How Muscles Work and How They Respond to Resistance Exercise

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Muscle contraction isn't just brawn. It isn't just mass that allows bodybuilders and powerlifters to perform Herculean lifts. Contraction of muscle, and strength in general, is much more than just size but also involves the muscle response to resistance exercise.

Anatomy and Physiology

There are three types of muscle in the body:

  • Skeletal (voluntary muscles that move the body, arms and legs)
  • Smooth (involuntary, found within the walls of internal organs, like stomach, intestine, bladder and blood vessels)
  • Cardiac (the muscle of the heart)

This article discusses skeletal muscle.

Skeletal muscle allows the body to move. The 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 that contain 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 allows the body to move. A muscle attaches on each side of a joint and when the muscle contracts or shortens, the joint moves. For example, the bicep muscle crosses the front part of the elbow. When you do a bicep curl, the muscle contracts, the elbow flexes and the weight is lifted.

Origins, Insertions, and Contraction Types

Where a muscle attaches to the bone closest to the center of the body is called its origin. The insertion of a muscle is where it attaches to bone farthest from the center of the body. The biceps origin is in the scapula of the shoulder, and its insertion is in radius bone of the forearm.

When a muscle contracts or shortens, it pulls on both its origin and insertion in bone and causes the joint to move. To return the joint to its original position, the reciprocal muscle on the other side of the joint must contract and shorten. Muscles don't push joints, they only shorten and pull. It is up to both reciprocal muscle groups to work together to move the body. For instance, your biceps shortens and bends your elbow, while the triceps on the other side of the arm shortens and returns the elbow to its original 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. Pulling down the bar uses the following muscle groups: biceps, latissimus dorsi, posterior deltoids, and rhomboids. All these muscles located in the back and arms contract and shorten, moving the shoulder and arm. These are concentric contractions.

To return the bar and lower the weight stack requires all those muscles to next lengthen and allow the bar to return to its starting position over your head. You don't just let go and allow the bar to fly up and the weight stack to crash down. Instead, you return the bar slowly by both contracting those muscles and allowing them to lengthen. This is an eccentric contraction, where there is shortening and tension within the muscle associated with lengthening.

Eccentric contractions are also called "negative" work. For example, suppose you lift the final biceps curl of your set with the assistance of your spotter and then lower it slowly on your own. During this lowering, or negative eccentric phase, the biceps is contracting to lower it slowly and prevent the dumbbell from falling, but it's lengthening at the same time to allow your arm to straighten and return to the starting position.

Eccentric contractions can generate more force and strength than concentric contractions. Eccentric contractions can also make your muscles more sore than concentric contractions, probably because of the greater force generated and because of the simultaneous lengthening and shortening of the muscle. Walking down stairs or going downhill is an eccentric stress on the quadriceps muscles of the thigh while going up is concentric. That is why your quads hurt more going downhill.

Skeletal Muscle Control

Skeletal muscles are voluntary muscles stimulated and controlled by the brain and the somatic nervous system. Your brain is the central processing unit (like your computer). Nerve fibers from the brain run down the spinal cord and branch out in networks to every skeletal muscle that moves (like wires connected to light bulbs and outlets in your home). A small gap where the nerve meets the muscle is called the neuromuscular junction. This is where the nerve impulse fires and causes the release of chemical neurotransmitters including acetylcholine and electrolytes like sodium and calcium to stimulate the muscle to contract.