Abstract
Submaximal vertical hopping capitalizes on the strain energy storage-recovery mechanism associated with the stretch-shortening cycle and is emerging as an important component of progressive rehabilitation protocols in Achilles tendon injury and a determinant of readiness to return to sport. This study explored the reliability of transmission mode ultrasound in quantifying the instantaneous modulus of elasticity of human Achilles tendon during repetitive submaximal hopping. A custom-built ultrasound transmission device, consisting of a 1 MHz broadband emitter and four regularly spaced receivers, was used to measure the axial velocity of ultrasound in the Achilles tendon of six healthy young adults (mean ± SD; age 26 ± 5 years; height 1.78 ± 0.11 m; weight 79.8 ± 13.6 kg) during steady-state unilateral hopping (2.5 Hz) on a piezoelectric force plate. Vertical ground reaction force and lower limb joint kinematics were simultaneously recorded. The potential sensitivity of the technique was further explored in subset of healthy participants (n = 3) that hopped at a slower rate (1.8 Hz) and a patient who had undergone Achilles tendon rupture-repair (2.5 Hz). Reliability was estimated using the mean-within subject coefficient of variation calculated at each point during the ground-contact phase of hopping, while cross-correlations were used to explore the coordination between lower limb kinematics ground reaction forces and ultrasound velocity in the Achilles tendon. Axial velocity of ultrasound in the Achilles tendon was highly reproducible during hopping, with the mean within-subject coefficient of variation ranging between 0.1 and 2.0% across participants. Ultrasound velocity decreased immediately following touch down (−19 ± 13 ms–1), before increasing by 197 ± 81 ms–1, on average, to peak at 2230 ± 87 ms–1 at 67 ± 3% of ground contact phase in healthy participants. Cross-correlation analysis revealed that ultrasound velocity in the Achilles tendon during hopping was strongly associated with knee (mean r = 0.98, range 0.95–1.00) rather than ankle (mean r = 0.67, range 0.35–0.79) joint motion. Ultrasound velocity was sensitive to changes in hopping frequency in healthy adults and in the surgically repaired Achilles tendon was characterized by a similar peak velocity (2283 ± 13 ms–1) but the change in ultrasound velocity (447 ± 21 ms–1) was approximately two fold that of healthy participants (197 ± 81 ms–1). Although further research is required, the technique can be used to reliably monitor ultrasound velocity in the Achilles tendon during hopping, can detect changes in the instantaneous elastic modulus of tendon with variation in hopping frequency and tendon pathology and ultimately may provide further insights into the stretch-shortening cycle and aid clinical decision concerning tendon rehabilitation protocols and readiness to return to sport.
Highlights
Hopping capitalizes on the strain energy storage-recovery mechanism associated with the stretch-shortening cycle (Cavagna, 1977) and is emerging as an important component of progressive rehabilitation protocols in Achilles tendon injury and a determinant of readiness to return to sport following anterior cruciate ligament reconstruction (Noyes et al, 1991; Sancho et al, 2019; Silbernagel et al, 2020)
This study explored the reliability of transmission mode ultrasound in quantifying the instantaneous modulus of elasticity of human Achilles tendon during repetitive submaximal hopping
Measures of the elastic modulus of the Achilles tendon during hopping have typically been derived secondarily from estimates of tendon displacement and loading using B-mode ultrasound and indirect techniques, including inverse dynamics, and electromyography-to-force processing (Lichtwark and Wilson, 2005; Bogey and Barnes, 2017). Such indirect approaches are based on estimates of joint centers and tendon moment arms and require assumptions regarding the contribution of agonist and antagonist muscles to the net ankle joint moment, which may result in overestimation of tendon loads by as much as 50% compared to direct measures (Gregor et al, 1991; Fukashiro et al, 1993) and tendon work loops that are physiologically implausible (Zelik and Franz, 2017)
Summary
Hopping capitalizes on the strain energy storage-recovery mechanism associated with the stretch-shortening cycle (Cavagna, 1977) and is emerging as an important component of progressive rehabilitation protocols in Achilles tendon injury and a determinant of readiness to return to sport following anterior cruciate ligament reconstruction (Noyes et al, 1991; Sancho et al, 2019; Silbernagel et al, 2020). The technique has been shown to be sufficiently sensitive to detect changes in the instantaneous elastic modulus of Achilles tendon with changes in walking speed (Brauner et al, 2017), there is debate as to whether the Achilles tendon normally operates within the elastic “toe” region under conditions of physiological loading or the “linear” elastic region, where Young’s elastic modulus is traditionally measured in vitro and ultrasound transmission velocity would approach a constant value (Maganaris and Paul, 1999; Biewener and Roberts, 2000) This is further compounded by direct measures of physiological loading in the Achilles tendon during submaximal hopping in which peak loads were found, rather unexpectedly, to be approximately twice those observed walking (Komi et al, 1992) and jumping (Fukashiro et al, 1995). All participants attended the laboratory on a single occasion for testing and provided written informed consent to the procedures of the study, which received approval from the University Human Research Ethics Committee
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