Variation in human 3D trunk shape and its functional implications in hominin evolution

Abstract

This study investigates the contribution of external trunk morphology and posture to running performance in an evolutionary framework. It has been proposed that the evolution from primitive to derived features of torso shape involved changes from a mediolaterally wider into a narrower, and antero-posteriorly deeper into a shallower, more lightly built external trunk configuration, possibly in relation to habitat-related changes in locomotor and running behaviour. In this context we produced experimental data to address the hypothesis that medio-laterally narrow and antero-posteriorly shallow torso morphologies favour endurance running capacities. We used 3D geometric morphometrics to relate external 3D trunk shape of trained, young male volunteers (N = 27) to variation in running velocities during different workloads determined at 45-50%, 70% and 85% of heart rate reserve (HRR) and maximum velocity. Below 85% HRR no relationship existed between torso shape and running velocity. However, at 85% HRR and, more clearly, at maximum velocity, we found highly statistically significant relations between external torso shape and running performance. Among all trained subjects those with a relatively narrow, flat torso, a small thoracic kyphosis and a more pronounced lumbar lordosis achieved significantly higher running velocities. These results support the hypothesis that external trunk morphology relates to running performance. Low thoracic kyphosis with a flatter ribcage may affect positively respiratory biomechanics, while increased lordosis affects trunk posture and may be beneficial for lower limb biomechanics related to leg return. Assuming that running workload at 45-50% HRR occurs within aerobic metabolism, our results may imply that external torso shape is unrelated to the evolution of endurance running performance.

Figure 1

Frontal, lateral and posterior views of the 3D landmarks on the trunk surface. Red dots are fixed landmarks (Supplementary Table 1) and anatomically homologous between subjects, blue dots are curve semilandmarks, and green dots are surface semilandmarks. After resliding the semilandmarks are mathematically homologous among subjects.

Figure 2

Torso shapes (160 lms) and thin-plate splines warped to the highest and lowest velocity at maximum intensity and running speed. (a) Non-muscular torso shape (142 lms) on maximum velocity. (b) Full torso shape (160 lms) on maximum velocity (c) Full torso shape warped to the configuration of lowest (left) and highest (right) maximum velocities. Upper panel left lateral view, lower panel frontal view. Note that flatter ribcages, narrower trunks with low thoracic kyphosis and more pronounced lumbar lordosis correlate significantly with higher velocities at maximum intensity. (Magnification factor from left to right: − 7.5; − 5; + 5, + 7.5, for better visualization).