Concurrent repetitions overestimate hamstring:quadriceps ratios at more extended knee joint positions: implications for clinical practice

INTRODUCTION:
Recent research suggests that return-to-play protocols may benefit from the inclusion of angle-specific hamstring:quadriceps (H:Q) ratios to accurately understand muscle balance through the ROM (Lunn et al. 2023). In undertaking angle-specific analyses, most isokinetic knee extension-flexion protocols are conducted using concurrent repetitions (CON) whereby active knee extension is immediately followed by active knee flexion. To reduce the influence of the stretch-shortening cycle and limit axis misalignment, isolated repetitions (ISO) have been recommended (Alt et al. 2014) whereby extension and flexion are completed separately. To inform athlete screening protocols, this study examined the effect of CON and ISO protocols on discrete and angle-specific hamstring:quadriceps ratios.
METHODS:
Fifteen healthy males (age: 27 ± 4 years; height: 184 ± 9 cm; body mass: 80 ± 9 kg) performed isokinetic tests of the knee flexors and extensors (60 deg/s) using CON and ISO repetitions while sagittal-plane kinematics were captured (100 Hz) to quantify axis misalignment. Statistical parametric mapping then enabled the effects of protocol type (CON vs. ISO) and axis misalignment (uncorrected vs. corrected data) to be compared.
RESULTS:
The use of uncorrected data resulted in an underestimation of discrete conventional (−10.2%, p < 0.001) and functional (−9.2%, p < 0.05) H:Q ratios with these differences being observed for all angle-specific ratios (p < 0.001). The use of concurrent repetitions resulted in a significant overestimation of the H:Q ratio (+7.4%, p < 0.05) with the differences being most prevalent at extended knee joint positions. Despite the main effect(s) of protocol type and axis misalignment, no significant interactions were observed.
CONCLUSION:
Practitioners should be mindful that the use of concurrent repetitions will result in a significantly higher conventional H:Q ratio which will particularly influence angle-specific ratios in more extended knee joint positions. This may increase the likelihood of “false negative” injury risk categorisation during “time-efficient” protocols involving concurrent repetitions. Whilst the use of corrected knee moment data is preferable, practitioners should be mindful that the use of isolated repetitions and corrected joint moment data does not lead to significant differences in discrete or angle-specific H:Q ratios when compared with uncorrected data obtained from concurrent repetitions.

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