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Part 02

Muscle fatigue and workload

An MSD shows up when the load placed on the body repeatedly goes beyond its ability to recover.

What increases the load

  • Force, repeated movements, long hours
  • Awkward postures, vibrations
  • Cold, stress, high pace

What restores capacity

  • Enough good-quality sleep
  • Hydration, food, physical activity
  • Breaks, stress management

Sensing the effort: the rating of perceived exertion

The same force can feel like a very different effort from one person to the next. The rating of perceived exertion (also called the Borg scale) helps you put a number on what your body feels, from 0 (no effort) to 10 (maximum effort).

Place your effort on the scalePut a number on the effort (0 to 10) to spot fatigue before it sets in.

The consequences of muscle fatigue

When it's used over and over without recovery, the muscle wears out. Three consequences follow one after another:

Muscle fatigue leads to a higher risk of injury.
Drop in muscle strength
Less strength to do the same task: the body compensates by forcing somewhere else.
Change in the biomechanics of your movements
Less efficient movements and compensations that load the wrong structures.
Drop in joint stability
Less control over your joints: awkward movements and a higher risk of injury.
Acting early lets you preserve your abilities, lower the risk of injury and protect your health over the long run.

Muscle work: dynamic or static

The muscle needs blood to work. When the contraction is held, blood flow drops. Compare the two:

Static
Held contraction
Blockage and compression of blood flow: the vessel is squeezed by the contraction
Test tubes: the demand for blood is high but blood flow is reduced
  • The muscle stays contracted.
  • Blood flow is squeezed.
  • Fatigue comes faster.
  • The risk of pain goes up.
Normal contraction

Good blood supply: oxygen and nutrients reach the muscle.

Prolonged contraction

Reduced blood supply: less oxygen, fewer nutrients.

Even contractions as low as 2% of maximum force can reduce the blood supply.
What prolonged contraction causes (click for the details)

  • Starved of blood, the muscle runs short on oxygen and fuel. It contracts less well and tires out much faster, even for a moderate effort.

  • The constant tension and lack of blood flow wake up the muscle's pain sensors: it pulls, burns or throbs. It's the first warning sign.

  • With no circulation to flush them out, the waste from the effort (lactic acid, ions) builds up in the muscle. It keeps the fatigue and the tissue irritation going.

  • Small areas of the muscle stay knotted up and contracted. Tender to the touch, they can send pain elsewhere in the body (referred pain).

  • The muscle ends up staying clenched all the time, painful at the slightest touch. By this stage, rest alone isn't always enough to release it.

Switch up your tasks, change position often and take micro-breaks.
Vary your movements
Move around often
Take your time, don't rush
Stay hydrated and stretch

Ref.: Baillargeon et Patry, 2003; CCOHS; INRS.

Staying frozen, sitting or standing, tires you out too: change position every 20 to 30 minutes.

Prolonged sitting work

Even when you're sitting, your muscles work statically to hold you in position. Sitting for a long time tires the body and increases the pressure on the discs in your lower back.

Pressure changes

help bring water, nutrients and oxygen into the disc.

Movement = better diffusion
The disc stays well hydrated and healthy.

Prolonged static pressure

squeezes the disc and limits how much water, nutrients and oxygen get in.

Static pressure = less diffusion
Less hydration, fewer nutrients: the disc weakens.
Moving feeds your discs: varying the pressure draws in water, nutrients and oxygen, while prolonged static pressure starves them Enlarge

Discs feed on movement: varying the pressure draws in water, nutrients and oxygen; prolonged static pressure starves them.

Moving often helps protect the discs in your lower back.

Control measures

Simple moves to limit muscle fatigue and recover through the day.

Control measures: micro-breaks, task rotation, stretching, switching postures Enlarge
1

Micro-breaks

15 to 30 seconds, 3 to 4 times an hour, to relax the muscles.

2

Task rotation

Switch tasks to vary the muscles you use.

3

Stretching

Warm up and stretch before and during work.

4

Switch postures

Avoid staying too long in the same position.