Differences in knee isokinetic strength under experimentally controlled cervical forward-flexion movement in university soccer players: a within-participant study using wearable flexible strain sensors

Background isokinetic strength testing is the international gold standard for quantitative muscle function assessment. However, the absence of standardized cervical positioning protocols introduces a critical confounding variable that compromises the validity of knee strength measurements. This study aimed to verify the impact of compensatory cervical forward flexion on knee isokinetic outcomes using wearable flexible strain sensors and to establish an evidence-based postural standardization protocol. Methods This self-controlled trial enrolled 39 university soccer players. A high-resolution wearable flexible strain sensor combined with self-developed computer software named Resistance Monitor was used to collect and process real-time angular data of the first to the seventh cervical vertebra, so as to quantify cervical forward flexion range of motion and identify flexion compensation patterns. Combined with the A8-2 isokinetic dynamometer, knee torque indicators including knee extensor and flexor relative peak torque, hamstring-to-quadriceps peak torque ratio, average peak torque and other primary measures were measured at 30°/s and 60°/s, whereas cervical forward flexion range of motion was assessed at 180°/s. All intergroup comparisons, correlation analyses and repeatability tests were performed using SPSS 27.0 and OriginPro 2025. Results Compensatory cervical forward flexion induced significant alterations in both extension and flexion parameters of right knee isokinetic assessments at 30°/s (all p < 0.01). At 60°/s, significant changes were only observed for right knee extension parameters (all p < 0.05), whereas flexion parameters remained unaffected (all p > 0.05). Bonferroni correction was applied for all statistical analyses in this study. At 180°/s, correlation analysis was conducted on simultaneously acquired cervical range of motion and relative resistance variation, yielding a Spearman’s ρ of 0.925 ( p < 0.001). Conclusions These findings demonstrate that cervical forward flexion compensation serves as an overlooked biomechanical confounder that systematically biases knee isokinetic extensor and flexor measurements. Consistent maintenance of cervical neutral positioning is recommended to eliminate this systematic bias. This work provides empirical foundation for postural standardization in knee isokinetic protocols and expands the translational application of the wearable flexible strain sensor in rehabilitative biomechanics and sports medicine. Trial registration ClinicalTrials.gov, ChiCTR2600127415. (Current Controlled on June 11, 2026).