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.

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.

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.

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.

Research shows that eccentric contractions can generate more force and strength than concentric contractions. Eccentric contractions can also make you sorer than concentric contractions, probably because of the greater force generated and because of the simultaneous lengthening and shortening of the muscle. Walking down stairs is eccentric for the quadriceps, and that’s why when your legs are sore it’s more painful to walk down the stairs than up (up is concentric).

Skeletal muscles are controlled and stimulated by the nervous system that we control (somatic nervous system). They are not involuntary muscles like smooth or cardiac muscle (autonomic nervous system ), and so they don’t have a «mind of their own» and cannot contract without orders from higher up in our conscious brain. It’s designed like this. Your brain is the central processing unit (like your computer CPU). 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 fibers (motor neurons) meet the muscle fiber is called the neuromuscular junction. It’s at the neuromuscular junction that the nerve impulse fires and causes the release of the neurotransmitter acetylcholine and electrolytes like sodium and calcium to stimulate the muscle to contract. The neuromuscular junction is like the space in a light socket where the electrical wires meet the light bulb, just without the biological stuff like neurotransmitters and electrolytes.

Movement works like this. You think about moving, your brain processes the thought and figures out which muscles are necessary to make the movement happen, and then it sends impulses via the nerves to the muscles necessary for the movement. If you decide right now to get up and walk across the room, your brain would need to send signals to the muscles in your legs to lift you up out of the chair and then signal the correct muscles in your legs to walk you across the room. It’s an exquisitely sensitive and finely tuned system far more complex than the computer you’re reading this at. Scientists could build robots that move as smoothly as we do if it were simple.

Strength is both a function of mass and the amount of neurological patterning to the muscle fiber. We’ve all known someone who isn’t huge in terms of mass, but who has lots of strength. That strength is somewhat a function of mass, but it also comes from recruitment patterns in the nervous system that connect to muscle fibers. You’ll generate more strength in your biceps if you can recruit and fire 50,000 muscle fibers than if you can only recruit 40,000 fibers. The reason people get so much stronger in just the first few weeks of a new strength training program without increases in mass is that they have tapped in to new patterns of muscle recruitment via the nervous system. This comes from routinely lifting weights and recruiting new patterns of communication between the brain, nerves, neuromuscular junction, and muscle fibers. Every time you lift weights and light up those muscles, you lay down new neuromuscular patterns and get stronger.

Research on this subject is extremely interesting. Motor neurons in the muscle and nervous system die as we get older but don’t regenerate, and as a result, we lose strength. But the process can be reversed. Research shows that motor neuron firing can increase by as much as 20%, with strength increases as high as 35%, in just six weeks of weight training in men as old as 80 years of age!

Muscle hypertrophy (increase in size) is a separate mechanism from the nervous system that I just described. Sure, you need the nervous system to fire up the muscles to stimulate hypertrophy, but hypertrophy works differently. When you lift weights, you cause microscopic damage (microtears) to the myofibrils inside the muscle fiber. This isn’t the type of damage that you go to the doctor for, but normal catabolic damage that the body repairs. The microtears stimulate white blood cells, protein, testosterone, and other nutrients to flood the muscle cells and repair the damage, and they also stimulate more myofibrils to grow. The growth in the number of myofibrils swells inside the muscle fiber and you get pumped up. Importantly, muscle fibers don’t grow in number; they just swell as the number of myofibrils increases.

Recent advances in molecular biology and microscopic technologies have allowed scientists to peer into the lives of cells hundreds of times smaller then the head of a pin. Physiologists recently published stunning electron microscopic images of muscle satellite stem cells in myofibrils after they were stimulated by muscle contractions. You can see in the images how the contractions stimulated immature cells to grow into mature myofibrils, thus causing muscle fiber hypertrophy. These images were of muscles in men and women 65 to75 years of age who were weight lifting. In addition, the researchers were able to «tag» these satellite cells with special tracer molecules that can be seen under a microscope. The tags clearly show increases in activity of the satellite cells by as much as 30%, proving that activities like weight lifting have a profound effect on growth and development no matter what the age of the individual. Our body is an awesome biological system that responds positively whenever we use it.

There’s ample evidence that resistance exercise is effective at any age. And the biological changes that occur in the muscle when you lift weights ought to convince you that this type of activity is beneficial for a lifetime. We’ve got the stuff to get stronger no matter how old we are or how sedentary we’ve been. I encourage you to stick with muscle-building exercise if you’re already doing it, and get started if you’re not. Enjoy your workouts!