Increased muscular activity, which results in an increase in metabolic rate, heart rate, and oxygen uptake. Exercise also causes an increase in anaerobic respiration in order to compensate for the oxygen debt, which results in a build-up of lactic acid in the tissues. There are two main types of exercise. Strength exercise (or anaerobic exercise), such as weightlifting, repeatedly contracts certain muscles to induce the formation of new actin and myosin filaments, and thus increase the maximum force they can exert. Endurance exercise (or aerobic exercise) sustains a given load over a certain time. This increases the oxidative capacity of the muscle by boosting the numbers of mitochondria, developing the network of capillaries that deliver blood to muscle tissue, and increasing the concentration of the oxygen carrier myoglobin. Consequently, the workload can be maintained for successively longer periods.