Cédric Schwartz

Purpose : The objective of this study was to compare the long-term effects of running retraining and minimalist footwear on running economy (RE), foot and ankle adaptations, and running biomechanics over a 12-month period. Method : In this randomized controlled trial, 61 habitual rearfoot runners were allocated to 3 groups: minimalist footwear (n = 19), running-retraining (n = 18), and control (n = 24). Participants were evaluated at baseline, 2, 6, and 12 months. Outcome measures included RE and biomechanics assessed on a treadmill at self-selected comfortable speed, as well as foot and ankle characteristics. Results : RE improved over time in all groups, with no clear between-group differences at any time point. RE significantly improved from baseline in both the minimalist group (+4.9% at 6 mo and +9.3% at 12 mo; P = .03 and P < .001, respectively) and the running-retraining group (+5.6% at 6 mo and +10.5% at 12 mo; P < .001 for both). The control group showed an improvement in RE (+4.7%) at 12 months compared to baseline ( P = .01). No significant changes were observed in foot and ankle characteristics in any group across the follow-up period. Conclusion : RE improved similarly across all groups, suggesting that neither running retraining nor transitioning to minimalist footwear provides additional benefits beyond the control condition over a 12-month period. Although descriptive trends indicated greater improvements in the intervention groups, substantial interindividual variability highlights the need to identify responders to these interventions in future studies.

BACKGROUND:: Isokinetic dynamometry is widely used to assess muscle fatigability, particularly in athletes, due to its high reproducibility and sensitivity. However, the clinical and practical relevance of fatigability parameters remains debated, especially regarding their added value compared to simpler strength indicators. Objectives: The objectives of this study were to determine (1) the redundancy of measured knee extensor fatigability parameters, (2) the feasibility of predicting them from the best repetition (BR ₁ ), and (3) the relationship between initial strength and fatigability. Methods: 189 healthy male volunteers (sedentary to elite athletes) performed 30 maximal concentric knee flexions-extensions at 180°.s ⁻¹ . Pearson correlations, linear regressions, and k-fold cross-validation (k = 10) assessed parameter relationships and derived predictive equations. Results: Very strong correlations (PCC > 0.80) linked BR ₁ to most fatigability parameters, indicating high redundancy. Predictive equations based on BR ₁ accurately estimated total/partial sums (R ² > 0.95, ICC > 0.90). Moreover, stronger individuals exhibited steeper performance declines (PCC < −0.80 between BR ₁ and slope of the regression of the performance). Conclusions: Fatigability parameters offer limited added value, as they are highly redundant with BR ₁ . Simplified BR ₁ -based protocols suffice for clinical use, while full isokinetic testing remains essential for elite athletes or (neuro)muscular pathology requiring precision.

Landing tasks are a major contributor to knee injuries in sport. While biomechanical determinants of knee joint loading have already been studied, less attention has been given to the role of inter-segmental timing during sport-specific landings. This study aimed to examine both biomechanical and temporal predictors of knee joint loading during horizontal and vertical phases of stop-jump landings. Thirty-three male volleyball and basketball players performed stop-jump tasks while lower-limb kinematics, kinetics, patellar tendon forces, and anterior tibial shear forces were calculated. Statistical Parametric Mapping and Linear Mixed Models were used to identify phase-specific differences and predictors of knee loading. Inter-segmental timing, quantified as foot-contact delay between body segments, emerged as a significant predictor of knee joint loading across conditions. This finding extends previous work by highlighting inter-segmental timing as an additional factor influencing knee joint loading alongside established biomechanical predictors such as displacement velocity, body mass, and knee range of motion. Horizontal landing phases showed a stiffer strategy compared to vertical phases, with reduced joint range of motion (40–83%) and increased knee valgus. These differences were associated with higher anterior tibial shear (121%) and patellar tendon (156%) forces. These results may be particularly relevant in sport situations where landings are not fully planned and can be altered by external perturbations such as opponents’ deceiving actions.

This research investigates the ability of humans to synchronize their jumping motion with an external audible stimulus, providing insight into the nonlinear dynamics of human rhythmic movement. Experiments were conducted in which individuals were asked to jump in rhythm with the periodic click of a metronome. Ground reaction forces and body kinematics were measured to characterize the dynamic response of the body and identify key features of the coordination mechanism. The observations suggest that the human neuromuscular control system can be idealized as a nonlinear dynamical system exhibiting a limit cycle, representing the stable periodic motion of the jumping activity. When exposed to an external auditory cue, the system demonstrates phase entrainment, i.e., the synchronization of the internal oscillation with the external frequency. This entrainment capability was clearly observed experimentally and successfully reproduced by a reduced-order nonlinear model, confirming the theoretical predictions of synchronization phenomena in human movement. Although the current work focuses on single individuals, it establishes a robust experimental and modeling framework for future extensions to groups of jumpers, where mutual coupling effects and collective synchronization dynamics will be explored. Such developments have potential applications in biomechanics, crowd dynamics, and vibration mitigation of flexible structures.

Context : A previous study has shown that strengthening the foot muscles could decrease the risk of running-related injuries by 2.4 times in endurance runners. However, it remains unclear whether runners with stronger foot muscle have less risk of running-related injuries. The aim of this study was to compare foot–ankle characteristics between runners with and without a history of running-related injury. Design : Retrospective case–control study. Methods : This study enrolled 225 runners in this retrospective study. The participants filled out a questionnaire related to their history of running-related injury during the previous year. A foot morphology screening and a maximal voluntary isometric strength of ankle plantar flexor, hallux, and lesser toe flexor were performed. The footstrike pattern and cadence were determined using a high-speed camera during a self-paced run on a treadmill. Results : Runners with and without a history of overuse running-related injury had no significant differences in foot–ankle muscle strength. However, faster runners were more likely to report a history of overuse foot injury (odds ratio: 1.51, 95% CI, 1.14 to 2.00; P = .004). Runners with a history of tibia injury were less experienced (odds ratio: 0.83, 95% CI, 0.69 to 0.99; P = .038) and had lower cadence (odds ratio: 0.84, 95% CI, 0.88 to 0.99; P = .042). Conclusions : Runners with a history of overuse running-related injury, whether affecting any anatomical region, or specifically the foot do not exhibit reduced foot–ankle muscle strength. Runners with and without a history of running-related injury had no differences in foot–ankle muscle strength, suggesting that foot–ankle strength alone was not associated with previous injury status in this cohort.

Context : Accurate assessment of ankle evertor strength is essential for guiding rehabilitation and return-to-sport decisions after lateral ankle sprain, yet the reliability and measurement error of commonly used assessment instruments remain unclear. Objective : To synthesize the literature evaluating the reliability and measurement error of instruments used to assess ankle evertor strength. Evidence Acquisition : A systematic search was conducted in MEDLINE, Embase, Scopus, and DiTA without date restrictions. Studies assessing voluntary instrument-measured ankle evertor strength were included and evaluated using the COSMIN risk of bias tool. Two reviewers independently screened articles and extracted data on reliability (intraclass correlation coefficient or Pearson r ) and measurement error (minimal detectable change, limit of agreement, or coefficient of variation). Sufficient reliability was defined as intraclass correlation coefficient or r ≥ .70, and sufficient measurement error was define as minimal detectable change, limit of agreement, or coefficient of variation ≤20%. Key protocol characteristics, including contraction velocity, knee position, stabilization methods, visual feedback, and verbal encouragement, were extracted. Evidence synthesis : From 4047 records, 23 studies were included. Instruments used were isokinetic dynamometers, hand-held dynamometers, vertical pulley, and custom instruments. Most participants were healthy (90%), while 10% had lateral ankle sprain. Methodological quality was rated adequate (35%), doubtful (39%), or inadequate (26%). Reliability was sufficient in 83% of extracted values, whereas measurement error was sufficient in 34%. Protocol parameters did not significantly influence test–retest reliability; however, contraction velocity, knee position, and verbal encouragement influenced test–retest measurement error, and knee position and stabilization influenced interrater reliability. Conclusion : Isokinetic dynamometry, especially in isometric testing, demonstrated the most consistent reliability and the lowest measurement error. When unavailable, hand-held dynamometers offers a reliable field alternative for isometric testing. One-repetition maximum methods may be used for dynamic strength evaluation, despite higher measurement error. Protocol parameters influencing measurement error and interrater reliability should be standardized to improve measurement reproducibility.

Running-related injuries (RRIs) have been linked to high-frequency components of impact-related variables. The influence of running retraining interventions on low- (LoF) and high-frequency (HiF) components of impact-related variables remains unexplored. This study investigated the effects of different retraining strategies on LoF and HiF components of impact-related variables. Twenty-six habitual rearfoot runners completed four overground running conditions: no instructions (CON), forefoot strike (FOR), increased cadence by 10% (CAD), and a combination of both (CADFOR). Three-dimensional kinematics were recorded, while ground reaction forces were processed to separate LoF and HiF signals (10 Hz as cut-off value). Compared to CON, HiF impact peak was reduced in FOR (p d = 2.0), CAD (p = 0.006, d = 0.6), and CADFOR (p d = 2.3). HiF instantaneous vertical loading rate was significantly reduced only in FOR (p d = 0.6) and CADFOR (p d = 0.7). Notably, adopting a forefoot strike pattern reduced HiF components of impact-related variables nearly three times more than cadence increase alone. The combination of forefoot strike and increased cadence produced the largest reduction in HiF impact components, suggesting it may be the most effective retraining strategy for potentially lowering RRI risk.

While inertial measurement unit (IMU)-based systems have shown their potential in quantifying medically significant gait parameters, it remains to be determined whether they can provide accurate and reliable parameters both across various walking conditions and in healthcare settings. Using an IMU-based system we previously developed, with one IMU module on each subject’s heel, we quantify the gait parameters of 55 men and 46 women, all healthy and aged 40–65, in normal, dual-task, and fast walking conditions. We evaluate their intra-session reliability, and we establish a new reference database of such parameters showing good to excellent reliability. ICC(2,1) assesses relative reliability, while SEM% and MDC% assess absolute reliability. The reliability is excellent for all spatiotemporal gait parameters and the stride length (SL) symmetry ratio (ICC ≥ 0.90, SEM% ≤ 4.5%, MDC% ≤ 12.4%) across all conditions. It is good to excellent for the fast walking performance (FWP) indices of stride (Sr), stance (Sa), double-support (DS), and step (St) times; gait speed (GS); and the GS normalized to leg length (GS_n1) and body height (GS_n2) (ICC ≥ 0.91, |SEM%| ≤ 10.0%, |MDC%| ≤ 27.6%). Men have a higher swing time (Sw) and SL across all conditions. The following parameters are gender-independent: (1) Sa, DS, GS_n1, GS_n2; (2) the symmetry ratios of SL and GS, as well as Sa% and Sw% (representing Sa and Sw as percentages of Sr); and (3) the FWPs of Sr, Sa, Sw, DS, St, cadence, Sa% and Sw%. Our results provide reference values with new insights into gender FWP comparisons rarely reported in the literature. The advantages and reliability of our IMU-based system make it suitable in medical applications such as prosthetic evaluation, fall risk assessment, and rehabilitation.

While inertial measurement unit (IMU)-based systems have shown their potential in quantifying medically-significant gait parameters, it remains to be demonstrated that they can provide accurate and reliable parameters both across various walking conditions and in healthcare settings. Using an IMU-based system we have previously developed, with one IMU module on each subject’s heel, we quantify gait parameters in 55 men and 46 women, all healthy and aged 40-65, in normal, dual-task, and fast-walking conditions. We evaluate their intra-session reliability, and we establish a new reference database of such parameters showing good to excellent reliability. ICC(2,1) assesses relative reliability, while SEM% and MDC% evaluate absolute reliability. The reliability is excellent for all spatiotemporal gait parameters and stride length (SL) symmetry ratio (ICC≥0.90, SEM%≤4.5%, MDC%≤12.4%) across all conditions. It is good to excellent for fast-walking performance (FWP) indices of stride (Sr), stance (Sa), double-support (DS), and step (St) times; gait speed (GS), and GS normalized to leg length (GSn1) and body height (GSn2) (ICC≥0.91, |SEM%|≤10.0%, |MDC%|≤27.6%). Men have higher swing time (Sw) and SL across all conditions. The following parameters are gender-independent: (1) Sa, DS, GSn1, GSn2; (2) symmetry ratios of Sa, Sa ratio (Sa%), Sw, Sw ratio (Sw%), SL, GS; and (3) FWPs of Sr, Sa, Sw, DS, St, cadence, Sa%, Sw%. Our results provide reference values with new insights into gender FWP comparisons rarely reported in the literature. The advantages and reliability of our IMU-based system make it suitable in medical applications such as prosthetic evaluation, fall-risk assessment, and rehabilitation.

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