SEARCH

SEARCH BY CITATION

Keywords:

  • ankle joint;
  • joint mechanics;
  • mechanical advantage;
  • power arm length;
  • prosimian

Abstract

  1. Top of page
  2. Abstract
  3. Introduction
  4. Methods
  5. Results
  6. Discussion
  7. Concluding remarks
  8. References

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.


Introduction

  1. Top of page
  2. Abstract
  3. Introduction
  4. Methods
  5. Results
  6. Discussion
  7. Concluding remarks
  8. References

The ankle joint is a structural unit between the leg and the foot. The joint is generally considered to be a conservative feature among primates, but each bone composing the joint varies in shape (Turley et al. 2011). Interspecific variation has been assessed to correlate ankle morphology with positional behaviour (Gebo, 1986; DeSilva, 2009; Pina et al. 2011; Turley et al. 2011) and/or determine phylogenetic relationships (Strasser, 1988; Gebo, 1989). Numerous studies have suggested the existence of kinematic differences in ankle joint movements during walking and climbing among many primates, resulting from different locomotory modes or adaptations to different habitats (Gebo, 1986; Meldrum, 1991; DeSilva, 2009; Pina et al. 2011).

Prosimians are one of the most suitable models within the primate order for studying functional anatomy for two reasons. First, prosimians are characterised by remarkable diversity in locomotor modes, habitat, and diet, despite close phylogenetic relationships among species (Crompton et al. 1987). Secondly, a great deal of behavioural information, including habitat use and vertical strata preferences in the forest, provides evidence for locomotor repertoire and utilisation (Napier & Walker, 1967; Ward & Sussman, 1979; Dykyj, 1980; Crompton et al. 1987; Gebo, 1987; Nash et al. 1989; Off & Gebo, 2005). This ecological information is valuable for functional interpretations of morphological characteristics. Among prosimians, the slow loris (Nycticebus coucang) and Garnett's galago (Otolemur garnettii) are particularly remarkable. The loris is exclusively a canopy dweller that habitually climbs very slowly on fine branches (Grand, 1967; Crompton et al. 1987). The galago hops and leaps and also exhibits quadrupedalism (Crompton et al. 1987; Nash et al. 1989; Gebo, 2011). In contrast, the ring-tailed lemur (Lemur catta) is a relatively more generalised prosimian that is terrestrial and arboreal and demonstrates more labile behaviour (Crompton et al. 1987). Understanding the structure of the locomotor apparatus and its joint mechanics is of interest in these ecologically diverse primates.

A joint is composed of bones, ligaments, and muscles that work together as a mechanical unit (Zajac, 1992; Gebo, 1993). Although a great deal of attention has been paid to bone morphology, muscle layout has been ignored despite its correlation to bone morphology. Interspecific variation in foot morphology can be expected to alter the layout of lower leg muscles, changing the mechanics of the lever system around the joint. Therefore, differences in joint mechanics probably reflect differences in locomotor modes.

Joint mechanics are most simply approximated by a lever system. In a standing animal, external forces compel the ankle joint to flex dorsally. To maintain a standing posture, the animal must resist the external force by exerting muscle force. In this case, the degree of muscle force exerted depends on the mechanical properties of the ankle joint, which can be represented as power arm length (PAL) and load arm length (LAL). PAL is defined as the shortest distance between the joint axis and the line of muscle action (Zajac, 1992; Gebo, 1993; Thorpe et al. 1999; Payne et al. 2006). LAL is defined as the shortest distance between the joint axis and the point at which an external force is applied (Zajac, 1992; Gebo, 1993). In addition, mechanical advantage (MA), calculated as the ratio of PAL to LAL, is an index representing the mechanical properties of the joint. When a muscle contracts, a higher MA indicates a higher efficiency of joint moment relative to the muscle power exerted. Inversely, a lower ratio indicates a higher efficiency of angular excursion of the distal segment relative to muscle excursion (Kumakura, 1989; Zajac, 1992; Gebo, 1993).

The purposes of this study were (i) to demonstrate the diversity of ankle joint mechanics among primates and (ii) to elucidate how functional demand, possibly associated with positional behaviour, moulds joint mechanics.

Methods

  1. Top of page
  2. Abstract
  3. Introduction
  4. Methods
  5. Results
  6. Discussion
  7. Concluding remarks
  8. References

Specimen preparation and measurement

Five slow loris (Nycticebus coucang), two ring-tailed lemur (Lemur catta), and four Garnett's galago (Otolemur garnettii) specimens, including males and females, were studied (Table 1). All subjects were adults and showed no obvious pathological or morphological abnormality. The specimens were preserved in a 10% formalin solution at the Laboratory of Biological Anthropology of Osaka University, Japan. We focused on 12 lower leg muscles; the muscles and the abbreviations used in this paper are shown in Table 2. The reported body weight of Garnett's galago is about 0.7 kg, that of the slow loris is about 0.6 kg, and that of the ring-tailed lemur is about 2 kg (Smith & Jungers, 1997) .

Table 1. Specimens
SpeciesnFibula length (mm)
  1. n, number of specimens.

  2. Fibula lengths are presented as mean ± SD.

Nycticebus coucang575 ± 1.9
Lemur catta2110
Otolemur garnettii467 ± 3.2
Table 2. Abbreviations of lower leg muscles in this paper
Abbreviations
TATibialis anterior
EHLExtensor hallucis longus
EDLExtensor digitorum longus
FLFibularis longus
FBFibularis brevis
FDTFlexor digitorum tibialis
FDFFlexor digitorum fibularis
TPTibialis posterior
SLSoleus
LGLateral head of gastrocnemius
MGMedial head of gastrocnemius
PTPlantaris

The hind limbs were removed from the pelvis of each subject, and the muscle tissues were removed. To preserve the fine anatomy around the ankle joint, the tendons and retinacula were preserved.

To estimate the MA of lower leg muscles and compare ankle joint mechanics among the studied primates, we constructed three-dimensional lower leg muscle layout models because direct measurement of the PALs of the muscles proved difficult. PALs around the dorsi-plantarflexion and mediolateral rotation axes and LALs were computed. To quantify the muscle layouts as three-dimensional coordinates, the origins and insertions of all muscles and bony landmarks were represented as points and marked with fine pins. Muscle origins were defined as the midpoints of areas attached to the tibia or fibula. Muscle insertions were defined as points where their tendons attached to the bones of the foot. The following bony landmarks were also marked: in the thigh segment, the tip of the greater trochanter, medial epicondyle, and lateral epicondyle; in the lower leg segment, the head of the fibula, medial malleolus, and lateral malleolus; in the foot segment, the calcaneal tuberosity and the heads of the five metatarsals. In addition, intermediate points were marked in muscles that changed directional paths, such as the tibialis anterior (TA) and fibularis longus (FL). In muscles with continuous straight paths, such as the lateral head of the gastrocnemius (LG) and soleus (SL), intermediate points were not marked.

The specimens were fixed in a measurement apparatus with stands and clamps. Because hind limb postures differed among specimens, the flexion-extension angle of the knee was fixed at about 90° and the adduction-abduction and mediolateral rotation angles were fixed at about 0°. Also, the dorsi-plantarflexion angle of the ankle was fixed at about 90°, and the mediolateral rotation angle was fixed at about 0°. Fixing of the angles was accomplished by manual adjustment. The marked points on the specimens were digitised with MicroScribe M (Revware Inc., Raleigh, NC, USA).

Segment definition

Coordinate systems were defined for each segment using the bony landmarks. Hind limb posture was defined by the relationships among the thigh, lower leg, and foot coordinate systems. For detailed definitions of the coordinate systems, see Fig. 1 and Table 3. The relationships among coordinate systems and layouts of the lower leg muscles are described in Fig. 2.

Table 3. Definitions of segment coordinate systems
Coordinate systems
  1. a

    The y axis of the foot coordinate system indicates that the ‘a vector’ is oriented to the dorsal side of the foot and perpendicular to a plane specified by the three bony landmarks: the calcaneus and second and fifth metatarsals.

Thigh coordinate system
OriginMidpoint of the lateral and medial epicondyles.
X axisA unit vector from the origin to the medial epicondyle in the right limb or lateral epicondyle in the left limb.
Y axisA proximally oriented unit vector defined by the outer product of the x and z axes.
Z axisAn anteriorly oriented unit vector perpendicular to a plane specified by the greater trochanter and lateral and medial epicondyles.
Lower leg coordinate system
OriginMidpoint of the lateral and medial malleoli.
X axisA unit vector from the origin to the medial malleolus in the right limb or the lateral malleolus in the left limb.
Y axisA proximally oriented unit vector defined by the outer product of the x and z axes.
Z axisAn anteriorly oriented unit vector perpendicular to a plane specified by the head of the fibula and the lateral and medial malleoli.
Foot coordinate system
OriginMidpoint of the lateral and medial malleoli.
X axisA unit vector defined by the outer product of the a vectora and the z axis.
Y axisA dorsally oriented unit vector defined by the outer product of the x and z axes.
Z axisA unit vector oriented from the origin toward the 2nd or 3rd metatarsal.
image

Figure 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.

Download figure to PowerPoint

image

Figure 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.

Download figure to PowerPoint

We defined the ankle as a biaxial joint because adduction-abduction at this joint was very limited during dissection. The joint axes were matched to the axis of each coordinate system. In the thigh and lower leg coordinate systems, the x axes were matched to the flexion-extension axis of the knee joint and the dorsi-plantarflexion axis of the ankle joint. In the foot coordinate system, the z axis was matched to the mediolateral rotation axis of the ankle joint. These axes were first computed to define each coordinate system because they were defined to pass directly through the measured points. The other axes were computed by the outer product so that they did not pass through the measured points, such as the y and z axes of the thigh and lower leg coordinate systems or the x and y axes of the foot coordinate system.

Standardisation of posture

To take measurements, we manually manipulated the various hind limb postures; however, the specimens were in slightly different postures. As PAL varies depending on joint angle (Zajac, 1992; Payne et al. 2006), the limb postures were mathematically standardised using the spatial relationships among the three segment coordinate systems. First, a rotational matrix of the lower leg coordinate system relative to the thigh reference frame was computed, and its inverse matrix was calculated. The coordinates of the measured points in the lower leg and the foot were multiplied by the inverse matrix. Because the flexion-extension angle of the knee joint was altered to 180° through this process, the coordinates were rotated by 90° around the flexion-extension axis of the knee joint. Next, a rotational matrix of the foot coordinate system relative to the lower leg coordinate system and its inverse matrix were computed. The coordinates of the measured points in the foot were multiplied by the inverse matrix. These procedures mathematically standardised the limb postures.

PAL, LAL, and MA

We computed PALs using the following formula (Ogihara et al. 2009):

  • display math

where v is the unit vector defining the direction of muscle force, a is the unit vector defining the direction of the joint axis, and r is the vector from the joint to a starting point of vector v. PALs were normalised with the linear dimension using fibula lengths, which were computed as the distance from the head of the fibula to the lateral malleolus. This procedure allowed us to compare PALs as dimensionless values. The signs of the PALs represent the direction of motion. For dorsi-plantarflexion, a positive value represents dorsiflexion of the ankle joint and a negative value represents plantarflexion. For mediolateral rotation, a positive value represents medial rotation and a negative value represents lateral rotation.

Load arm length corresponds to the functional axis, which acts as a lever during locomotion. Morton (1922) defined this axis using a line through the longest metatarsal. In the present study, the functional axis was defined by a line passing through the head of the third metatarsal in the lemur and galago. However, the lateral digits of its specialised feet are extremely abducted in the loris, and Grand (1967) reported the presence of an axis through the second metatarsal during grasping in this primate. For this reason, the second metatarsal was used to define the functional axis in the loris. LAL was computed as the distance from the dorsi-plantarflexion axis to the head of the metatarsal.

Mechanical advantage is the ratio of PAL to LAL. Thus, MAs were computed from PAL and LAL values. Non-paired t-tests were performed using pasw statistics 18 (SPSS Inc., Chicago, IL, USA) to evaluate differences in the MAs and PALs of the loris and galago. The number of lemurs was insufficient for statistical analysis.

Results

  1. Top of page
  2. Abstract
  3. Introduction
  4. Methods
  5. Results
  6. Discussion
  7. Concluding remarks
  8. References

Power arm length

The PALs of the 12 lower leg muscles in all species as a whole were mainly characterised by superficial muscles, such as the TA, LG, or MG (Fig. 3). Generally, the loris and lemur showed similar PAL distributions along the dorsi-plantarflexion PAL axes and the mediolateral rotation PAL axes (Fig. 3), whereas that of the galago was unique, characterised by short PALs of the TA and extensor hallucis longus (EHL) and long PALs of the SL, LG, medial head of the gastrocnemius (MG), and plantaris (PT).

image

Figure 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.

Download figure to PowerPoint

Several statistically significant differences were found between the loris and galago. The dorsiflexion PALs of the TA and EHL showed significant differences and the plantarflexion PALs of the LG, MG, and PT also showed significant differences (Fig. 4A,D and Table 4). For dorsiflexion, the loris had longer PALs than the galago except for the extensor digitorum longus (EDL) muscle. In contrast, the galago had longer plantarflexion PALs than the loris except for the flexor digitorum tibialis (FDT) and tibialis posterior (TP; Fig. 4C,D and Table 4). In the lemur, the dorsiflexion PALs of the TA and EHL were the longest of all specimens investigated, and the lemur also had plantarflexion PALs for the SL, LG, MG, and PT that were intermediate between the galago and loris (Fig. 4A,D and Table 4).

Table 4. Dorsi-plantarflexion power arm lengths
 Loris (mm)Lemur (mm)Galago (mm)
  1. Power arm lengths are presented as mean ± SD. Positive and negative values represent dorsiflexion and plantarflexion, respectively. The number of lemurs was insufficient for the calculation of SDs.

TA3.5 ± 0.36.41.8 ± 0.7
EHL3.2 ± 0.46.21.4 ± 0.6
EDL2.0 ± 0.63.93.1 ± 0.6
FL−0.2 ± 1.6−0.5−0.6 ± 0.8
FB−0.6 ± 1.4−1.6−0.7 ± 0.4
FDT−2.9 ± 1.4−5.2−3.0 ± 0.5
FDF−5.0 ± 0.4−7.8−6.1 ± 0.9
TP−2.5 ± 1.4−2.9−2.0 ± 0.4
SL−4.7 ± 0.9−9.6−8.4 ± 0.9
LG−4.8 ± 0.8−9.7−8.6 ± 0.6
MG−5.4 ± 0.8−9.9−8.6 ± 0.7
PT−5.2 ± 0.9−10.1−8.7 ± 0.6
image

Figure 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.

Download figure to PowerPoint

For the rotation PALs, only the lateral rotation PAL of the TA showed a significant difference between the galago and loris (Fig. 4B and Table 5). The muscle layout of the EHL was similar to that of the TA and no significant differences were found in the rotation PALs. The lemur and loris had similar values for the lateral rotation PAL of the TA.

Table 5. Mediolateral rotation power arm lengths
 Loris (mm)Lemur (mm)Galago (mm)
  1. Power arm lengths are presented as mean ± SD. Positive and negative values represent medial and lateral rotation, respectively. The number of lemurs was insufficient for the calculation of SDs.

TA−1.7 ± 0.3−2.7−0.5 ± 0.7
EHL−0.8 ± 0.2−2.4−0.4 ± 0.4
EDL2.1 ± 0.22.82.9 ± 0.7
FL5.0 ± 1.97.95.4 ± 0.4
FB4.5 ± 1.07.65.1 ± 0.2
FDT−3.3 ± 0.5−5.0−3.1 ± 0.4
FDF−0.5 ± 1.20.60.1 ± 0.9
TP−3.3 ± 1.2−6.3−4.1 ± 0.2
SL2.5 ± 1.93.82.6 ± 1.3
LG3.8 ± 1.83.72.9 ± 0.8
MG2.5 ± 1.83.02.5 ± 0.8
PT3.4 ± 1.63.92.6 ± 1.2

Mechanical advantage

For most of the MAs, the loris had higher values than the galago (Fig. 5). In the more generalised lemur, most of the MAs were between those of the loris and the galago. There were three significant differences between the galago and the loris. Two of these differences involved dorsiflexors, namely the TA and EHL (Fig. 5A). The other difference involved a plantar flexor, namely the flexor digitorum fibularis (FDF; Fig. 5B). For the TA and EHL, the galago had low MAs compared with the loris and lemur. For plantar flexors, the MA of the FDF of the loris was significantly higher than that of the galago (Fig. 5B). No other significant differences were found, although the TP and FDT showed major differences between the loris and the galago (Fig. 5C).

image

Figure 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.

Download figure to PowerPoint

Discussion

  1. Top of page
  2. Abstract
  3. Introduction
  4. Methods
  5. Results
  6. Discussion
  7. Concluding remarks
  8. References

Interspecific differences in dorsiflexors

The galago has unique PALs for the dorsiflexors. The galago had shorter PALs and lower MAs of the TA and EHL than those of the loris and lemur. A morphological interpretation is that drastically extended tarsal bones alter the lines of actions of the dorsiflexors. The insertion of the TA lies in the navicular bone, and the EHL tendon reaches the big toe via a retinaculum near the insertion of the TA (Stevens et al. 1982). Therefore, the elongated tarsal bones displace the insertion of the TA and the intermediate point of the EHL toward the distal part of the foot, resulting in the lines of action forming sharper angles relative to the plane of the foot (Fig. 6). Because PAL is defined as the shortest distance between the line of action and the joint axis (Zajac, 1992; Gebo, 1993; Thorpe et al. 1999; Payne et al. 2006), the PALs are affected by the angle between the lines of actions of the dorsiflexors and the plane of the foot. In this case, the foot morphology of the galago results in the short PALs of the dorsiflexor, and MAs are relatively low compared with the others.

image

Figure 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.

Download figure to PowerPoint

When leaping, the galago extends its hind limb joints more than the lemur does (Günther et al. 1991). This is especially true for Galago senegalensis, which is an extremely specialised leaper with small body size (Crompton et al. 1987; Günther et al. 1991). The angular excursions of the hind limb joints are the most pronounced in the small galago during the final stage of take-off (Günther et al. 1991). Although Galago garnettii has a larger body size than the small galago, the angular excursions of the joints are larger and faster than those of the lemur (Günther et al. 1991). It is likely that the difference in the angular velocity also partly reflects an interspecific difference in the distal segment weight of the hind limb in each species. However, for the galago to prepare to land securely on substrates, the large ankle joint extension would need to be recovered quickly or dorsiflexed immediately after take-off. In that case, the relatively low MAs of the dorsiflexors would be advantageous in their locomotion.

Interspecific differences in planterflexors

A general trend was found in the interspecific differences among PALs of most plantarflexors in the studied groups. In descending order of PAL, plantarflexors (i.e. SL, LG, MG, and PT) values were distributed as follows: galago, lemur, and loris. However, the trend was reversed for MAs. This result suggests that the LALs of these species also vary, as do the PALs, and that the MAs are affected not only by PALs but also by foot morphology in these species.

The loris moves with slow quadrupedalism in the canopy and uses horizontal supports (Grand, 1967; Crompton et al. 1987). Gebo (1987) reported that the loris uses bridging and suspensory behaviour to fill the gaps between supports much more frequently than does the lemur in arboreal environments. In addition, lemurs do not using cantilevering, in which a primate orients outward from a vertical support and holds a horizontal position. In contrast, the loris performs this positional behaviour with its hind limbs while foraging in an arboreal environment (Gebo, 1987). In this posture, the centre of mass is located away from the base of support, where the feet are firmly gripping twigs. In the horizontal posture, the moment of external force that the ankle joint must bear is proportional to the distance between the feet and the centre of mass; therefore, the muscle should produce a high joint moment to stabilise this posture. Thus, the loris would have high MAs for plantarflexion to respond to this mechanical demand. In addition to stabilising the joint, the loris must also grasp twigs firmly in this positional behaviour. Interestingly, the flexors of digits, particularly the FDF, yielded significantly higher MAs in the loris than in the other species.

The MAs of most plantar flexors were higher in the lemur than in the galago. The lemur and galago have similar locomotor repertoires, which include quadrupedal walking and running, leaping, and hopping (Gebo, 1987; Nash et al. 1989). However, the morphologies of these two species reflect different adaptations. In the galago, the calcaneus and navicular bones are extended toward the distal foot as a morphological adaptation for leaping (Hall-Craggs, 1965; Gebo, 1993). In contrast, the lemur lacks the specialised foot morphology of the galago and is considered a generalist with labile behaviour (Ward & Sussman, 1979; Crompton et al. 1987).

Demes et al. (1999) compared the kinetics of leaping between leapers and generalists according to the classification of Napier & Walker (1967), in which some prosimians were assigned to a vertical clinging and leaping group based on their positional behaviours and morphological characteristics. Although very few data are available for Garnett's galago, and most members of the leaper group are of the Propithecus genus, take-off force is higher in the generalist (including Lemur catta) than in a leaper of comparative body weight leaping the same distance between compliant poles designed to mimic natural conditions (Demes et al. 1999). Leaping distance depends on take-off force, acceleration distance, and take-off angle (Crompton et al. 1993; Demes et al. 1999). A leaper leaps a given distance effectively, with a long acceleration distance due to long hind limbs and with a low take-off force, whereas a generalist leaps the distance with a short acceleration distance and high take-off force (Demes et al. 1999). Although the locomotor repertoires of the lemur and galago do not differ greatly, given the suggestions by Demes et al. (1999) and other morphological studies of prosimian hind limbs (Hall-Craggs, 1965; Oxnard et al. 1981), the results of our study might reflect the difference between a leaper and a generalist, which means that the galago has developed a mechanical system enabling elongation of the acceleration distance or speed-oriented mechanics.

However, comparisons of species of different body weights, such as Galago garnettii and Lemur catta, must account for the scaling effect. The force during locomotion is proportional to body weight2/3; thus, the relative force (force/body weight) should be proportional to body weight−1/3 or follow negative allometry (Alexander, 1985; Demes et al. 1999). The smaller galago is expected to produce a higher force relative to the lemur, but the galago has speed-oriented ankle joint mechanics. This discrepancy might be mediated by muscle size and architecture. Unfortunately, no data on muscle weight or fiber arrangement in Galago garnettii and Lemur catta are available, but the more specialised leaper Galago senagalensis has been demonstrated to have relatively heavier superficial plantarflexors than those of quadrupedal primates (Demes et al. 1999) . Consequently, in terms of force production, the speed-oriented joint mechanics of the galago might be complemented by muscle size and/or other joint power. In the loris, the flexors of the digits account for most of the total lower leg muscle weight (Grand, 1967). This arrangement is consistent with the high MAs of digit flexors, which exacerbates the interspecific difference and confirms our identification of power-oriented ankle joint mechanics in the loris.

Concluding remarks

  1. Top of page
  2. Abstract
  3. Introduction
  4. Methods
  5. Results
  6. Discussion
  7. Concluding remarks
  8. References

Our study of prosimian ankle joints demonstrates mechanical diversity across three species. The galago extends its hind limbs much faster and for a greater distance relative to the lemurs to accelerate the centre of mass. We believe that the galago evolved speed-oriented joint mechanics characterised by low MA. This mechanical solution may also be present in more specialised leapers, such as Galago senegalensis or Tarsius. In contrast, the loris ankle joint requires power-oriented joint mechanics to hold unstable postures on supports. Lemurs generally showed intermediate PAL and MA values relative to a speed- or power-oriented ankle system. The main findings of this mechanical study are that joint mechanic diversity corresponds to positional behaviour in primates, and that PALs of lower leg muscles should be considered in examinations of primate locomotor adaptation.

References

  1. Top of page
  2. Abstract
  3. Introduction
  4. Methods
  5. Results
  6. Discussion
  7. Concluding remarks
  8. References