Comparison between the kinematics for kangaroo rat hopping on a solid versus sand surface

In their natural habitats, animals move on a variety of substrates, ranging from solid surfaces to those that yield and flow (e.g., sand). These substrates impose different mechanical demands on the musculoskeletal system and may therefore elicit different locomotion patterns. The goal of this study is to compare bipedal hopping by desert kangaroo rats (Dipodomys deserti) on a solid versus granular substrate under speed-controlled conditions. To accomplish this goal, we developed a rotary treadmill, which is able to have different substrates or uneven surfaces. We video recorded six kangaroo rats hopping on a solid surface versus sand at the same speed (1.8 m/s) and quantified the differences in the hopping kinematics between the two substrates. We found no significant differences in the hop period, hop length, or duty cycle, showing that the gross kinematics on the two substrates were similar. This similarity was surprising given that sand is a substrate that absorbs mechanical energy. Measurements of the penetration resistance of the sand showed that the combination of the sand properties, toeprint area, and kangaroo rat weight was likely the reason for the similarity.

Methods
Video data was digitized for joint centers. Joint angles were calculated from the digitized data. For the sand data, a penetrator was connected to a servo motor to measure force and penetration depth. Resistance to penetration was estimated by dividing change in force by change in depth.