Joined: January 16, 2005
From: St. Petersburg, FL
Ergonomics and Muscle Fatigue
When skeletal muscle is continually stimulated, the force or tension that is developed by the muscle fibers diminishes. This failure of muscle fiber to maintain tension as a result of contractile activity is known as muscle fatigue. The onset of fatigue depends on both the type of skeletal muscle fibers as well as the intensity and duration of the muscle contractions. The red muscle fibers, or the ?slow twitch? fibers appear to have better blood flow and therefore oxygen supply to maintain aerobic metabolism. In the slow twitch muscle fibers, fatigue develops more slowly. These muscles fibers are used mostly during long duration, low intensity activities. The white muscle fibers, also called ?fast twitch? fibers, appear to rely more upon anaerobic metabolism. These fibers fatigue more rapidly, and are used more for short duration, high intensity activities. The development of muscle fatigue corresponds to four events that occur in working muscles:
1.) The depletion of the concentration of ATP. The rate of ATP utilization exceeds the rate of production. The muscle cannot contract without ATP.
2.) Increased amounts of intracellular acidity due to the rise in lactic acid levels. This increased hydrogen ion concentration affects the contractile proteins of the muscle fibers, decreasing the force generated by the muscle fibers.
3.) The depletion of muscle glycogen levels. As the amount of available glycogen diminishes, the muscle can no longer sustain a contraction.
4.) Levels of other metabolic waste products, including Carbon Dioxide, increase within muscle cells. If levels of acid and carbon dioxide waste products build up, this will slow aerobic metabolism, resulting in less efficient metabolism.
If muscle fatigue sets in and the muscle is no longer able to sustain work efficiently, the muscle becomes overloaded resulting in micro trauma to the muscle fibers. If this fatigue and overloading is repetitive or long term in nature the resulting microtrauma becomes cumulative and pathology or injury occurs. Local muscle fatigue is suspect to contribute to work-related Cumulative Trauma Disorders. In order to avoid the adverse effects of muscle fatigue, a sufficient supply or flow of blood to the working muscles is critical.
Since aerobic metabolism generates almost 20 times as much ATP for energy as does anaerobic energy, the effects of muscle fatigue can be minimized by ensuring work load intensity is low enough so that adequate oxygenation, or blood flow to the active working muscles is achieved. If heavy workloads are required, they should be brief in duration, lasting less than a few seconds or minutes, which reduces the effects of prolonged anaerobic metabolism, and maximizes metabolic efficiency.
Summary The most important factor in ergonomic job design or modification is to promote aerobic metabolism and adequate blood flow, resulting in a high metabolic efficiency. This will maintain adequate blood flow to working muscles, prevent fatigue and allow maximal performance. Dynamic muscle contractions are always preferred over static muscle loading situations. Work-rest cycles should provide sufficient recovery times to sufficiently perfuse active muscles with blood. Jobs should be designed or modified to minimize or reduce the requirements for static contractions, such as static grips, extended reaches and extreme postures.