Muscle activation and strain in the guinea pig hindlimb
Muscle activity and length-change studies in a diverse group of quadrupeds (rats, goats, dogs, and horses) have highlighted the similarities and differences in hip and knee extensors. Specific hindlimb muscles, the vastus lateralis and the anterior biceps femoris, exhibit similar activation periods and display biceps shortening, which mirrors hip extension during stance. However, the vastus strain does not correspond to knee excursions. The major problem in forming conclusions from this data is that these species are not closely related, which is why electromyography (EMG) and sonomicrometry were used to study guinea pig hindlimb extensors. The guinea pig data were compared to data collected from rats to find out if the trend within rodents was similar to that found within the larger group of quadrupeds. As expected, the EMG activity and biceps strain of guinea pigs mimicked that of rats. In guinea pigs, the knee joint is a good indicator of the vastus length-change. For example, the knee joint extended more than it flexed during stance, thereby giving rise to slightly more vastus shortening than lengthening. The major trend within rodents reveals that the vastus strain follows knee joint excursions, but the vastus of larger quadrupeds (dogs, goats, horses) has different strain patterns than expected based on knee joint kinematics. Variations in muscles and tendons may bring about these differences in vastus strain.