Muscles of the shoulders and arms are not constantly active but are used intermittently, usually for short periods, to produce large amounts of tension, as in lifting and throwing. These muscles have a higher proportion of Type I and Type IIB fibres. Most skeletal muscles within the body however are a mixture of all three types of fibre, with the proportion varying according to the action of the muscle. For example, the postural muscles of the neck, back and legs have a higher proportion of Type I fibres, which means they are aerobic in nature and have a high resistance to fatigue. This allows them to function at a constant activity level; they are often referred to as tonic muscles.