EFFECTS OF STRETCHING ON LENGTH, RESTING TENSION, AND FLEXIBILITY OF THE HUMAN HAMSTRINGS

INTRODUCTION: The effects attributed to stretching exercises are: lowering muscular resting tension, increase of range of motion (ROM), prevention of muscle tightness, precaution against developing short muscles and increase of muscular performance. Experimental verification of those effects, however, has so far only been obtained successfully for enlargement of joint’s ROM. The rest of the mentioned effects are merely deduced from the observation, that flexibility is increased after stretching exercises. Therefore, the purpose of this study was to investigate the effects of short-term and long-term stretching training on ROM, resting tension (RT) and muscle length (ML) of the human hamstrings. METHODS: An experimental station was constructed to determine ROM, end ROM torque (ERT), RL, and ML of the human hamstrings (in detail described in Wiemann et al. 1997). Hip ROM was determined by the passive straight leg raising test. RT i.e. passive muscle stretching tension was defined as the mechanically measured resistance of the passive muscle against the stretching test procedure. ERT was determined by the torque that had to be generated to reach ROM. ML of the hamstrings was determined as the origio-insertio distance, in which the maximum of isometric voluntary contraction force (MVC) was generated. The following experimental groups were investigated: Short-term stretching group (SG: 14 male students), control group (CG: 15 male students), long-term stretching groups (FLG: 12 female students, and MLG: 11 male students). Short-term stretch training consisted of a 15 min lasting static stretching program. The control group remained resting between pretest and posttest for 15 min. Long-term stretch training included 3 short-term stretching programs per week for 10 weeks. RESULTS: After the training periods, ROM as well as ERT were significantly increased in SG, FLG and MLG. In CG, ROM as well as ERT had not changed. RT measured in a hip flexed position of 80° remained unchanged in SG, CG, and MLG, and increased significantly in FLG. ML did not change in FLG and MLG. DISCUSSION AND CONCLUSION: The increase of ROM effected by short-term and long-term stretching training must be related to the enhanced ERT that the subjects were willing to tolerate during the posttest. The constancy of RT may be explained by the recent findings that the source of the resting tension can be seen in the high elastic stiffness of the titin filaments within the sarcomeres. The increase of the RT in FLG indicates a hypertrophy of the hamstrings i.e. an increase of the thin and thick filaments and - connected with this - an increase of titin filaments. This hypertrophy may be caused by the mechanical stress placed on the sarcomeres during the stretching exercises. This suggestion can be supported by the significant increase of the hamstrings’ MVC in FLG. The constancy of ML may be attributed to the functional constraints of the subjects’ daily movement behaviour which demands presumably a constant optimal length of the hamstrings. REFERENCES: Wiemann, K. et al. (1997). Int. J. Sports Med. 18, 340-346