If your most vigorous form of exercise is watching football on telly, then there's good news to you. Imagining you are exercising can help build muscle strength - you don't have to lift a finger to improve fitness.
Dave Smith, Paul Holmes and David Collins from Manchester Metropolitan University and Katherine Layland of United Norwest Cooperatives investigated the effect of physical practice, mental practice and no practice on the finger strength of 18 male students. Over four weeks the finger strength of the no-practice group did not change, but the mental-practice group increased their strength 16 per cent - half the improvement achieved by those who actually performed the finger contractions.
It is thought that mental practice strengthens memory traces involved in performing physical activity and that mental practice may therefore have a significant effect on sports performance.