The estimated mechanical advantage of the prosimian ankle joint musculature, and implications for locomotor adaptation

Abstract

In this study we compared the power arm lengths and mechanical advantages attributed to 12 lower leg muscles across three prosimian species. The origins and insertions of the lower leg muscles in Garnett's galago, the ring-tailed lemur, and the slow loris were quantified and correlated with positional behaviour. The ankle joint of the galago has a speed-oriented mechanical system, in contrast to that of the slow loris, which exhibits more power-oriented mechanics. The lemur ankle joint exhibited intermediate power arm lengths and an intermediate mechanical advantage relative to the other primates. This result suggests that the mechanical differences in the ankle between the galago and the lemur, taxa that exhibit similar locomotory repertoires, reflect a difference in the kinematics and kinetics of leaping (i.e. generalised vs. specialised leapers). In contrast to leaping primates, lorises have developed a more power-oriented mechanical system as a foot adaptation for positional behaviours such as bridging or cantilevering in their arboreal habitat.

Fig. 1

Orientation and position of each coordinate system. The thigh, lower leg, and foot coordinate systems are abbreviated TCS, LCS, and FCS, respectively. For consistency, the x axis is always oriented to the left side of the body, and the left or right limb is used. The y axis is oriented to the proximal part of each segment. The z axis is anteriorly oriented. The x axes of the TCS and LCS correspond to the flexion-extension axis of the knee joint and the dorsi-plantarflexion axis of the ankle joint, respectively. The z axis of the FCS corresponds to the mediolateral rotation axis of the ankle joint.

Fig. 2

Muscle attachment sites and relationships between muscle layouts and coordinate systems of the right hind limb of the lemur, viewed from various angles. The muscles, which have insertions on the phalanges, are traced up to the heads of the metatarsals. The attachment sites and intermediate points of the lower leg muscles are represented by black points. The muscle paths are represented as black solid lines; dotted lines in (A) indicate that the muscles pass behind other anatomical structures in this view. The lower leg coordinate systems are indicated by grey solid arrows. (A) Superior anteromedial view. (B) Superior anterolateral view. (C,D) Inferior posteromedial views.

Fig. 3

Bivariate plots for the dorsi-plantarflexion power arm lengths (PALs) and the mediolateral rotation PALs for the 12 lower leg muscles. The positives on the vertical axes or horizontal axes represent dorsiflexion PALs or medial rotation PALs, respectively. The negatives represent plantarflexion PALs or lateral rotation PALs, respectively.

Fig. 4

Power arm lengths (PALs) of the lower leg muscles. (A) Dorsiflexion PALs of the TA, EHL, and EDL. (B) Lateral rotation PALs of the TA and EHL. (C) Plantarflexion PALs of the FL, FB, FDT, FDF, and TP. (D) Plantarflexion PALs of the SL, LG, MG, and PT. * P < 0.05.

Fig. 5

Mechanical advantages (MAs) of the lower leg muscles. (A) MAs of the TA, EHL, and EDL. (B) MAs of the FL, FB, FDT, FDF, and TP. (C) MAs of the SL, LG, MG, and PT. * P < 0.05.

Fig. 6

The line of action of the TA. In the galago, the elongated tarsal bones displace the insertion of the TA to the distal part of the foot. The acute angle between the line and long axis of the foot results in the short PAL of the TA. The star symbol represents the angle.