Muscle Mechanics: Decoding Dynamic Locomotion with the Avatar Method

Muscle mechanics refers to the study of how muscles produce force and motion in the body. This field encompasses understanding the physical and biochemical processes that occur during muscle contraction, relaxation, and overall functioning.  Through studying muscle mechanics, researchers and clinicians can gain a deep understanding of how muscles work in normal conditions. Many diseases and conditions, such as muscular dystrophies, myopathies, and nerve disorders, directly affect muscle functioning. Understanding muscle mechanics helps in diagnosing these conditions accurately and managing them effectively. Moreover, knowledge of muscle mechanics is vital in designing rehabilitation programs and physical therapies for patients recovering from injuries, surgeries, or dealing with chronic conditions affecting muscle function. Furthermore, muscle mechanics is important in sport medicine for optimizing athletic performance, preventing injuries, and aiding in recovery processes. Additionally, understanding how drugs affect muscle function is important in developing medications for muscle-related conditions.

Traditional ex vivo methods, such as isometric force-length and isotonic force-velocity relationships, have been instrumental in advancing our understanding of muscle function. However, these methods have limitations, especially in replicating the dynamic strain and loading conditions experienced by muscles during in vivo locomotion. This discrepancy necessitated the development of new methodologies and theoretical frameworks to better understand muscle mechanics under more realistic, variable conditions. In a new study published in the peer-reviewed Journal of Experimental Biology by Nicole Rice, Caitlin Bemis, and led by Professor Kiisa Nishikawa from the Department of Biological Sciences at Northern Arizona University in collaboration with Professor Monica Daley from the Department of Ecology and Evolutionary Biology at University of California Irvine, aimed to bridge the gap between traditional muscle mechanics studies conducted ex vivo and the dynamic conditions of in vivo locomotion. They introduced an innovative ‘avatar’ methodology. The term ‘avatar’ was used to describe the use of mouse extensor digitorum longus (EDL) muscle as a proxy for studying another muscle in an experimental setting. In this case, the mouse EDL muscle acted as an avatar for the guinea fowl lateral gastrocnemius (LG) muscle. This approach addresses the challenge of direct in vivo force measurements, which are often difficult to obtain, especially in humans and certain animal species​​. The EDL muscles were surgically removed from mice and prepared for ex vivo experiments. These muscles were then subjected to various strain trajectories and stimulation patterns to simulate the in vivo conditions experienced by the guinea fowl LG muscle during unsteady locomotion on a treadmill with obstacle perturbations​​. The researchers used detailed in vivo data from guinea fowl LG muscles, including strain trajectories during running over obstacles at different speeds. These in vivo strain patterns were replicated in the ex vivo experiments on mouse EDL muscles. The goal was to mimic the natural conditions as closely as possible, including variations in muscle loading and strain rate​​. The authors used different stimulation patterns to emulate the guinea fowl’s muscle activation during running. These patterns were designed to reflect the timing and duration of muscle activation in vivo, considering the delay between activation onset and force production​​. The forces produced by the mouse EDL muscle under these simulated conditions were compared to the actual in vivo forces of the guinea fowl LG muscle to assess the accuracy of the avatar method.

One of the significant findings was that the forces produced by the EDL muscle under in vivo strain trajectories were more similar to the in vivo forces of the guinea fowl LG muscle, especially when compared to forces produced under sinusoidal trajectories. This highlights the importance of strain transients (abrupt changes in strain rate) in understanding muscle function during variable loading conditions​​. The authors demonstrated that the timing and pattern of muscle stimulation significantly influence muscle force production and the work performed per cycle. Different stimulation patterns produced varying effects on the muscle’s work output, emphasizing the role of neural activation in muscle function​​. The new avatar method showed that muscle function is not only a matter of length or velocity but also of how muscles respond to dynamic changes in strain and loading. This finding challenges some traditional views of muscle mechanics and emphasizes the need to consider the variability inherent in natural locomotion​​. Different strain trajectories and stimulation patterns led to significant variations in peak stress and work per cycle. This finding highlighted the complexity of muscle mechanics under varying physiological conditions and the importance of strain trajectory and stimulation pattern in determining muscle performance​​.

In summary, the authors developed a novel avatar approach, a significant methodological advancement in muscle mechanics research. It offers a novel way to study muscle function under dynamic, variable conditions that more closely mimic in vivo situations. This could lead to more accurate models and simulations of muscle function, particularly in the context of unsteady locomotion and variable loading conditions.