BACKGROUND: "Flexibility" is defined as "the range of movement or motion of a single or multiple joints." Its limits decline significantly with age, reaching maximum flexibility in the mid-to-late twenties for males and females, respectively. Conclusions regarding appropriate stretching time duration are mainly based on mechanical factors such as range of motion (ROM) and flexibility and tend to ignore the adverse neural mechanical tension that may be created during stretching exercises. It appears that longer-duration stretching increases flexibility for geriatric populations. AIM: To explore the effect of variable stretching intervals on neural function and ROM. DESIGN: Double blind randomized controlled trial. SETTING: University research laboratory. POPULATION: One hundred participants, 60 to 65 years old, with a diagnosis of tight hamstring muscles, bilaterally, were randomly assigned to either a control group or one of three intervention groups. METHODS: Participants who were randomly placed in one of the three intervention groups, were further randomized by selection of right or left limb for intervention. Intervention groups consisted of either 15-, 30-, or 60-second stretches to hamstring muscles; whereas the control group was given a sham stretch for 20 seconds. Main Outcome Measures included the neurophysiological outcome measures; peak to peak amplitude of somatosensory evoked potential for dermatomes L4, L5, and S1. Secondary outcome measures included knee ROM. All outcome measures were assessed before, immediately after, and 24 hours after the treatment session. Mixed linear model analysis was used to evaluate group, time, and group x time interaction effects for outcome measures. RESULTS: Stretching for 30 and 60-seconds gave significant increase in ROM compared to control (4.64 (95% CI 3.35 to 5.93); p < 0.01) (10.30 (95% CI 9.01 to 11.6); p < 0.01) and the improvement was persistent at 24-hours' follow-up (p < 0.01). However, the analysis showed significant reduction in dermatomal somatosensory evoked potentials' amplitudes for L4 (-1.19 (95% CI -1.35 to 1.02); p < 0.01), L5 (-1.34 (95% CI -1.56 to -1.13); p < 0.01), S1 (-0.99 (95% CI -1.16 to -0.83); p < 0.01) after 60-seconds static hamstring muscle stretch. The reduction was persistent at 24-hours' follow-up (p < 0.01). CONCLUSIONS: Stretching hamstring for 30-seconds was optimal in increasing the knee range of motion and minimizing the negative effects on the neural function of the involved nerve roots. Thus, 60-seconds static hamstring muscle stretching, places increased stress and strain on the nervous system and should be avoided.

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