Hey there Grace,
Technically, as stated above: 24 hours for small muscle groups and 48 hours for large BUT: This also depends on what is going on: is the person not moving and laying down, or still moving.
How long they have been training according to studies, is also very relevant to how quickly muscles “seem” to atrophy and how quickly they can return to their previous level of conditioning.
Sometimes a break is positive, needed and won’t hurt the athlete in the big picture as long as they are still moving.
The most recent information that I have seen shows that atrophy/strength loss (which may also be due to nuerologically response) occurs after 96 hours without training. If training is re-instated at this point, return to the previous trained state will be very quick (two to three training sessions). The longer the time before returning to training, the more deconditioning that will occur. And the longer it will take to return to the previous level of conditioning.
that depends a lot of what you are doing during the break. In a worst case scenario (bed rest), muscle atrophy starts almost immediately. Here is a quote from a study published by the NIH:
” In the absence of a robust countermeasure (nutritional, exercise, or pharmacologic), lean tissue loss is largely inevitable during prolonged bed rest. Inactivity-induced loss of muscle mass predominantly affects the lower body musculature and is most rapid during the initials days/weeks of inactivity [9-11]. In young, healthy adults subjected to bed rest, the loss of lower body lean mass appears to be on the order of 100-200 g┬Àwk-1. LeBlanc et al. reported a total body lean mass loss of 2.6 kg following 119 days of bed rest in healthy young males; of this, 2.4 kg was from the legs (0.14 kg┬Àwk-1) . In a similar cohort, Paddon-Jones et al. reported a loss of 0.4 kg of lean leg mass following 28 days of bed rest (0.10 kg┬Àwk-1) . More recently, Trappe et al. reported a 21% (quadriceps) and 29% (triceps surae) decrease in muscle volume after 60 days of bed rest in young females .”
Here is a link to the entire publication http://www.ncbi.nlm.nih.gov/pmc/articles/PMC3276215/.