The Musculo-skeletal System

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The primary structures of the human locomotive (movement) system are the skeleton, the muscles, and the joints (Figure 2.2). These structures combined allow the human body to move, withstand physical loading and recover when the body’s abilities have been exhausted.

These are the structures that are mainly active when performing physical work, although other systems (such as the nervous system, the respiratory system and the circulatory system) that are all very important for the human being’s ability to function are naturally also affected by physical work. However, this webpage will focus on movement, physical loading and what the locomotive system requires in order to function optimally.

Figure 2.2: The musculo-skeletal (or locomotive) system

Together, the skeleton, muscles and joints allow the human body to turn chemical energy into motion, to withstand physical forces and perform physical work in a way that is simultaneously dynamic, stable, flexible and adaptive. All of the structures are made up of living materials, so our body is constantly adapting to the loading and movements that we expose it to, making it better suited to perform those activities by becoming stronger and more stable. Unfortunately, it is also possible to load the body in such a way that we wear down or break the structures that make up our locomotive system. In order to avoid this and ensure that we design work and work systems that allow the human body to perform at its strongest, we need to know something about how each of these structures are shaped, how they move, how they respond to loading and regenerate.

The Muscles

There are many different types of muscles in the human body, as shown in Figure 2.3. In the locomotive system, skeletal muscles convert chemical energy into contractions, thereby producing motion and mobility, stabilizing body positions, producing heat and helping to return deoxygenated blood to the heart. As the name suggests, most skeletal muscles are attached to the skeleton (via fibrous tissues at the ends called tendons) and are dedicated to moving it. This differentiates skeletal muscle tissue from cardiac muscle and smooth muscle, which are not under our voluntary control.

Figure 2.3: Muscle types – skeletal, smooth and cardiac

Some skeletal muscles have specific functions, for example the postural muscles keep the posture of our body and head upright while we are awake, without any need for active control from our brain (although in states of extreme fatigue, we lose control over our postural muscles, which explains the term “nodding off” to sleep). It is our skeletal muscles that allow us to transfer loads and torques, and the strength of our muscles varies depending on our age, sex, genetic heritage and training habits.

Definitions of how to count the number of muscles in the body vary, but they number in the hundreds (about 600 indiviidual muscles) and they make up 40 to 50% of our body weight. Many muscles function in opposing pairs called antagonists, meaning that their contractions result in movements that work against each other. So when one anatagonist is maximally contracted, the other one is – by definition – in a state of relaxation to allow the movement (see Figure 2.4). Examples of antagonists at work include bending and straightening of the knee or the arm, pointing and flexing the foot, or alternately bending the back outward and inward. For high-precision movements, the body controls a sophisticated and sensitive balance of contraction and relaxation between antagonists.

Figure 2.4-Antagonistic pair – biceps and triceps

To stay balanced and well-aligned, the body generally needs to develop equal strength between antagonists; for example, some symptoms of back problems may actually have to do with weak stomach, or core musculature, rather than just the back muscles.

Healthy muscle tissue has four characteristics:

  1. Excitability, which is the ability to respond to stimuli
  2. Contractility, which is the ability to shorten and thicken (contract) when stimulated
  3. Extensibility, which is the ability to stretch without being damaged
  4. Elasticity, which is the ability to return to its original shape after any form of physical loading
Figure 2-5-skeletal muscle structure

On a cellular level, muscles consist of clusters of long, thread-like cells called muscle fibres that measure about 2 – 150 mm long and 10 – 110 micrometres thick; see Figure 2.5.

Muscle fibres are in turn bundled into motor units, a group of cells that respond to voluntary signal impulses from the brain by contracting until the motor unit is fully, 100% contracted. This reaction of a motor unit (sometimes called “firing”) can never be partial, so it is said that motor units are recruited one by one by the brain, until there are enough to perform the task. Motor units vary in how many muscle fibres they contain, and what type of movements and force generation they are adapted to. Each motor unit consists of one motor neuron and all the muscles fibres they contain, and what type of movements and force generation they are adapted to. Each motor unit consists of one motor neuron and all the muscle fibres it contracts. The structure of a motor neuron is shown in Figure 2.6.

A contraction of a muscle can be explained as a chemical process that leads to a shortening and thickening of each motor unit, resulting in the production of force exertion and heat.

Figure 2-5-skeletal muscle structure

Generally, muscle fibres behave differently when stimulated by nerve impulses and can be classified into two types: Type 1 (slow-twitch fibres, suited for prolonged work and high endurance) or Type IIa or IIb (fast-twitch muscle fibres, suited for quick, explosive, brief movements). They are characterized by the type of physical loading or movement that they are best adapted to. Most people are born with a genetically determined proportion of Type I and Type II muscle fibres, but it is possible through physical training and nutrition to influence the proportions of different muscle fibre types. The differences in characteristics of these muscle fibres are described in Table 2.2.

Table 2.2 | Main differences between different muscle fibre types.
Type I: Slow-twitch, aerobicType IIa: Fast-twitch A, intermediateType IIb: Fast-twich B, anaerobic
  • Adapted to high-endurance continuous contractions, work for a long period of time
  • Low force production
  • Slow contraction time
  • Small motor unit size
  • Not fatigued easily
  • Plenty of blood vessels and myoglobin = good supply of oxygen to generate muscle fuel
  • Red in colour (due to high blood vessel content
  • Predomiant in marathon runners and cyclists
  • Adapted to fast, short-term contractions
  • High force production
  • Fast contraction time
  • Large motor unit size
  • Quickly fatigued
  • Intermediate number of blood vessels and myoglobin
  • Mix of aerobic and anaerobic processes to generate muscle fuel
  • Red in colour
  • Adapted to extremely fast, explosive contractions
  • Very high force production
  • Very fast contraction time
  • Large motor unit size
  • Fatigues very quickly
  • Low number of blood vessels and myoglobin
  • Completely anaerobic processes (no oxygen), burns glycogen to generate muscle fuel
  • White in colour
  • Predominant in sprinters, high jumpers
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  • References
    • Bevan, S. (2013). Reducing Temporary Work Absence Through Early Intervention: The case of MSDs in the EU.
    • Kroemer, K.H. E. & Grandjean, E. (1997) Fitting the Task to the Human: A Textbook of Occupational Ergonomics. London; Bristol, PA: Taylor & Francis.
    • Kuorinka & Forcier (Eds.). (1995). Work Related Musculoskeletal Disorders (WMSDs): A Reference Book for Prevention. London: Taylor and Francis.
    • OSHA. (2007). MSDs. [Online].
    • Palmer, K. T., Walsh, K., et al. (2000). Back pain in Britain: comparison of two prevalence surveys at an interval of 10 years. BMJ, 330: 1577 – 1578.
    • Tortora, G. J. & Grabowski, S. R. (2004). Introduction to the Human Body: The Essentials of Anatomy and Physiology, 6th edition. Hoboken, NJ:J. Wiley & Sons.

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