Q: Is it okay to strength-train every day?
A: Muscles generally need to rest for at least 48 hours, during which time they fully replenish their energy stores and increase protein synthesis, thus growing in size and strength. This recovery period also helps reduce the risk of injury and excessive soreness.
Most people need to train just two or three times a week, in any case. If you want to work out on consecutive days, you can do upper-body training one day and lower-body the next.