When the different functional tissues of the locomotive system work together, the body generates movement. The study of human movement is known as kinesiology. There is some useful standard terminology that is used in medical science to describe different types of movement, in terms of directions and orientation. Most human movements consist of bending or twisting motions that change the joint angles between different body segments. Some movements are coupled, in the sense that pairs of muscles work against each other to “do and undo” each other’s respective movements (for example, bending and straightening the arm is, simply put, the work of two antagonistic muscles; the biceps and the triceps). The following terms are a helpful framework to describe motions.
When muscles use chemical energy and nerve impulses to pull together muscle fibre components so that the muscle becomes shorter and thicker, generating force exertion, movement and heat.
When the contraction of muscle fibres is released, so that the muscle fibre components disengage from each other and the muscle becomes longer, more elastic and stops exerting force.
Another useful distinction between movements is whether they are static or dynamic. Static movement (or loading) usually means that the muscles’ motor units are engaged for a long, sustained period of time (or in frequent repetition) without rest and recovery, until the point of fatigue (which occurs after a long time). Staticmovements are especially hazardous when they occur at low intensity, because then it is easy to ignore them or write them off as “not such a big load”. Static work can involve keeping body parts still, small movements of part of t he body for a long time while carrying out a task, or upholding an external load. Examples include working with arms above shoulder height, using a computer mouse and carrying heavy grocery bags.
Dynamic movement, on the other hand, is characterized by large, swiftly changing movements that may often involve great speed and/or large force exertions. While this type of movement may be a bigger risk for sudden trauma to the locomotive tissues (such as torn muscles or ligaments), this type of loading is also characterized by much more loading variation, leading to relatively frequent rest and recovery while different muscles take turns being loaded. In comparison, static loading can gradually wear down locomotive abilities due to the constant loading of the same muscles, pressure on the same body structures, etc.
A good rule of thumb from a health perspective is that both these movement types can help the body become stronger and more prepared for high loading, provided that there is sufficient rest and recovery for the body to replenish the needed oxygen and energy via the blood, and to transport waste products from the locomotive tissues. This is what distinguishes risky workloads from intentional physical training: although both can push the body to its loading limits, a work environment may sometimes allow little or no time for recovery, while physical training is designed to alternate between loading the body and letting it recover.
Simply put: Dynamic loading with breaks and variation is mostly good and strengthening, while static loading with few breaks risks being harmful and weakening.