Exercise-induced muscle damage (EIMD) is characterized by loss of strength, increase in muscle stiffness, swelling, and soreness. The aim of this study was to assess the effects of flexibility training of the hamstring muscle group on symptoms of EIMD. Fourteen males (mean +/- standard deviation: age 20.6 +/- 0.8 years; mass 77.3 +/- 10.4 kg; height 1.77 +/- 0.05 m) were randomly assigned to a 5-week flexibility (proprioceptive neuromuscular facilitation (PNF)) training or control group. Pre- and post-measures of peak static muscle force at knee joint angles representing decreasing muscle lengths of the hamstrings (25 deg, 30 deg, 40 deg, 80 deg) and flexibility were taken. Flexibility increased significantly following PNF flexibility training (pre-test 19.4 +/- 6.2 cm, post-test 26.6 +/- 6.9 cm), whereas it stayed constant in the control group (pre-test 16.0 +/- 9.3 cm, posttest 16.7 +/- 7.6 cm). Both groups underwent a damaging exercise protocol incorporating six sets of 10 isokinetic eccentric contractions of the hamstrings. Measurements of isometric strength, flexibility, and perceived soreness were recorded prior to and 1, 24, 48 and 72 hours after damage. There was a tendency for the PNF group to recover from strength loss earlier at longer muscle lengths (25 deg, p = 0.06; 30 deg, p = 0.05), but not at shorter muscle lengths. There was no evidence of a protective effect of PNF training on flexibility and soreness. In conclusion, an increase in flexibility of the hamstring muscle group led to some protection from strength loss at long muscle lengths following EIMD.

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