Support size and thumb position preferences in humans during suspension and vertical climbing: implications for hominin hand evolution

Exploring how the human hand interacts with the support substrate during arboreal-like activities can provide insight into whether, and how, early hominins exploited arboreal habitats. Previous ergonomic work suggests that supports 30-40mm in diameter provide the greatest grip force and comfort and are optimal for the human hand. In this study, participants (n=28) completed static postural and dynamic vertical climbing and suspensory activities on three diameters (45mm, 80mm, and 105mm), on a custom-built apparatus. Hand posture (thumb adducted and abducted) and diameter preference data were collected for each activity. Data was analyzed using exact binomial tests with Bonferroni corrected alpha values. Reported significance are alpha values below the Bonferroni corrected alpha value. In suspension the 45mm diameter, closest to the human hand optimal grasping diameter, was significantly preferred, despite hand size variation. In both suspension and vertical climbing the largest diameter was significantly the least preferred. Thumb posture preference was analyzed separately for the static condition and dynamic locomotion on each diameter for suspension and vertical climbing. No preference was found for thumb position across all suspension activities. In climbing, there was a significant preference for an abducted thumb during static and dynamic activities on the 45mm diameter, despite hand size differences. These results suggest a support size preference during arboreal-like locomotion, but it varies by locomotion type. Participants generally had no strong thumb position preference during these arboreal-like activities, which suggests arboreal locomotion had minimal influence on thumb evolution within the hominin clade.