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.
Purpose: 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.
Methods: 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.
Results: 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.
Conclusions: 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.
Flexibility is most commonly defined as maximal joint range of motion across a joint or series of joints (1,8,25). Alternatively, it has been suggested that passive muscle flexibility can be defined as the length-tension relationship of the muscle when it is passively stretched (10,11). However, it should be noted that studies on human skeletal muscle flexibility have largely been limited to measurements of goniometric joint range of motion. Consequently, there is scarce information on the effect of stretching on the length-tension relationship in human skeletal muscle.
A common stretch procedure is the static stretch, which is performed by stretching the target muscle to a given length (dynamic phase) where it is held for some time (static phase) before returning to the starting position (9,32). In the dynamic phase, the slope of the of the length-tension curve represents the stiffness, and the area under the curve is a measure of the energy absorption of the muscle-tendon unit (22,23). In the static phase, the nonlinear decline in passive tension with time represents viscoelastic stress relaxation (6). The passive behavior of the muscle in both the dynamic and static phase implies viscoelastic behavior, which is a combination of viscous rate dependent properties, and elastic load dependent properties (6,33). Previous studies of passive properties in passive skeletal muscle have mainly been limited to animal studies (29,31,32). However, recently it was demonstrated that static stretching of human skeletal muscle, in vivo, will yield a viscoelastic stress relaxation (22,24). Further, it has been shown that five repeated static stretches of human skeletal muscle, each lasting 90 s and separated by 30 s, altered the passive properties on a short term basis, as evidenced by a reduced resistance with each successive stretch (22). It should be noted that for most athletes a total of 7.5 min (5·90 s) of stretching for a single muscle group is a very rigorous, time consuming, and hence an unrealistic stretching program. A more common regimen is one to four consecutive stretches lasting 10-60 s (1,2,30,32). It has been demonstrated that in a single static stretch the resistance to stretch abates for up to 45 s (21). Therefore, the purpose of the present investigation was to examine whether three repeated 45-s static stretches had a measurable effect on the passive properties of the muscle-tendon unit.