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.

Detraining can resulting 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 Padillam, S, Muscular characteristics of detraining in humans. Medicine & Science in Sports & Exercise. 33(8):1297-1303, August 2001.

Loss of over 24 lb loss 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 fibres (slow- twitch) versus glycolytic (fast-twitch) fibres in elite power lifters and bodybuilders.

J Appl Physiol 1994; 77:1532-1536