The effects of electric and magnetic fields on single intact and chemically skinnedmuscle fibres of frog are comparativelystudied. The results show that alternative pulse electric field obviously decreases the depolarization ratio of the diffracted light from intact fibers but it does not change that from skinned fibres. Static magnetic field does not affect the depolarization ratio from either intact or skinned fibers. It is suggested that changes of spatial disposition but not conformation of the contractile proteins take place during muscle contraction caused by electric field and that during the transition from weakly-binding state to strongly-binding state the crossbridges (particularly subsegment-2) move towards actins，thus the average tilt of crossbridges relative to the backbone of thick filaments is increased. It is also shown that static magnetic field with field strength could not exert apparent influence on both the electric properties of cell membrane and the contractile protein movement within skeletal muscle fiber.