Normal Muscle Function

Normal muscle activity is a combination of contraction and relaxation, technically referred to as facilitation and inhibition, respectively. When running, for example, contraction and relaxation occur continuously throughout the body. When muscles contract, they get tighter and provide more work, and when relaxed they work with less force and allow their opposite muscle to contract better.

The best way to explain normal muscle function is for you to feel it yourself. Let’s use the biceps muscle on the front of the upper arm and the triceps muscle on the back of the arm. The contraction and relaxation of these two muscles, which usually work together, can provide an accurate view of how muscles normally work throughout the body. Let’s try this experiment:

广告：个人专属 VPN，独立 IP，无限流量，多机房切换，还可以屏蔽广告和恶意软件，每月最低仅 5 美元

First, in a relaxed, sitting position, with your left hand feel your right biceps muscle on the front of your upper arm. Then feel the right triceps muscle on the back of your upper arm. At rest, they should both be relatively relaxed—firm but neither tight nor too loose.

Next, place your right hand under your thigh, then pull upward as if trying to lift your thigh; in doing so you contract the biceps muscle. Now feel the biceps muscle again with your left hand, and it should feel noticeably tighter. This is how a contracted muscle (one that is normally facilitated by the brain) feels.

While continuing to lift up on your thigh, now feel the triceps muscle on the opposite side of the arm. This should feel much looser than the biceps and even a bit looser (depending on how much you pull up on your thigh) than when at rest. This is how a muscle relaxes itself more to allow the opposing muscle to contract. The biceps muscle is contracted (or facilitated), and the triceps is in a state of inhibition. In fact, without this extra relaxation (inhibition) by the triceps, the biceps could not properly contract.

During a run, this same facilitation and inhibition takes place constantly in opposing muscles, just like the biceps and triceps. It occurs in the quadriceps (front of the thigh) and hamstrings (back of the thigh), the anterior tibialis muscle (front of the leg) and calf muscles (including the gastrocnemius and posterior tibialis), the pectoralis muscles (upper chest) and latissimus (middle of back), and so on.

Normal muscle function is the optimal state of the neuromuscular system. It provides the best balance of the physical body—with the right combinations of inhibition and facilitation during physical activity.