The effects of stretching rate on plantar flexor neuromechanical properties and maximum ankle range of motion

INTRODUCTION: Maximal joint range of motion (max-ROM) and resistance to tissue elongation (flexibility) are important physical attributes influencing performances in athletic tasks and activities of daily living. Max-ROM tests are typically performed by rotating a joint on the dynamometer at slow angular velocities, which might be of little functional relevance for most daily and sports activities that are performed at faster angular velocities. However, the effects of stretching velocity on max-ROM and the potential neuromechanical factors influencing it have been overlooked. The aim of this study was to test the effects of stretching velocity on plantar flexor (PF) neuromechanical properties and max-ROM, and to assess relationships between them.

METHODS: Fifteen participants attended two familiarisation sessions followed by one experimental session. Testing included the performance of PF max-ROM tests on an isokinetic dynamometer at 5, 30 and 60 deg/s, interspersed by 90 s, whilst joint position, joint moment, and surface electromyograms (sEMG) were recorded synchronously. Max-ROM was determined at the end of the isovelocity phase to account for potential errors in max-ROM estimates between stretching velocities.

RESULTS: Stretches performed at 30 and 60 deg/s resulted in significantly greater max-ROM (23.5% and 19%), peak passive moment (i.e. stretch tolerance, 68% and 71.1%), elastic energy storage (area under the moment-angle curve, 25.6% and 15.7%), slope of the joint moment-angle relation (musculo-articular (MAC) stiffness) calculated in the ranges 0-10 and 0-20 deg, and earlier EMG onset compared to stretches performed at 5 deg/s. Only MAC in the ranges 0-10 and 0-20 deg differed between 30 and 60 deg/s. Overall, no significant correlations between max-ROM and MAC stiffness and sEMG onset angle were found for any stretching velocity tested.

CONCLUSION: Greater max-ROM can be achieved at faster compared to slower stretching velocities, although no statistical difference was observed in max-ROM between stretches at 30 vs. 60 deg/s. In addition, greater strength tolerance, energy storage and stiffness were attained at faster velocities, which is likely explained by musculo-articular tissues being viscoelastic (i.e. rate-dependent) and reflexive activation (sEMG gain) increasing with stretch speed. Earlier PF sEMG onsets were correlated with stiffer MAC at all stretching velocities. However, sEMG onset and MAC stiffness were not correlated with max-ROM, suggesting that the neuromechanical variables tested in this study were not determinant factors affecting max-ROM. The greater stretch tolerance in faster stretches, where greater max-ROM was attained, may also indicate that stretch tolerance is not a factor limiting max-ROM. Because these are commonly believed to affect max-ROM and could not explain these results, further research will need to be undertaken. The results of the present study have important practical and clinical implications that will improve future assessment of max-ROM.