**4.2. Relationships between muscular force and EMG**

Muscular force is the amount of force a muscle can produce with a single maximum effort. Enhanced muscular force can lead to improvements in the areas of performance, injury prevention, body composition, self-image, lifetime muscle and bone health, and chronic disease prevention [16]. There are many cases in which knowledge of the relationship between EMG and force is desired. If the relationship between force and EMG amplitude is simply linear, a direct regression equation yields a relatively simple technique to control prosthetic limb function [19].

Ergonomists could assess the load on various muscles by monitoring the EMG activity. The relationship between EMG and force also seems to depend on the nature of the muscle studied, since some investigators have reported a linear relationship for the adductor pollicis and first dorsal interosseous and soleus, and a nonlinear relationship for the biceps and deltoid [68]. There have been other, numerous examples of observations of nonlinear relationships between force and EMG amplitude [69,70].

It is clear that when considering the possible shape of the EMG force relationship, one needs to consider various features of the movement, such as the type of muscle contraction; the size and location of the active muscles; their role as agonists, synergists, or antagonists; air temperature [71,19] and the numerous other physiological and technical factors that affect the electromyogram [19].
