Until recently such effects were attributed only to motor learning occurring in the central nervous system. Long-term effects of previous training on the muscle fibers themselves, however, have recently also been observed related to strength training.
Until recently it was generally assumed that the effects of exercise on muscle was reversible, and that after a long period of de-training the muscle fibers returned to their previous state. For strength training this view was recently challenged by using in vivo imaging techniques revealing specific long lasting structural changes in muscle fibers after a strength-training episode. Thats also probably the reason I leveled so viciously fast in the last four weeks since I began once again. It definitely gave me a tremendous advantage. Hell yeah :D