Grizzly bear (Ursus arctos horribilis) locomotion: forelimb joint mechanics across speed in the sagittal and frontal planes

Abstract

The majority of terrestrial locomotion studies have focused on parasagittal motion and paid less attention to forces or movement in the frontal plane. Our previous research has shown that grizzly bears produce higher medial ground reaction forces (lateral pushing from the animal) than would be expected for an upright mammal, suggesting frontal plane movement may be an important aspect of their locomotion. To examine this, we conducted an inverse dynamics analysis in the sagittal and frontal planes, using ground reaction forces and position data from three high-speed cameras of four adult female grizzly bears. Over the speed range collected, the bears used walks, running walks and canters. The scapulohumeral joint, wrist and the limb overall absorb energy (average total net work of the forelimb joints, −0.97 W kg −1). The scapulohumeral joint, elbow and total net work of the forelimb joints have negative relationships with speed, resulting in more energy absorbed by the forelimb at higher speeds (running walks and canters). The net joint moment and power curves maintain similar patterns across speed as in previously studied species, suggesting grizzly bears maintain similar joint dynamics to other mammalian quadrupeds. There is no significant relationship with net work and speed at any joint in the frontal plane. The total net work of the forelimb joints in the frontal plane was not significantly different from zero, suggesting that, despite the high medial ground reaction forces, the forelimb acts as a strut in that plane.

Spring-loaded inverted pendulum goes through two contraction-extension cycles during the single support phase of walking

“…First, VGRF rarely goes below 30% of the weight of the animal (Hubel and Usherwood, 2015; Bishop et al, 2018). Constraint on VGRF was implemented through the ‘VGRF ratio’, G r ≡(minimum VGRF)/ mg , and we imposed the constraint:For mammals, including humans (Cavagna and Margaria, 1966; Lee and Farley, 1998), certain birds (Bishop et al, 2018; Daley and Birn-Jeffery, 2018) and some quadrupeds (Shine et al, 2017; Bobbert et al, 2007; Kaya et al, 2006) the VGRF has a mid-stance minimum and is flanked on each side by two local maxima thereby producing a characteristic M-shape (see Fig. 1C).…”

Spring-loaded inverted pendulum goes through two contraction-extension cycles during the single stance phase of walking

“…Second, for mammals, including humans [12,29], certain birds [4,14], and most quadrupeds [43,9,27], the VGRF has a mid-stance minimum and is flanked on each side by two local maxima thereby producing a characteristic M-shape, see Fig. 1C.…”

Polar bear paw pad surface roughness and its relevance to contact mechanics on snow

“…These observations suggest that brown bears' paws slide on mud and snow, but polar bears are able to walk easily on slippery surfaces. Potential explanations for this observation can come from a combination of differences in running mechanics [38,[46][47][48], paw sizes [49][50][51] and paw pad microstructures [26].…”