Above: image © istockphoto.com/leremy

Almost everyone has experienced sore muscles. They could be the result of a weekend soccer tournament, resistance training, hiking, or any other any activity that works your muscles more than usual. An important thing to remember is that mild muscle soreness is a common part of adapting to new kinds of physical activity. It actually means that your muscles are getting stronger.

How muscles work

Skeletal muscle is the type of muscle used during exercise. It is composed of fibres (or cells) that run the entire length of the muscle. Muscle fibres have a thin, threadlike shape and contain two unique structures not found in other kinds of cells:

Muscle fibre organization and structure. Click image to enlarge (Mfigueiredo)

1. T-tubules (transverse tubules) run at right angles to the muscle fibre. They allow electrical signals coming from nerve cells to enter the interior of the fibre.

2. A modified cytoskeleton serves as a kind of scaffolding or skeleton for the muscle fibre. It consists of very long and thin rods or threads, called myofibrils, which run the length of the muscle fibre. In turn, myofibrils are made up of even smaller structures called myofilaments. Myofilaments are separated into small units called sarcomeres, which have the ability to contract.

When a muscle fibre is stimulated by a nerve cell, the T-tubules carry an electrical impulse from the nerve into the interior of the fibre. This causes calcium to be released, which results in shortening of sarcomeres. This causes the myofilaments to shorten along with the muscle fibre, leading to muscle contraction.

Muscle damage and repair

Did you know? The only voluntary muscle tissue in the human body is skeletal muscle. It controls every action you consciously perform.

Muscle fibres get damaged when an applied force causes them to lengthen. For example, your quadriceps (the muscles at the front of your upper leg) normally contract in order to straighten your leg. But when you land from a jump, your quadriceps are activated in order to slow the bending of your leg. As a result, the muscle fibres are forced to lengthen and suffer damage.

In the same way that your body reacts to a cut or scrape, damaged muscle fibres attract various types of special cells sent by the immune system to clean up the damage and protect the area from further injury. A by-product of this immune response is an increase in fluid and temperature, which helps additional immune cells arrive at the damaged area to help with the repair process.

Furthermore, when muscles are at rest, calcium levels normally decrease and the fibre relaxes. However, when muscle fibres have been damaged during exercise, calcium levels remain higher than normal. This activates enzymes that create chemical reactions in the body. These reactions are thought to cause even more damage and may contribute to pain.

Soreness and strength

Did you know? Examples of resistance training include free weights, weight machines, resistance bands, and body weight exercise.The increase in fluid and temperature associated with the repair process is believed to activate pain receptors inside the muscle tissue. The resulting pain tends to peak 24-48 hours following the activity. This is commonly referred to as delayed onset muscle soreness (DOMS).

Along with those sent by the immune system, special cells called muscle cell precursors also arrive at the damaged area. These cells have the ability to fuse to each other and to the damaged muscle fibres. In doing so, they cause the muscle to grow in size and strength by increasing the size and number of myofibrils.

No pain, no gain

Did you know? Your heart is the only muscle that works constantly, every second of your entire life.When you begin a new activity or return to an old one, you often experience pain, since the demands on your muscles are much greater and different than what you are accustomed to. As you become more adapted to a particular activity, the amount of muscle damage progressively becomes smaller, to the point where you may not feel sore anymore. So the old saying “no pain, no gain” is only true in the short term.

Strategies for coping with DOMS include gradually easing into new activities, stretching, maintaining proper nutrition, and participating in light exercise to relieve some of the soreness. Always give yourself time to recover between highly demanding activities. This will give your muscles time to recover and prevent injury to other parts of your body while your muscles are not working at 100%.

So enjoy your muscle soreness. It’s a sign of muscle growth and it won’t be around forever!

References

General information

Delayed Onset Muscle Soreness (DOMS) (American College of Sports Medicine) Muscle Pain and Soreness After Exercise (Elizabeth Quinn, About.com) Muscular System: Facts, Functions & Diseases (Kim Ann Zimmermann, LiveScience) Sore Muscles? Don’t stop Exercising (WebMD.com) Why Delayed Onset Muscle Soreness is a Good Thing (Mackenzie Lobby, Running Times)

Scholarly publications and textbooks

Charge, SBP. 2004. Cellular and Molecular Regulation of Muscle Regeneration. Physiological Reviews. 84(1):209–238. Cheung K, Hume P, Maxwell L. 2003. Delayed onset muscle soreness: treatment strategies and performance factors. Sports medicine (Auckland, New Zealand). 33(2):145–164. Kamen G. 2001. Foundations of Exercise Science. Lippincott Williams & Wilkins. Kenney WL, Wilmore J, Costill D. 2012. Physiology of Sport and Exercise. Human Kinetics. Lewis PB, Ruby D, Bush-Joseph CA. 2012. Muscle Soreness and Delayed-Onset Muscle Soreness. Clinics in Sports Medicine. 31(2):255–262. Patton KT, Thibodeau GA. 2010. Anatomy & physiology. Mosby/Elsevier, St. Louis, Mo.

Nathan Michaels

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