Wilmore, J., & Costill, D. (1988). Physiological adaptations to physical training. In Training for sport and activity, Chapter 11. Dubuque, IA: Wm. C. Brown

"Recent studies have made it clear that a few days of rest or a reduction in training will not impair, but may even enhance performance. . . . However, at some point a reduction in training or complete inactivity will produce a deterioration in performance." (p. 200)

The point is made that it is important to distinguish research that involves bed rest, and research that involves a reduction in specific physical activity (e.g., when an athlete is injured, during an off-season).

Loss of Muscle Strength and Power

Skeletal muscles undergo a considerable decrease in size with inactivity, and are accompanied by a loss in strength and power. Reduced activity over a long period of time can cause small diminutions to accumulate so that eventually they will become substantial.

Reductions are relatively small during the first few months following cessation of training. Some researchers have shown:

(a) no loss of strength was noted after cessation of a three-week training program; and
(b) only 45% of the original strength gained from a 12-week training program was lost after one year's removal from the program.

Similar results have been found with muscular endurance. Swimmers were shown to retain shoulder strength after four weeks of the termination of training.

There is the possibility that losses in strength, power, and muscular endurance depend upon the activity used for training and testing. For example, swimmers have been shown to have no change in strength or power on a land-based power bench, but did decrease in power by 8 to 13.5% during four weeks of reduced activity. This suggests that functional strength may be lost relatively quickly while general strength is retained.

Nonspecific strength/power measures may not reflect the loss of specific functional use in performance. It suggests that swimmers by detraining may lose the ability to apply force during swimming, that is, they lose the "feel" for the water.

For swimmers, it has been shown that strength and muscular endurance gained during a training period may be fully retained for periods of up to six weeks, and approximately 50 percent will be retained for up to a year following the cessation of training. (p. 202)

There is far less effort required to regain strength, power, and muscular endurance than that required to first develop it.

Any loss in strength or power may be caused by a loss of ability to activate some muscle fibers. This theory is supported by the fact that a sizable strength gain is achieved with only a few training sessions, a period too brief to accommodate any significant structural development. These losses may be interpreted better by considering them as neuromuscular forgetting and reminiscence.

Only a minimal stimulus is required to retain the strength, power, endurance, and size of a muscle or muscle group.

Land-based strength can be maintained by one full workout every 10 to 14 days. However, water-based strength would need to be stimulated more frequently because the specificity of the water-action would not be remotely stimulated by any non-water activity.

For injured athletes, any stimulation will be beneficial to the affected areas. Isometric contractions have been found to be very effective for retaining strength and muscle tone.

Loss of Muscular Endurance

When swimmers stop training there is no change in their muscle glycolytic enzymes (phosphoralase and PFK) for at least four weeks. On the other hand, the oxidative energy system declines much more rapidly. This explains why sprint times are virtually unaffected by brief lay-offs (up to a month) but endurance performances decline significantly within a period as short as two weeks.

Muscle fiber composition does not appear to change during short periods of inactivity. The major change that does occur is an alteration in glycogen content. Endurance-trained muscles store substantially more glycogen than untrained muscles.

". . . the muscle's oxidative and anaerobic energy systems change rather slowly, and are probably unaffected by only a few days of rest. Only during periods of complete inactivity (immobilization) do the changes impair performance within the first week or two." (p. 204)

Loss of Speed, Agility, and Flexibility

The loss of speed and agility with physical inactivity is relatively small. Peak levels can be maintained with limited amounts of training. (p. 204)

Flexibility is very transitory, and must be trained for the whole year. Reduced flexibility may leave athletes more susceptible to injury.

Loss of Cardiovascular Endurance

The cardiovascular system detrains rapidly with inactivity. The decline is due largely to a reduction in blood volume which diminishes the stroke volume of the heart. That decline reduces the VO2max.

Highly trained individuals will not be able to afford long periods of inactivity with little or no endurance training. An abstinence from training following a full season will produce much fitness regression and will require much training to recover the previous season's level of fitness.

The reduction in aerobic adaptation is considerably greater than for the other performance capacities.

Training stimulus intensity plays the major role in maintaining aerobic adaptation during periods of reduced activity. The level of beneficial volume can be reduced to as little as 60% of the original without diminution occurring. However, when intensity is reduced, VO2max declines rapidly. Efforts requiring 90-100% of VO2max are needed to maintain VO2max.

For land-based activities, three intense workouts per week will probably maintain endurance adaptation. However, in the water, the number would have to be increased because of the lack of stimulation of the capacity that occurs during normal land-based activity.

The maintenance of endurance capacity is an important objective for periods of reduced training. It is lost quickly, but takes considerably more time to regain and so is a capacity that should be maintained at a high level all year.

Changes in Body Composition

Lean body weight decreases and total body fat increases with inactivity. Substantial variations in skinfold thickness occurs with these changes. Optimal body composition levels can be maintained during periods of reduced training by engaging in a moderate level of activity and controlling the diet. Athletes must watch their weight carefully when the demands of training are reduced.

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