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MAGNUSSON, S. P., P. AAGAARD, and J. J. NIELSON. Passive energy return after repeated stretches of the hamstring muscle-tendon unit. Med. Sci. Sports Exerc., Vol. 32, No. 6, pp. 1160–1164, 2000. It has been shown that five repetitive static stretches of human hamstring muscle, each lasting 90 s and separated by 30 s, altered the passive properties on a short-term basis. However, a total of 7.5 min (5·90 s) of stretching for a single muscle group may be an unrealistic stretching program.The present investigation examined whether three repeated 45-s static stretches had a measurable effect on the passive properties of the hamstring muscle-tendon unit, in vivo.Resistance to stretch was defined as the passive moment (Nm) offered by the hamstring muscle group during passive knee extension using a KinCom dynamometer as previously described (Kinetic Communicator, Chattecx Corp., Chattanooga, TN). The static stretch exercise was administered to the left lower extremity of all subjects and consisted of a dynamic phase of passive knee extension to a predetermined final joint angle followed by a 45-s static phase. The procedure was repeated for a total of three 45-s static stretches with 30-s rest period between stretches.There was a significant decline in resistance over the 45-s the static phase in stretch 1 (20 ± 3%) and stretch 3 (18 ± 3%), P < 0.001. Further, the absolute or relative decline in resistance over time stretch 1 and 3 were equivalent. The mean resistance in stretch 1 and 3, expressed as the log(e) of time, yielded an equal and highly linear relationship (r2 = 0.96 ± 0.01); the slope and intercept did not differ. In the dynamic phase of the stretch, the energy of stretch 1 and 3 were similar.These data suggest that the static stretching protocol used in the present study had no short-term effect on the viscoelastic properties of human hamstring muscle group.