a hook grip on a horizontal support following (32). The points of application, spatial orientations, and magnitudes of external and internal force vectors, as well as moment arm lengths, are hypothetical. Joint reaction forces are not depicted here for the sake of clarity. SRF, support reaction force (near proximal, middle, and distal phalanges); R, external moment arm; r, internal moment arm; FFD, muscle force vector associated with the flexor digitorum muscles (m. flexor digitorum superficialis and m. flexor digitorum profundus); FFDP, muscle force vector associated with the m. flexor digitorum profundus. Flexor forces applied to the support are the vertical components of FFD and FFDP and the horizontal component of FFD′. (B) Major axis of variance derived from a PCA on six logged geometric mean-standardized measurements (MC5 HML, MC5 HDP, PP3 TML, PP3 TDP, IP3 TML, and IP3 TDP). Positive values of PC1 are associated with decreased MC5 head breadth, increased PP3 TDP, and increased IP3 TDP.