Abstract: Based on biomechanics and sports training theory,the recruitment of trunk and lower limb muscles and its relationship were investigated while squatting under stable and multi-level unstable conditions with bodyweight and 30% Repetition Maximum(RM),in order to provide scientific basis for core strength training and functional strength training.Fourteen soccer professional male players volunteered to participate in the study,EMG of eight muscles of trunk and lower limb were tested during squatting.The results indicated that,all the RMS value of the muscles did not change significantly while squatting with body weight under stable and unstable conditions(P>0.05),and all the RMS value of the muscles did not change significantly while squatting with body weight or 30% RM under the conditions of different unstable levels(A,B and C)(P>0.05).But the RMS value of most muscles(seven of eight muscles)significantly increased when the external weight increased from body weight to 30% RM(P<0.05).It indicated that the loading factor for muscle contraction was significantly greater than the influence of unstable factors.Under unstable conditions,30% RM could significantly increase the RMS value of most muscles(P<0.05),however,according to previous studies,muscle?maximal strength showed substantial decline while squatting under unstable conditions.Therefore, " unstable" condition couldn't be simply regarded as unidirectional increase or reduction of muscle strength training methods.With the increase of unstable extent,the RMS value of gluteus maximus near the hip were dramatically increased,while the RMS value of tibialis anterior near the ankle were decline,to some extent,the result showed that the " ankle strategy" and " hip strategy" to maintain the balance of the human body.