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 of Skeletal Muscles

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.

Muscle 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 lateral pull-down exercise (lat pull-down). 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.

How do muscles move?

Movement requires the whole system to work.

  • When you think about moving, your brain decides which muscles are necessary to make that movement happen.
  • Electrical impulses are sent via the spinal cord and nerves to the appropriate muscles.
  • Adequate supplies of neurotransmitters need to be present to stimulate the muscle to contract.
  • Once the movement has started, there are feedback mechanisms to allow the brain to monitor the movement. This is called proprioception: the sense of where one body part is located in relation to others and in relation to gravity.

How can I build muscle strength?

Strength is both a function of mass and the amount of neurological patterning of the muscle fiber. We've all known someone who isn't huge in terms of mass or body size but who has lots of strength. While there is a relationship between mass and strength, the power to move also comes from recruitment patterns in the nervous system that connect to muscle fibers. People generate more strength in their biceps if they can recruit and fire 50,000 muscle fibers than if they can only recruit 25,000 fibers. Muscle recruitment allows people to get so much stronger in the first few weeks of a new strength training program without increasing the mass of muscle. Routinely lifting weights recruits new patterns of communication between the brain, nerves, neuromuscular junction, and muscle fibers. Every time someone lift weights and engage up those muscles, he or she lays down new neuromuscular patterns and gets stronger.

Motor neurons in the muscle and nervous system die as people get older and do not regenerate, and as a result, people lose strength. Exercise can reverse that process. Geriatric patients can increase motor neuron firing by as much as 20% and increase stretch by more than one-third in just six weeks of weight training.

What is muscle hypertrophy?

Muscle hypertrophy (increase in cell size) is a separate mechanism that increases muscle strength. While the nervous system and neuromuscular junctions are needed to fire muscles to contract, hypertrophy works differently. When people lift weights, microscopic damage (microtears) occurs to the myofibrils within the muscle fiber. These microtears stimulate the body's repair response. The body delivers nutrients that flow to the muscle cells to repair the damage and to stimulate more myofibrils to grow. The increased number of myofibrils causes muscle fibers to enlarge, increasing their volume and size. It is important to remember that no new muscle fibers are created; they just swell as the number of myofibrils increases.

Molecular biology and microscopic technologies allow scientists to look into the lives of cells, hundreds of times smaller than the head of a pin. They can see how the muscle fiber 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.

Weight lifting is beneficial at any age

Weight lifting and other resistance exercises are effective at any age and are beneficial for a lifetime. Everybody has the right stuff to get stronger no matter how old or how sedentary. Muscle-building exercises have their benefit beginning with the first workout. Sticking with it will add not only strength but also quality of life. Enjoy your workouts!

Medically reviewed by John A. Daller, MD; American Board of Surgery with subspecialty certification in surgical critical care


Buford, T.W., et al. "Optimizing the Benefits of Exercise on Physical Function in Older Adults." PM&R. Available online 19 December 2013. Elsevier Press.

Krist, L., et al. "Can progressive resistance training twice a week improve mobility, muscle strength, and quality of life in very elderly nursing-home residents with impaired mobility? A pilot study." Clin Interv Aging 8 (2013): 443-448.

Medically Reviewed by a Doctor on 12/12/2016

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