Two types of protein in your muscles, actin and myosin, contribute to the basic mechanisms of isotonic contraction. During this type of contraction, thick strands of myosin and thin strands of actin slide over each other, producing shortening in each individual muscle cell and the muscle as a whole. Most of the exercises and daily activities you perform rely on isotonic changes in your muscle tissue. Depending on the amount of force working against your body, these changes appear in one of two basic forms: concentric or eccentric contractions