STUDY DESIGN: Repeated-measures experimental design. OBJECTIVE: To examine the acute effects of different durations of passive stretching on the time course of musculotendinous stiffness (MTS) responses in the plantar flexor muscles. BACKGROUND: Stretching is often implemented prior to exercise or athletic competition, with the intent to reduce the risk of injury via decreases in MTS. METHODS AND MEASURES: Twelve subjects (mean ± SD age, 24 ± 3 years; stature, 169 ± 12 cm; mass, 71 ± 17 kg) participated in 4 randomly-ordered experimental trials: control with no stretching, 2 min (2_min), 4 min (4_min), and 8 min (8_min) of passive stretching. The passive-stretching trials involved progressive repetitions of 30-second passive stretches, while the control trial involved 15 minutes of resting. MTS assessments were conducted before (prestretching), immediately after (poststretching), and at 10, 20, and 30 minutes poststretching on a Biodex System 3 isokinetic dynamometer. RESULTS: MTS decreased (P<.05) immediately after all stretching conditions (2_min, 4_min, and 8_min). However, MTS for the 2_min condition returned to baseline within 10 minutes, whereas MTS after the 4_min and 8_min passive-stretching conditions returned to baseline within 20 minutes. CONCLUSIONS: Practical durations of passive stretching resulted in significant decreases in MTS; however, these changes return to baseline levels within 10 to 20 minutes. LEVEL OF EVIDENCE: Level 5.

J Orthop Sports Phys Ther. 2008;38(10):632-639, published online 11 July 2008. doi:10.2519/jospt.2008.2843  

KEY WORDS: compliance, passive, stiffness, strain injury, stretch