The more relaxed the muscles are, the more energy can flow through the body. Using muscular tensions to try to 'do' the punch or attempting to use brute force to knock someone over will only work to opposite effect.
Bruce LeeThe athlete who is building muscles though weight training should be very sure to work adequately on speed and flexibility at the same time. In combat, without the prior attributes, a strong man will be like the bull with its colossal strength futilely pursuing the matador or like a low-geared truck chasing a rabbit.
Bruce Lee