GILLIS, Gary B.; Mount Holyoke College: Strain and Activation in the Thigh Muscles of Guinea Pigs During Level, Incline and Decline Locomotion

There is a well-documented relationship between body size and limb posture in mammalian quadrupeds. Larger animals adopt a more upright limb configuration during locomotion whereas smaller animals� limbs are held in more flexed positions. It is unclear whether these differences in limb posture impact the strain regimes of the underlying muscles. To address this issue, the actions of homologous muscles in animals of different size must be compared. In this study, sonomicrometry, electromyography and high-speed video are used to characterize the patterns of length change and muscle activity in two muscles of the guinea pig hindlimb: the vastus lateralis, a knee extensor, and the anterior biceps femoris, a hip extensor. These data can be compared to data from the same muscles in other mammalian species to determine if a pattern between size and muscle function emerges. In the guinea pig, both the biceps and vastus are activated near the start of stance in each stride and activity extends through 70-80% of stance in both muscles. EMG intensity increases with locomotor speed, and at a given speed, is highest on an incline and lowest on a decline. Strain in the biceps consists mainly of shortening during stance (14-17% of rest length) as the hip joint extends. The vastus undergoes a stretch-shorten cycle during stance as the knee flexes then extends. The amount of stretching and shortening are nearly equivalent (6-10% of rest length). When compared to other taxa, guinea pig EMG patterns are comparable for both muscles, as are strain regimes in the biceps. However, vastus strain patterns appear to differ among animals of different size, with smaller animals (e.g., rats and guinea pigs) exhibiting as much or more stretching than subsequent shortening during stance and larger animals (e.g., goats, dogs and horses) more shortening than initial stretching.