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Canadian Fitness Education Services (CFES)
Fitness Knowledge Homestudy Program

Chapter 4 Muscular Structure and Function

Factors Affecting Muscle Performance

The muscles ability to generate force is dependent on a number of factors, including muscle fiber recruitment, muscle fatigue and soreness.


Muscle Fiber Recruitment

The body learns how to generate the right amount of force for various activities. With weight lifting, for example, the neuromuscular system adjusts so the right number of muscle fibers are recruited for the resistance or load being applied. It does this by increasing the rate (speed) of nervous stimulation to the working muscles or by increasing the number of motor units being stimulated to contract. Recruitment patterns are trained quickly, often within the first two to four weeks. This is observable as a change from jerky or wobbly movements to smooth, coordinated movement patterns. When new exercises are introduced, one can expect a period of adjustment time for this adaptation to take place.


Local Muscular Fatigue

When a muscle is subjected to prolonged stimulation, it loses its ability to contract and is unable to produce the same work. This can be due to many factors including:

1.   The type of fiber being stimulated (fast twitch fatigue more easily).

2.   Decreased blood supply to the muscle.

3.   Lower pH (due to lactic acid) which inhibits enzymes and chemicals involved in muscular contraction.

4.   Depletion of fuel stores such as glycogen or creatine phosphate.

5.   Inability of the motor nerve to stimulate contraction of the muscle fiber.

6.   Physical pain.


Characteristics of a fatigued muscle fiber:

-     decreased irritability (ability to receive nervous impulse)

-     decreased conductivity (ability to relay the impulse)

-     decreased contractibility (ability to contract)

-     slower shortening and relaxing phases



While fatigue is a normal part of physical conditioning, the following steps should be taken to reduce its limiting effects:

-     a proper warm-up

-     a proper training intensity

-     adequate nutrition to supply the required sources of energy

-     adequate hydration to ensure a proper fluid balance


Muscle Soreness

Muscle soreness is also a normal part of physical conditioning. It is important, however, that individuals understand the difference between muscle stiffness and extreme pain in the muscle or joint. Extreme pain could be an indication of an injury (which may require medical assistance). Muscle stiffness, on the other hand, should be expected with resistance training. There are two main classifications of soreness, acute and delayed.


Acute Soreness: This is the pain felt during or immediately after high intensity exercise and is caused by a lack of blood flow and the presence of lactic acid in the muscle. As normal blood flow is restored, this pain quickly subsides.


Delayed Onset of Muscle Soreness (DOMS): This is the pain felt 24 - 48 hours after exercise and is believed to be caused by microscopic tearing of the muscle fibers or the swelling of the muscle fibers which decreases circulation and causes muscle spasm. Eccentric contractions increase the likelihood that this type of soreness will occur. If pain and stiffness continue for longer than a couple of days, medical attention should be recommended.



- adequate warm-up prior to exercise

- appropriate volume and intensity of training

- a gradual progression in intensity of exercise

- avoiding excessive eccentric muscular contractions

- stretching


Recovery: If the soreness is not extreme, modified activity may be helpful as it increases blood flow and removal of waste products. Do light aerobic exercise (walking, cycling, swimming) followed by gentle stretching. Massage may also be helpful.


Muscle Cramps and Spasms

Muscle cramps and spasms are involuntary contractions of muscle fibers. They appear to start off with one motor unit contracting and then quickly spread to other motor units in an area of the body (often the hamstrings or calves). They are usually quite painful but can be quickly treated by gently stretching and massaging the affected muscle.


Possible causes:

- over-activity or spontaneous discharge of motor nerves

- chemical imbalance (calcium ions staying in the muscle)

- abnormal interaction between Actin/Myosin contractile filaments

- hyperexcitability of muscle fiber membrane


Predisposing factors:

- sudden or abrupt movements

- extreme fatigue with accumulation of waste products

- strong sustained muscular contractions

- inadequate blood supply

- electrolyte, fluid and mineral imbalances



- consume plenty of fluids before, during and after exercise

- eat a balanced diet

- ensure adequate warm-up prior to strenuous activities



- gently stretch and massage the muscle group affected