Understanding Muscle: The Ultimate Actuator

It is safe to say that no man-made actuator surpasses muscle. For example, humans can contract their muscles to perform delicate tasks like writing characters on a grain of rice, as well as lift a 200 kg barbell. Blue whales use their muscles to swim through the great oceans with their massive 190,000 kg bodies, while mosquitoes, weighing only 2 mg, have muscles within them that they use to fly. A single type of man-made actuator cannot handle such a wide range of scales.

Muscles not only generate force but also have the properties of a spring and a dashpot (which generates a resistive force proportional to velocity, like friction) (Fig. 1). Because muscles have the properties of a spring and a dashpot, we can, for example, land softly when jumping from a high place by bending our knees and hips to absorb the impact with our muscles. Humans must dynamically change the spring and dashpot properties of their muscles, but these properties could only be measured with fingers, hands, or feet fixed to a measuring instrument.

How can we understand the properties of muscles during various human-like movements without fixing them to a measuring instrument? We measured electromyography (muscle activity) while walking or cycling (Fig. 2), and at the same time, we applied electrical stimulation to the muscles to measure muscle sounds (vibrations during muscle contraction), joint angles, forces, and torques. Since the subjects are walking or cycling, the vibrations (Fig. 3a) and forces from these activities are also measured. We remove these. By doing so, we can isolate only the muscle sounds induced by electrical stimulation (Fig. 3b). Once this is achieved, since the input and output are known, we can determine the internal properties—that is, the properties of the spring and dashpot.

For example, when cycling, how do the properties of the muscles change between pedaling quickly with light resistance and pedaling slowly with heavy resistance (assuming the power output is the same)? When we examined the spring properties of the thigh muscles, we found that the spring became stiffer as the pedaling speed increased. On the other hand, the average electromyography value per pedal rotation did not change. This suggests that although the overall muscle activity does not change, fast-twitch muscle fibers become active instead of slow-twitch muscle fibers as the rotation speed increases.

Now, when humans maintain an upright posture, they must also skillfully combine the force generated by muscles and their spring properties, but the control method for this is not understood. When considering control methods, a detailed model of the subject is necessary. We are currently investigating the relationship between the sway of the center of gravity and muscle properties while maintaining an upright posture. The questions are endless: what happens when the freedom of the ankle joint is restricted by wearing high-heeled shoes, or when visual information is unavailable because the eyes are closed?