The Influence Of Flexibility On The Stretching-Induced Force Deficit And Maximal Joint Range Of Motion.

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.” It has also been suggested that the stretching-induced force deficit may be joint angle-specific and influenced by initial level of flexibility. To examine the effects of initial flexibility on the stretching-induced force deficit in the plantar flexor muscles at multiple joint angles and ankle joint range of motion (ROM). Fourteen men with limited dorsiflexion ROM (mean age ± SD = 21 ± 2 yrs; stature = 173 ± 9 cm; mass = 75 ± 12 kg; dorsiflexion ROM = 11 ± 6°) and 14 men with a normal ROM (22 ± 2 yrs; 177 ± 8 cm; 75 ± 14 kg; 28 ± 6°) (Roaas et al. 1982, Acta Othropedic 53(2): 205 - 208) performed stretch tolerance assessments and isometric maximal voluntary contractions (MVCs) at -19°, -9°, 1°, and 12° of dorsiflexion (0° = neutral ankle joint angle) designed to examine the ROM and strength, respectively. Testing was performed on a custom-built load cell apparatus attached to a calibrated isokinetic dynamometer before and after a bout of passive stretching. To assess stretch tolerance, the dynamometer lever arm passively dorsiflexed the foot at 5°·s^-1 from -20° of dorsiflexion to the maximum tolerable ROM (as acknowledged by the subjects). 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 position signal (°) from the dynamometer and force signal (N) from the load cell were sampled at 1 KHz during the flexibility and strength assessments, respectively. The passive stretching protocol consisted of nine 135-s constant-torque passive stretches, which was held by the dynamometer at each subject's maximum tolerable passive stretching force (the point of discomfort). Five to 10 s was allowed between each stretching repetition. A three way mixed factorial ANOVA [time × angle × group; 2 × 4 × 2] was used to analyze the MVC data, while a two way mixed factorial ANOVA [time × group; 2 × 2] was used to analyze the ROM data. There was a 9% decrease in plantar flexor MVC strength from pre- to post-stretching across all joint angles for both groups (p < 0.001). Conversely, there was a 4% increase in dorsiflexion ROM from pre- to post-stretching for both groups (p < 0.001), however, the normal ROM group had higher dorsiflexion ROM values than the limited ROM group (p < 0.001) at all time points. The magnitude of change for MVC strength and dorsiflexion ROM from pre- to post-stretching was not affected (p > 0.05) by the joint angle or limitations of ROM. These findings suggested that the initial level of flexibility does not influence the force deficit or increases in dorsiflexion ROM following an acute bout of passive stretching in the plantar flexor muscles. These results also indicated that the stretching-induced force deficit occurs at all the ankle joint angles tested. These findings may be useful for strength and conditioning professionals or other allied health practitioners who may incorporate stretching prior to testing or performance. Regardless of initial flexibility, 20 min of passive stretching appears to increase dorsiflexion ROM, but decrease the maximal strength capabilities of the plantar flexors at all of the joint angles tested in the present study. [ABSTRACT FROM AUTHOR]