Research Article: Mechanics of Bipedal Hopping Across an Abrupt Upsloped Perturbation

Mitigating abrupt surface changes that can impede forward progression and stability during locomotion can be critical for survival. Joint-level mechanisms that contribute to dynamic stability under such conditions are still not thoroughly understood. This study examined how kangaroo rats Dipodomys deserti accommodate a decelerating perturbation – a sudden 10-degree upslope. We hypothesized that (1) hindlimb orientation mediates center of mass (CoM) pitch stability and (2) the ankle joint is the primary contributor to increased energy absorption demands caused by the decelerating perturbation. High-speed video and ground reaction forces (GRFs) were recorded as six desert kangaroo rats hopped at steady speed on our variable terrain rotatory treadmill instrumented with the perturbation. An inverse dynamics analysis was used to quantify whole-body and joint-level mechanical output during level and perturbed hopping conditions. Across both conditions, the average hindlimb angle was more retracted as the average orientation of the GRF vector became more accelerative. When acceleration was held constant however, the perturbed condition exhibited a more protracted hindlimb angle than level trials, likely due to anticipation by the animal exhibited through increased hindlimb protraction during the preceding aerial phase to set landing conditions. Analysis of angular impulse about the CoM revealed that the inclusion of the average hindlimb angle in the statistical model produced a positive relationship with both acceleration and condition. This result supports our first hypothesis indicating that the control and modulation of hindlimb orientation mediates pitch stability when animals encounter both challenges in acceleration and the additional decelerative forces associated with the perturbation. At the joint level, net work increased with acceleration at all joints except the metatarsophalangeal joint. Knee work increased at the fastest rate, followed by the ankle. Therefore, our second hypothesis was not supported, most likely due to the inclusion of the tarsometatarsal joint in the inverse dynamics analysis. Due to the upsloped nature of our perturbation, variation in vertical CoM momentum was observed and was modulated through decreased energy absorption at the ankle in the decelerative phase of stance during the perturbed condition. Consistent with prior work from our lab, these results highlight the ankle’s important functional role in the modulation of vertical CoM momentum.