Muscle atrophy and decreased strength with inactivity

When a normally active or highly trained person reduces his or her level of activity or ceases training altogether, major changes occur in both muscle structure and function. This is illustrated in the picture below by the results of two studies: studies in which entire limbs have been immobilized and studies in which highly trained people stop training.

Immobilization

When a trained muscle suddenly becomes inactive through immobilization, major changes are initiated within that muscle in a matter of hours. During the first 6h of immobilization, the rate of protein synthesis starts to decrease. This decrease likely initiates the start of muscular atrophy, which is the wasting away or decrease in the size of muscle tissue. Atrophy results from lack of muscle use and the consequent loss of muscle protein that accompanies the inactivity. Strength decreases are most dramatic during the first week of immobilization, averaging 3% to 4% per day. This is associated with the atrophy but also with decreased neuromuscular activity of the immobilized muscle.

Immobilization appears to affect both type I and type II fibers. From various studies, researchers have observed disintegrated myofibrils, streaming Z-disks(discontinuity of Z-disks and fusion of the myofibrils), and mitochondrial damage. When muscle atrophies, the cross-sectional fiber area decreases. Several studies have shown the effect to be greater in type I fibers, including a decrease in the percentage of type I fibers, thereby increasing the percentage of type II fibers.

Muscles can and often do recover from immobilization when activity is resumed. The recovery period is substantially longer than the period of immobilization but shorter than the original training period.

Cessation of training

Similarly, significant muscle alterations can occur when people stop training. In one study, women resistance trained for 20 weeks and then stopped training for 30 to 32 weeks. Finally they retrained for six weeks. The training program focused on the lower extremity, using a full squat, leg press, and leg extension. Strength increases were dramatic, as seen in the figure below. Compare the women’s strength after their initial training period(post-20) with their strength after detraining(pre-6). This represents the strength loss they experienced with cessation of training.

During the two training periods, increases in strength were accompanied by increases in the cross-sectional area of all fiber types and a decrease in the percentage of type IIx fibers. Detraining had relatively little effect on fiber cross-sectional area, although the type II fiber areas tended to decrease.

To prevent losses in the strength gained through resistance training, basic maintenance programs must be established once the desired goals for strength development have been achieved. Maintenance programs are designed to provide sufficient stress to the muscles to maintain existing levels of strength while allowing a reduction in intensity, duration, or frequency of training.

In one study, men and women resistance trained with knee extensions for either 10 or 18 weeks and then spent an additional 12 weeks with either no training or reduced training. Knee extension strength increased 21.4% following the training period. Subjects who then stopped training lost 68% of their strength gains during the weeks they didn’t train. But subjects who reduced their training(from three days per week to two, or from two to one) did not lose strength. Thus, it appears that strength can be maintained for at least up to 12 weeks with reduced training frequency.

Fiber type alterations

Can muscle fibers change from one type to another through resistance training? The earliest research included that neither speed(anaerobic) nor endurance(aerobic) training could after the basic fiber type, specifically from type I to type II, or from type II to type I. These early studies did show, however, that fibers begin to take on certain characteristics of the opposite fiber type if the training is of the opposite kind(e.g. type II fibers might become more oxidative with aerobic training).

Research with animals has shown that fiber type conversion is indeed possible under conditions of cross-innervation, in which a type II motor unit is artificially innervated by a type I motor neuron or vice versa. Also, chronic, low-frequency nerve stimulation transforms type II motor units into type I motor units within a matter of weeks. Muscle fiber types in rats have changed in response to 15 weeks of high-intensity treadmill training, resulting in an increase in type I and type IIa fibers from type IIx to type IIa and from type IIa to type I was confirmed by several different histochemical techniques.

Staron and coworkers found evidence of fiber type transformation in women as a result of heavy resistance training. Substantial increases in static strength and in the cross-sectional area of all fiber types were noted following a 20-week heavy resistance training program for the lower extremity. The mean percentage of type IIx fibers decreased significantly, but the mean percentage of type IIa fibers increased. The transition of type IIx fibers to type IIa fibers with resistance training has been consistently reported in a number of subsequent studies. More recent studies have shown that a combination of high-intensity resistance training training and short-interval speed work can lead to a conversion of type I and type IIa fibers.