Detraining is the partial or complete loss of training-induced adaptations, in response to an insufficient training stimulus. Detraining characteristics may be different depending on the duration of training cessation or insufficient training.
Mujika I, Padilla S. Detraining: loss of training-induced physiological and performance adaptations. Part I: short term insufficient training stimulus. Sports Med. 2000 Aug;30(2):79-87.
Squat strength of Olympic Weightlifters declined approximately 10% after 4 weeks of cessation of weight training.
Hakkinen K, Komi PV (1985). Changes in the electrical and mechanical behavior of leg extensor muscles during heavy resistance strength training. Scandinavian Journal of Sports Science 7: 55-64.
Detraining can result in a decrease in muscle mass.
American College of Sports Medicine. ACSM's Resource Manual for Guidelines for Exercise Testing and Prescription, 2005
Muscle fiber cross-sectional area declines rapidly in strength and sprint athletes (during detraining).
Mujika, I and Padilla, S, Muscular characteristics of detraining in humans. Medicine & Science in Sports & Exercise. 33(8):1297-1303, August 2001.
Loss of over 24 lbs of muscle after 7 months of detraining.
The effects of detraining on an elite powerlifter. Journal of Neurological Sciences, 51, 247-257.
Muscle mass returned to pretraining levels after 5 months of detraining.
Thorstensson, A. Observations on strength training and detraining. Acta Physiologica Scandinavica 100: 491-493, 1977.
…detraining appears to shift the contractile characteristics towards type IIb, although muscle atrophy is also likely to occur.
Ross A, Leveritt M. Long-term metabolic and skeletal muscle adaptations to short-sprint training: implications for sprint training and tapering. Sports Med. 2001;31(15):1063-82.
With detraining, however, muscle cross-sectional area decreases and there is an increase in the number of oxidative fibers (slow- twitch) versus glycolytic (fast-twitch) fibers in elite powerlifters and bodybuilders
J Appl Physiol 1994; 77:1532-1536