The effects of stretching rate on plantar flexor maximum range of motion and resistance to stretch
Date of Award
Master of Science (Sports Science)
School of Medical and Health Sciences
Professor Anthony J Blazevich
Professor Anthony D. Kay
Dr Jodie C. Wilkie
Maximal joint range of motion (max-ROM) and resistance to tissue elongation (components of ‘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 using systems such as isokinetic dynamometers at slow angular velocities (≤5°.s-1), which might be of little functional relevance for most daily and sports activities that are performed at faster angular velocities (≥20°.s-1). Therefore, the present research tested the feasibility and reliability of a laboratory-based set of tests performed on a commercially available dynamometer aiming to assess flexibility during both slow and faster ankle joint rotations. In addition, a major drawback of using isokinetic systems in such tests was identified, and the effects of joint angular velocity on the plantar flexor neuromechanical properties and max-ROM as well as their relationship were tested.
Fifteen participants attended two familiarisation sessions followed by two experimental sessions separated by ≥72 h. These included the performance of ankle joint max-ROM tests on an isokinetic dynamometer at 5, 30 and 60°.s-1, interspaced by 1.5 min, whilst joint position, joint moment, and surface electromyography (EMG) were recorded synchronously. In Study 1, max-ROM was defined as either the maximal position observed in the joint position trace (max-ROMPOS) or the position at which the angular acceleration signal first deflected below zero after the constant-velocity phase (max-ROMACC). Max-ROMACC was assumed to be indicative of the participant’s true volitional stretch termination because it represents the time at which the participant pushed the button to end the stretch; it thus removes the deceleration period of the dynamometer arm. In studies 2 and 3, max-ROM was determined as max-ROMACC. Max-ROM, peak passive joint moment (indicative of stretch tolerance), musculo-articular (MAC) stiffness and area under the joint moment-position curve (energy storage) were calculated in both studies. The joint angle at EMG onset and maximal amplitude of EMG were also quantified in Study 3.
In Study 1, the delay between button press and eventual stopping of joint rotation statistically affected max-ROM and peak passive joint moment in an angular velocity-dependent manner, which affected other variables calculated from the data. These effects were considered to be functionally relevant at the faster (30 and 60°⋅s-1) but not slower (5°⋅s-1) speeds. In Study 2, between-day relative (ICC2,1) and absolute reliabilities (standard error of measurement and minimal detectable change) for all variables, excluding EMG data, ranged from moderate to good (0.90.5) with an inverse relationship between ankle joint rotation velocity and reliability results. In Study 3, significantly greater max-ROMs were achieved at faster compared to slower joint rotation velocities, although no statistical differences were observed in max-ROM between 30 vs. 60°.s-1 joint rotations. Greater stretch tolerance, energy storage and MAC stiffness were observed at faster velocities. Earlier onset of plantar flexor EMG was correlated with stiffer MAC at all stretching velocities. However, neither earlier EMG onset nor MAC stiffness were correlated with max-ROM.
The present research shows that the rate dependence of max-ROM and MAC mechanical properties can be feasibly tested on commercially-available isokinetic dynamometers when ankle joint rotations are performed at 5, 30, and 60°.s-1 with moderate to good reliability. When high data accuracy is required, especially at fast joint rotation velocities (≥30°.s-1), max-ROM (and associated measures calculated from joint moment data) should be taken at the point of first change in joint angular acceleration rather than at the dynamometer’s ultimate (final) joint position. The greater peak passive moments at faster rotation velocities occurred alongside greater ROMs. Thus, participants did not cease the muscle stretch at a given joint moment (i.e. a given stretch tolerance level). In fact, the peak passive joint moment at slow speed was attained earlier (smaller ROM) in the fast stretches, yet subjects did not cease the stretch at that point and proceeded to greater ROMs. These findings show that ‘stretch tolerance’ changed with velocity and therefore was not an absolute predictor of joint ROM. The greater MAC stiffness at faster joint rotation velocities was associated with an earlier onset of plantar flexor EMG activity, indicating that the greater muscle activity might increase stiffness through active muscle force production; however, the viscoelastic properties of the tissues might also influence MAC stiffness and further research is required to determine the relative influence of each of these factors. Additionally, the neuromechanical variables measured in this study were not identified as factors limiting max-ROM, so further research is required to pinpoint these variables. Nonetheless, these results have important practical and clinical implications for the velocity-dependent assessment of max-ROM.
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Daros Pinto, M. (2019). The effects of stretching rate on plantar flexor maximum range of motion and resistance to stretch. https://ro.ecu.edu.au/theses/2277