In an effort to design machines that feel smooth, many companies have reduced the amount of mechanical friction within their equipment. The ramp analogy that was used earlier to explain the effects of internal muscular friction also applies to mechanical friction. For example, when you contract your quadriceps concentrically, you receive extra resistance from your hamstrings. At one time or another, most of us have experienced some degree of muscular soreness. The two types of muscle soreness are acute and delayed-onset.
Acute soreness occurs during and immediately following exercise.
It has been found that eccentric muscle contractions produce greater DOMS than either concentric or static contractions. The exact cause of DOMS is unknown. Perhaps muscular friction is at least partly responsible for muscular soreness. Any friction that involves movement of biological tissue across another surface will cause irritation. Too much irritation will injure the tissue.