Size and Shape: Morphology's Impact on Human Speed and Mobility

Abstract

While human sexual dimorphism is generally expected to be the result of differential reproductive strategies, it has the potential to create differences in the energetics of locomotion and the speed at which each morph travels, particularly since people have been shown to choose walking speeds around their metabolic optimum. Here, people of varying sizes walked around a track at four self-selected speeds while their metabolic rate was collected, in order to test whether the size variation within a population could significantly affect the shape of the optimal walking curve. The data show that larger people have significantly faster optimal walking speeds, higher costs at their optimal speed, and a more acute optimal walking curve (thus an increased penalty for walking at suboptimal speeds). Bigger people who also have wider bitrochanteric breadths have lower metabolic costs at their minimum than bigger people with a more narrow bitrochanteric breadth. Finally, tibia length significantly positively predicts optimal walking speed. These results suggest sex-specific walking groups typical of living human populations may be the result of energy maximizing strategies. In addition, testable hypotheses of group strategies are put forth.

Energetic Consequences of Human Sociality: Walking Speed Choices among Friendly Dyads

“…These results are consistent with other data that demonstrate that males walk faster than females both while walking alone [17], and while walking in single sex groups [25], [27]. The results further indicate that any difference in walking speeds between female dyads and male-female dyads is not significant [25].…”

“…An individual’s optimal speed has been shown to correlate strongly with mass and lower limb length, with longer lower limbed and larger individuals having faster optimal speeds [17]. Because of this, people of different masses and/or lower limb lengths will have different optimal speeds.…”

“…For humans traveling together this can pose significant problems. For instance, human sexual dimorphism in mass and lower limb length [23] leads to male and female differences in optimal speeds, with males having faster speeds than females [17]. If males and females are traveling together, it is impossible for all individuals to be walking at their own optima.…”

Reproductive costs for everyone: How female loads impact human mobility strategies

“…Whereas non-carrying females could feasibly walk either more slowly or more quickly with less of an increase in cost, frontal loads change these conditions, reducing speed flexibility. Both the energetic cost and preferred speed data presented here imply the speed of women walking together when at least one is pregnant or lactating is going to be slower than estimates of walking speed based on unloaded women walking alone (Wall-Scheffler, 2012a). …”

“…In terms of selection acting upon locomotor morphology, mass has previously been shown to increase the MinCoT and the curvature around it during unloaded walking (Wall-Scheffler, 2012a) suggesting selection for increased size may relate to other selection pressures (e.g., increased fecundity or increased infant size) and also suggesting that populations that maintain some amount of sexual dimorphism of size may have individuals with different mobility strategies (Wall-Scheffler, 2012a). Because absolutely longer lower limbs increase stride length and reduce steps taken (this study, Grieve and Gear, 1966), and relatively (to body mass) reduce energetic costs of walking (Steudel-Numbers and Tilkens, 2004), the appearance of longer lower limbs in the hominin lineage should be expected to reduce the energetic cost of bipedal walking (Pontzer et al, 2010).…”