Hamstrings-to-quadriceps isokinetic peak torque ratios have been used to assess injury risk in athletes with muscle strength imbalances. Since stretching may transiently decrease isokinetic peak torque, it is possible that hamstring stretching may adversely affect the hamstrings-to-quadriceps (H:Q) ratio. Accordingly, recent evidence has shown acute static stretching may decrease H:Q ratios. However, the effects of static stretching on the functional H:Q ratio, which uses eccentric hamstring muscle actions, have not been investigated. PURPOSE: To examine the acute effects of traditional hamstrings static stretching on leg extensor and flexor concentric peak torque, leg flexor eccentric peak torque, and the conventional and functional H:Q ratios during isokinetic muscle actions. METHODS: Twenty-two women (mean ± SD age = 20.6 ± 1.9 yrs; body mass = 64.6 ± 9.1 kg; height = 164.5 ± 6.4 cm) performed three maximal voluntary unilateral isokinetic leg extension, flexion, and eccentric hamstring muscle actions at the angular velocity of 60°·s−1 before and after a bout of hamstring stretching and a control condition. The stretching protocol consisted of one unassisted and three assisted static stretching exercises designed to stretch the posterior muscles of the thigh. Four repetitions of each stretch were held for 30 s with 15-s rest periods between repetitions. The control condition consisted of quiet sitting between pre- and post-tests. Conventional H:Q ratios were calculated by dividing the highest leg flexion concentric peak torque by the highest leg extension concentric peak torque. Functional H:Q ratios were calculated by dividing the highest eccentric hamstrings peak torque by the highest leg extension concentric peak torque. Five separate two-way repeated measures ANOVAs (time × condition) were used to analyze the leg extension, flexion, and eccentric peak torque as well as the conventional and functional H:Q ratio data. When appropriate, post-hoc t-tests were conducted for follow up analyses. RESULTS: Leg flexion peak torque decreased under both control (mean ± SE = 75.0 ± 3.9 to 71.5 ± 3.6) and stretching (76.1 ± 4.1 to 69.1 ± 3.7) conditions while eccentric hamstrings peak torque decreased only following the stretching (86.7 ± 4.1 to 70.8 ± 3.7) intervention (p ≤ 0.05). The results indicated no changes in leg extension peak torque (p > 0.05). Conventional H:Q ratios decreased under both control (0.58 ± .02 to 0.57 ± 0.02) and stretching (0.56 ± 0.02 to 0.52 ± 0.02) conditions, whereas functional H:Q ratios decreased only after stretching (0.64 ± 0.02 to 0.54 ± 0.02) (p ≤ 0.05). CONCLUSIONS: The results indicated that hamstrings static stretching caused hamstring concentric peak torque and conventional H:Q ratio decreases to a greater magnitude than a control condition (9.2% and 7.1% vs. 4.7% and 1.7%, respectively). In addition, hamstrings static stretching decreased eccentric hamstrings peak torque and the functional H:Q ratio. Practical Applications: These findings suggested that stretching of the hamstrings may adversely affect the conventional H:Q ratio as well as the functional H:Q ratio. Thus, strength and conditioning coaches, athletic trainers, and other allied health professionals may want to avoid using hamstring static stretching as a mean of injury risk prevention immediately prior to athletic activities.
© 2011 National Strength and Conditioning Association