How can the brain ensure that muscles are working efficiently? During running, for example, it seems that it would require exquisitely precise tuning of contraction timing, strength and duration to coordinate all the leg muscles. Does the brain orchestrate the split second coordination or is some other intrinsic property of the leg responsible for our sprinting performance? In his Journal of Biomechanics paper A. L. Hof suggests a simple way to ensure efficiency. Hof knew that two muscles in the human calf, the soleus and gastrocnemius, shorten very little during running, even though the ankle moves a lot. This is intriguing, because these muscles are most efficient when they shorten slowly and not very far. He describes his new theoretical model of the lower leg, which indicates that the spring-like Achilles tendon, to which both muscles attach, determines almost all ankle movement. Over a range of running speeds, the muscles can operate near maximum efficiency simply by turning on at the right time as the tendon stretches. So the intrinsic properties of muscles and tendons keep them working efficiently, almost independently of the brain's commands during fast activities, which means that the brain does not need to be very precise at all.

In most previous studies of neuromuscular control, the muscle and tendon properties are seen as secondary effects; they may modulate the commanded force, but the brain is ultimately in control. Surprisingly, Hof's new model implies that the tendon properties are more important than what the brain tells the muscles to do. Based on a linearised Hill model of muscle contraction, the model suggests that the muscle and tendon properties may be primary, and those of the brain only secondary. The model simplifies into a well-known physical system - a lightly damped spring - which describes the combined muscle-tendon force output at the ankle fairly well. Hof quickly realized that the spring, which represents the tendon, determines most of the force output, while the damper, which represents the muscles and the brain, is less important. In other words, once the tendon starts bouncing, it will keep bouncing for a long time without much external input. The brain doesn't need to do much to keep it bouncing and, as a side benefit, the large tendon length changes mean that the muscles usually contract slowly over a small distance, when they're most efficient.

The portion of the model that represents the muscle reveals another substantial benefit of this bouncy tendon: it prevents eccentric contractions. During an eccentric contraction, a muscle tries to contract against too much weight and ends up being forcibly lengthened, absorbing energy and resulting in negative efficiency. This happens when you slowly lower a heavy weight; your muscles are active to keep the weight from dropping rapidly, but are lengthening as you lower it. However, the model shows that if the commanded muscle force is above a certain level, all the lengthening occurs in the tendon. Even if the total muscle and tendon length increases, the muscles can still shorten slightly, as long as the commanded force is high enough. This reinforces the idea that the tendon keeps the muscles efficient, because they reach peak efficiency when contracting slowly. Ultimately, for efficient running, the brain is relatively unimportant - it's the tendon's bounciness that keeps us running effortlessly.