How Do Muscles Work?

By Sarah Robson, RMT

(Originally written for the Kal Rats Newsletter 2004)

There are three types of muscle tissue in the body: skeletal (attaching to bones), cardiac (heart), and smooth (along the digestive tract, in the skin, and in blood vessels. As triathletes, we focus most on skeletal muscle training, but also train the cardiac muscle through endurance (aerobic) exercise. This article will focus on skeletal muscles. Skeletal muscles do a lot more than just create movement and power. They store energy, help the flow of digestive contents, and create heat to maintain body temperature, to name a few functions.

Skeletal muscle is defined as voluntary type of muscle tissue because you consciously choose to move your limbs. However, there are involuntary characteristics to skeletal muscle, such as the diaphragm for breathing – most often an unconscious act -and muscle tone. Muscle tone means a small number of muscle fibers within the whole muscle are contracting at any given time to maintain a functional muscle length, so the muscle is always ready to be called into action. So although your muscles are never truly relaxed…they shouldn’t be rock hard at rest either! A healthy muscle should be supple and elastic. In a strong muscle, it is not the resting tone that is the defining factor of strength, but rather how that muscle contracts against resistance and how fatigue resistant it is under prolonged use.

A skeletal muscle has layers of fascia, the saran wrap of the body, which surrounds the microscopic compartments forming groups within the muscle and even surrounding the whole muscle itself. The functional part of the muscle is the contractile portion, in between the tendons, that shortens to move the bone it is attached to or resist a force on the bone as in carrying a weight. The tendon is made up of the continuation of the fascia that divides and groups the contractile elements of the muscle. When the contractile elements end, the fascia continues on as tendon and binds with the outer layers of the bone connecting the muscle to bone. The fascia is non-contractile, composed of fibers and a gel-like solution. The more dehydrated you are, the less gel-like it is and the fascia becomes rigid and “sticky”. This change in fascia can decrease the health and mobility of the muscle tissue itself, affecting the range of motion of your joints, the strength of the muscle, and its ability to resist fatigue. If your fascia is tight and restrictive it will increase pressure within the muscle. Your blood flows from high pressure to low pressure areas; therefore, when the pressure inside the muscle is very high, the muscle has decreased blood flow. This decreased blood flow decreases the oxygen and nutrient availability to the muscles and waste removal from the muscle. This series of events decreases the health of the tissue making it prone to injury and less able to perform to maximum capacity.

There are three types of skeletal muscle found in the body. The first are slow oxidative fibers that are the least powerful but contain the most capillaries (where oxygen and nutrients can diffuse from blood to the tissue) and oxygen-based energy producing mechanisms. These types of fibres are the endurance athlete’s favourite muscle type because they are very fatigue resistant. They take longer to reach peak tension but can create prolonged contraction for many hours. The second type of skeletal muscle tissue is fast oxidative fibers that have characteristics in between the slow oxidative and fast glycolytic fibres. These intermediate fibres have characteristics of both extremes. They are relatively fatigue resistant while being able to produce tension with both aerobic and anaerobic (non-oxygen) mechanisms. This muscle type can create peak tension faster than the slow oxidative fibers. The third and final fiber type is called fast glycolytic fibers and they create the most powerful contractions – strong and quick. They are more developed for short powerful anaerobic contractions and will fatigue quickly. These fibers are most useful in quick, fast, pick-ups and sprints, but not the long, sustained efforts most triathletes are used to. You are genetically given a percentage of these different fibres which does not change, but you can, however, maximize what you are given. The middle fiber type, the fast oxidative, can be “trained” to maximize the aerobic or anaerobic portion of this fibre type. Therefore, depending on your preferred activity you can increase your endurance or your quick and powerful strength.

When you strengthen your muscles you are not increasing the number of fibers in a muscle. You are actually damaging the existing fibers, forcing them to rebuild stronger, thicker, and larger. So as you are out running hills or biking intervals you are taxing your muscle to the point of microscopic injury. Your body’s natural healing mechanism will kick in to help mop up the damage, rebuild the fibers, and strengthen the fibers against future damage at the same intensity. It is important to include active recovery and stretching after you exercise because you want to maximize the proper healing of the fibers. The muscle will rebuild by forming strong scars that look like a tangled mess that leads to decrease elasticity and improper sliding of the muscle filaments during contraction. This will, in-turn, decrease the strength and efficiency of the muscle. When you stretch during the period when the tissue is healing you can help align the fibers so they continue to heal in a more parallel way, increasing the efficiency and sliding of the filaments. A muscle strain is what happens when you tear a large number of muscle fibres to the extent that is significantly more than what happens when strengthening the muscle. The degree of the muscle strain is determined by the number of fibres torn, the pain pattern, and the function lost. Muscle strains take a lot of time and rest to recover well. When you start training again post injury, you must start slowly and work your way back up to your original effort and intensity to prevent a re-injury. Often athletes do not take enough time off their sport, don’t get the right (or enough) treatment, and start back at the intensity they left off at, which usually overwhelms the muscle and leads to re-injury.

Muscles are a vast topic area and I’ve just begun to scratch the surface with this article. I hope that this has given you a basic understanding of your muscles.