Many previous studies have reported a temporary decrease in muscle strength following an acute bout of stretching, which has been termed the "stretching-induced force deficit." However, chronic stretching programs have reported mixed results demonstrating improvements or no changes in performance. PURPOSE: To examine the effects of chronic stretch training of the plantar flexors on muscle strength at multiple joint angles. METHODS: Thirty-one healthy men volunteered for this investigation and were randomly assigned to either a stretch training (STR) group (n = 17; mean age +/- SD = 21 +/- 2 yrs; stature = 175 +/- 8 cm; mass = 75 +/- 13 kg) or control (CON) group (n = 14; 21 +/- 2 yrs; 177 +/- 8 cm; 75 +/- 11 kg) for four weeks. At baseline (week 1) and post-testing (week 5) all subjects completed isometric maximal voluntary contractions (MVCs) at -19[degrees], -9[degrees], 1[degrees], and 12[degrees] of dorsiflexion (0[degrees] = neutral ankle joint angle). Testing was performed on a custom-built load cell apparatus attached to a calibrated isokinetic dynamometer. For the MVC assessments, subjects were instructed to provide maximal force of the plantar flexors for 4 s at each randomly-ordered joint angle. The force signal (N) from the load cell was sampled at 1 KHz during the strength assessments. The stretch training protocol included four 135-s constant torque passive stretches (9 min of time under stretch) performed 3 times per week which was held by the dynamometer at each subject's maximum tolerable passive stretching torque (the point of discomfort). Five to 10 s was allowed between each stretching repetition. A three way mixed factorial ANOVA [time x angle x group; 2 x 4 x 2] was used to analyze the MVC data. RESULTS: There was a 14% increase in plantar flexor MVC strength for the STR group (P<0.01) and no change (-1%) for the CON group (P = 0.87) from weeks 1 - 5 across all joint angles. In addition, isometric MVC force at 1[degrees] was greater than MVC force at -19[degrees], -9[degrees], and 12[degrees] (P<0.01) and -9[degrees] and 12[degrees] was greater than -19[degrees] (P<0.01) for all time points and groups. CONCLUSIONS: These findings suggested that chronic stretch training results in increases in muscle strength at all the joint angles tested in the current study. In addition, the stretch training protocol did not alter the joint angle at which peak isometric strength was produced. PRACTICAL APPLICATIONS: These findings may be useful for strength and conditioning professionals or other allied health practitioners who are interested in incorporating passive stretching routines into their training programs. It appears 9 min of passive stretching performed 3 times a week for 4 weeks results in significant (14%) gains in muscle strength.
(C) 2011 National Strength and Conditioning Association