Some of the loss of muscle performance (decreased force production) due to aging does not appear to be only due to the conversion of muscle fibers from one type to another, but largely due to a selective atrophy of certain populations of muscle fiber types. With aging, there is a progressive loss of muscle mass and maximal oxygen uptake, leading to a reduction in muscle performance and presumably some of the loss of function (decreased ability to perform activities of daily living) seen in elderly people. Age-related loss of muscle mass results primarily from a decrease in the total number of both type I and type II fibers and, secondarily, from a preferential atrophy of type II fibers. Atrophy of type II fibers leads to a larger proportion of slow type muscle mass in aged muscle, as evidenced by slower contraction and relaxation times in older muscle.
Fortunately, physical therapy interventions can affect muscle fiber types leading to improvements in muscle performance. Physical therapy interventions can be broadly divided into those designed to increase the patient’s resistance to fatigue and those designed to increase the patient’s force production.
Evidence is lacking to demonstrate that type II fibers convert to type I with endurance training, although there does appear to be an increase in the mixed type I and IIA fiber populations. Researchers have found that type I fibers become faster with endurance exercise and slower with deconditioning.
High-intensity resistance training (high-load–low-repetition training) results in changes in fiber type similar to those seen with endurance training, although muscle hypertrophy also plays an essential role in producing strength gains. Initial increases in force production with high-intensity resistance training programs are largely mediated by neural factors, rather than visible hypertrophy of muscle fibers, in adults with no pathology or impairments. Even so, changes in muscle proteins, do begin after a few workouts, but visible hypertrophy of muscle fibers is not evident until training is conducted over a longer period of time (>8 weeks).
Although the trends in fiber type conversions are similar for endurance training and resistance training, differences in physiological changes that occur with each type of exercise are also important. Endurance training increases the oxidative capacity of muscle, whereas training to increase force production of sufficient intensity and duration promotes hypertrophy of muscle fibers by increasing the volume of contractile proteins in the fibers.