Lack of Neuromuscular Origins of Adaptation After a Long-Term Stretching Program

in Journal of Sport Rehabilitation
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Context:

Static stretching is commonly used during the treatment and rehabilitation of orthopedic injuries to increase joint range of motion (ROM) and muscle flexibility. Understanding the physiological adaptations that occur in the neuromuscular system as a result of long-term stretching may provide insight into the mechanisms responsible for changes in flexibility.

Objective:

To examine possible neurological origins and adaptations in the Ia-reflex pathway that allow for increases in flexibility in ankle ROM, by evaluating the reduction in the synaptic transmission of Ia afferents to the motoneuron pool.

Design:

Repeated-measures, case-controlled study.

Setting:

Sports medicine research laboratory.

Participants:

40 healthy volunteers with no history of cognitive impairment, neurological impairment, or lower extremity surgery or injury within the previous 12 mo.

Intervention:

Presynaptic and postsynaptic mechanisms were evaluated with a chronic stretching protocol. Twenty subjects stretched 5 times a wk for 6 wk. All subjects were measured at baseline, 3 wk, and 6 wk.

Main Outcome Measures:

Ankle-dorsiflexion ROM, Hmax:Mmax, presynaptic inhibition, and disynaptic reciprocal inhibition.

Results:

Only ROM had a significant interaction between group and time, whereas the other dependent variables did not show significant differences. The experimental group had significantly improved ROM from baseline to 3 wk (mean 6.2 ± 0.9, P < .001), 3 wk to 6 wk (mean 5.0 ± 0.8, P < .001), and baseline to 6 wk (mean 11.2 ±0.9, P < .001).

Conclusions:

Ankle dorsiflexion increased by 42.25% after 6 wk of static stretching, but no significant neurological changes resulted at any point of the study, contrasting current literature. Significant neuromuscular origins of adaptation do not exist in the Ia-reflex-pathway components after a long-term stretching program as currently understood. Thus, any increases in flexibility are the result of other factors, potentially mechanical changes or stretch tolerance.

Hayes and Hicks-Little are with the Dept of Exercise and Sport Science, University of Utah, Salt Lake City, UT. Harter is with the Dept of Health and Human Performance, Texas State University, San Marcos, TX. Widrick is with the Muscle Cell Physiology Laboratory, Harvard Medical School, Boston, MA. Williams is with the Dept of Health and Physical Education, Northern State University, Aberdeen, SD. Hoffman is with the Dept of Exercise and Sport Science, Oregon State University, Corvallis, OR.