How Muscles Work and How They Respond to Resistance Exercise

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Author: Richard Weil, MEd, CDE
Medical Editor: William C. Shiel Jr., MD, FACP, FACR

Muscle contraction isn't just all brawn. You might look at bodybuilders and powerlifters and think that it's just all mass that allows them to do those Herculean lifts. But it's much more than that. Sure, mass is part of it, but the contraction of muscle, and strength in general, is much more than just size. I'll review the mechanisms of muscle contractions and how your muscles respond to resistance exercise in this article.

Anatomy and Physiology

To understand muscle contraction, it's important to know a little anatomy and physiology. To get started, you need to know that there are three types of muscle in your body: skeletal (voluntary, like the muscles that move your limbs), smooth (involuntary, like around organs), and cardiac (the heart). I'll discuss skeletal muscle in this article.

Skeletal muscle is contractile tissue made up of thousands of parallel, cylindrical fibers that run the length of the muscle (you could have 100,000 fibers in your biceps alone!). The fibers are made up of smaller protein filaments called myofibrils, and the myofibrils are made up of even smaller protein myofilaments called actin and myosin. The sliding filament theory of muscle contraction describes how actin and myosin slide over each other, causing the myofibrils to shorten, which in turn causes muscle fibers to contract.

Skeletal muscle was so named because it attaches to the bones in your skeleton. In fact, we're really just a bag of bones strung together by muscles! Most of the skeletal muscle in our body crosses a joint and attaches to a bone, and when muscles contract, or shorten, they pull on a bone and we move. For example, your biceps muscle crosses your elbow joint (a hinge joint), and when it contracts, your elbow flexes. When you do biceps curls, your biceps pulls on the bone in your forearm, your elbow bends, and you lift the weight (biceps actually cross the shoulder joint, too). The biceps couldn't bend your arm if your elbow wasn't a movable joint.

Origins, Insertions, and Contraction Types

The origin of a muscle is where it attaches to the bone closest to the center of the body, and the insertion is where it attaches to the bone furthest from the center of the body. The biceps origin is in the shoulder, and the insertion is in the forearm.

When the muscle contracts, it shortens and pulls on the bone. To return the bone to where it started, the reciprocal muscle on the other side of the bone must contract and shorten. Muscles don't push bones, they only shorten and pull. So, it's up to reciprocal muscle groups to work together to move us back and forth. For instance, your biceps shortens and bends your arm, but it's up to the triceps on the other side of the arm to shorten and pull the bone back to its original starting position. This "reciprocal" synergy between muscle groups is sometimes called the agonist/antagonistic system.

Concentric and eccentric contractions are two types of contractions that you use every time you lift weights. Concentric contractions are when a muscle shortens, and eccentric contractions are when the muscle shortens and lengthens at the same time. It sounds confusing, but here's how it works. Consider the lat pull-down exercise. You pull the bar down using the following muscle groups: biceps, lats, posterior deltoids, and rhomboids. All these muscles contract and shorten to pull on the bones in your back and arms. Those are concentric contractions. But now you must return the bar and lower the weight stack. That means all those muscles that pulled the bar must now lengthen to allow it to return to the starting position over your head. But you don't just let go and allow the bar to fly and the weight stack to crash. Instead, you hold on and return the bar slowly. To do that, all the muscles that pulled it down must now contract to prevent it from flying away, but the muscles must also lengthen to allow your arms to stretch out and return the bar to the starting position over your head. This is an eccentric contraction, where there is shortening and tension in the muscle but also lengthening.