Torsten Brauner

Purpose: This study analyzed the inter-session reliability of the parallel back squat during one-repetition-maximum (1RM) testing in elite youth soccer players and to exploratorily evaluate the consistency with which the prescribed squat depth is achieved. Methods: The sample comprised 47 male soccer players (age: 18.1 ± 2.3 years, body mass: 72.4 ± 7.7 kg, height: 1.83 ± 0.06 m) from German youth soccer academy teams competing in top junior leagues (U16-U19). The 1RM was assessed under standardized conditions, with squat depth monitored using a hybrid marker- and silhouette-based motion capture system. The inter-session reliability of 1RM load was quantified using the intraclass correlation coefficient ( ICC ). Systematic bias between sessions was evaluated using a paired-samples t -test (with Cohen’s d for effect size). Within-session deviations from the 90° squat-depth target were assessed using a one-sample t -test (with Cohen’s d for effect size). Results: The participants achieved an absolute 1RM of 94.9 ± 13.7 kg (relative: 1.32 ± 0.19). The 1RM test–retest (inter-session) reliability regarding load lifted yielded an ICC of 0.970 (95% CI: 0.948–0.982), with a mean bias of 1.71 kg ( P = 0.018, d = 0.39). An explanatory analysis of the kinetic data from each participant’s best trial (within-session) showed that squat depth deviations (thigh angle 89.7° ± 4.1°) did not differ significantly from the target depth of 90° ( P = 0.630, d = 0.15), where 0° corresponds to full knee extension. Conclusions: This study’s parallel back-squat 1RM testing protocol in national-level male youth soccer players demonstrated excellent load reliability. Squat depth may confound reliability, and its full impact on inter-session error warrants further study; nonetheless, familiarization, skilled spotters, and practiced participants can help limit its effect.

Speed strength performances are substantially dependent on maximum strength. Due to their importance, various methods have been utilized to measure maximum strength (e.g., isometric or dynamic) with discussed differences regarding transferability to sport-specific movements dependent upon the testing procedure. The aim of this study was to analyze whether maximum isometric force (MIF) during isometric back squats correlates with maximum strength measurements of the one repetition maximum (1RM) in the squat, with countermovement jump (CMJ) performance, and with drop jump (DJ) performances in elite youth soccer players (n = 16, 18.4 ± 1.5 [range: 17–23] years old). Additionally, concordance correlation coefficients (CCC, [ρ_c]) between isometric and dynamic measurements were calculated to verify whether one measurement can actually reproduce the results of the other. To improve comprehension, differences between isometric and dynamic testing values were illustrated by providing differences between both testing conditions. For this, the mean absolute error (MAE) and the mean absolute percentage error (MAPE) were calculated. To reach equality in scale, the 1RM measures were multiplicated by 9.81 to obtain a value of N. The 1RM demonstrated correlations of τ = |0.38| to |0.52| with SJ and CMJ performances, while MIF demonstrated correlations of τ = |0.21| to |0.32|. However, the correlations of both 1RM and MIF with the DJ reactive strength index (RSI = jump height/contact time) from different falling heights were of no statistical significance. The data showed significant correlations between both the absolute (τ = |0.54|) and the relative (τ = |0.40|) performances of 1RM and MIF, which were confirmed by CCC of ρ_c= |0.56| to |0.66|, respectively. Furthermore, the MAE and MAPE showed values of 2080.87 N and 67.4%, respectively. The data in this study show that, despite good correlations, there is no exact coincidence between isometric and dynamic strength measurements. Accordingly, both measurements may only represent an estimation of maximal strength capacity and cannot be substituted for each other. Therefore, maximal strength should be tested by using high similarity in the contraction condition, as it is used in the training process to counteract underestimation in strength because of unfamiliarity with the testing condition.

This study aims to analyze the influence of relative strength performance, determined by parallel back squats (REL SQ), on 30 m sprinting (LS) and on jumping performance (squat [SJ], countermovement [CMJ]) in a large sample (n = 492) of elite youth soccer players. The soccer players were divided into subgroups based on their strength performance: strength level 1 (0.0–0.5 REL SQ), strength level 2 (>0.5–1.0 REL SQ), strength level 3 (>1.0 to 1.5 REL SQ), strength level 4 (>1.5 to 2.0 REL SQ), and strength level 5 (>2.0 REL SQ). The results of this study show that REL SQ explains 45–53% (r = |0.67–0.73|) of the variance of SJ, CMJ, and LS for the total sample. Strength levels 2–4 showed similar coefficients of correlation in jumping performance (r = |0.42–0.55|) and strength levels 2 and 3 in sprint performance (r = |0.41|). The respective extreme strength levels showed lower coefficients of correlation with the sprinting and jumping performance variables (r = |0.11–0.29|). No coefficients could be calculated for strength level 5 because no athlete achieved an appropriate strength level (>2.0 REL SQ). The data from this study show a clear influence of REL SQ on sprint and jump performance, even in a large sample.

Progression of the difficulty of agility exercises in sport is often achieved by changing the stability of the support surface via graded sensorimotor training devices. However, little is known about the challenge imposed to postural equilibrium by these graded devices. This study quantified the instability provided by four sensorimotor training devices typically used to enhance athletic performance; three progressively unstable balance pads (ST1–3) and an oscillatory platform (PM). Twenty-five (13 female, 12 male) young adults (age, 26 ± 3 yr; height, 1.76 ± 0.10 m; and weight, 69 ± 12 kg), completed seven unipedal balance conditions involving stable and progressively unstable surfaces that involved four sensorimotor training devices (ST1-3, PM) and their combination (PM-ST1, PM-ST2). An inertial sensor, mounted over the lumbar spine, was used to monitor Centre of Mass (COM) displacement in each condition. Potential differences in COM displacement between conditions were assessed using a mixed-model analysis of variance. COM displacement differed between training devices; with a progressive, though non-linear, increase in COM displacement from the most (ST1) to the least (ST3) stable balance pad. However, there was no significant difference in COM displacement between the least stable balance pad (ST3) and the oscillatory platform used in isolation (PM) or in combination with balance pads (PM-ST1, PM-ST2). These novel findings have important practical implications for the design of progressive sensorimotor training programs in sport.

The capacity of foot-strike running patterns to influence the functional properties of the Achilles tendon is controversial. This study used transmission-mode ultrasound to investigate the influence of habitual running foot-strike pattern on Achilles tendon properties during barefoot walking and running. Fifteen runners with rearfoot (RFS) and 10 with a forefoot (FFS) foot-strike running pattern had ultrasound transmission velocity measured in the right Achilles tendon during barefoot walking (≈1.1 ms⁻¹) and running (≈2.0 ms⁻¹). Temporospatial gait parameters, ankle kinematics and vertical ground reaction force were simultaneously recorded. Statistical comparisons between foot-strike patterns were made using repeated measure ANOVAs. FFS was characterised by a significantly shorter stance duration (−4%), greater ankle dorsiflexion (+2°), and higher peak vertical ground reaction force (+20% bodyweight) than RFS running (P < .05). Both groups adopted a RFS pattern during walking, with only the relative timing of peak dorsiflexion (3%), ground reaction force (1–2%) and peak vertical force loading rates (22–23%) differing between groups (P < .05). Peak ultrasound transmission velocity in the Achilles tendon was significantly higher in FFS during walking (≈100 ms⁻¹) and running (≈130 ms⁻¹) than RFS (P < .05). Functional Achilles tendon properties differ with habitual footfall patterns in recreational runners.

This study used transmission-mode ultrasound (US) to evaluate the effect of footwear, with differing heel elevation, on Achilles tendon loading and basic gait parameters. Axial transmission velocity of US was measured in the right Achilles tendon of 20 healthy adults (10 male and 10 female; age: 31 ± 9 years; height: 1.72 ± 0.04 m; weight: 67.8 ± 14.2 kg) while walking barefoot (BF) and in four prototype athletic shoes (S1–S4) on an instrumented treadmill. Shoes were constructed from identical materials and differed only in heel elevation (S1: 0.4 mm; S2: 5 mm, S3: 9.9 mm, S4: 14.8 mm), which was achieved by varying the graded thickness of the midsole beneath the rearfoot. Temporospatial gait parameters, sagittal ankle movement, and vertical ground reaction force were simultaneously recorded at rate of 120 Hz. Shod walking conditions were characterized by a significantly lower cadence, longer stance duration, greater ankle dorsiflexion, larger peak vertical ground reaction force, and higher US transmission velocity in the Achilles tendon than BF walking (P < .05). Incremental heel elevation in footwear resulted in a progressive increase in peak vertical force, a reduction in peak external loading rate, and a small but progressive reduction in US transmission velocity in the Achilles tendon (P < .05). Simple regression modelling predicted that a shoe with 58 mm of heel elevation would effectively replicate values observed during BF walking. Peak US transmission velocity, and hence tensile load, in the Achilles tendon was higher during shod than BF walking and was partially countered by increasing the heel elevation of the shoe. These findings have important clinical implications for the use of footwear in performance applications and in the prevention and rehabilitation of Achilles tendon disorders.

Purpose This cross-sectional study used transmission-mode ultrasound to evaluate dynamic tendon properties during walking in surgically repaired and contralateral Achilles tendon (AT), with a median (range) post-operative period of 22 (4–58) months. It was hypothesised that the axial transmission speed of ultrasound (TSOU) during walking would be slower, indicating lower material stiffness in repaired compared with contralateral AT. Methods Ten patients [median (range) age 47 (37–69) years; height 180 (170–189) cm; weight 93 (62–119) kg], who had undergone open surgical repair of the AT and were clinically recovered according to their treating clinicians, walked barefoot on a treadmill at self-selected speed (1.0 ± 0.2 m/s). Synchronous measures of TSOU, sagittal ankle motion, vertical ground reaction force (GRF), and spatiotemporal gait parameters were recorded during 20 s of steady-state walking. Paired t tests were used to evaluate potential between-limb differences in TSOU, GRF, ankle motion, and spatiotemporal gait parameters. Results TSOU was significantly lower (≈175 m/s) in the repaired than in the contralateral AT over the entire gait cycle (P < 0.05). Sagittal ankle motion was significantly greater (≈3°) in the repaired than in the contralateral limb (P = 0.036). There were no significant differences in GRF or spatiotemporal parameters between limbs. Conclusions Repaired AT was characterised by a lower TSOU, reflecting a lower material stiffness in the repaired tendon than in the contralateral tendon. A lower material stiffness may underpin greater ankle joint motion of the repaired limb during walking and long-term deficits in the muscle-tendon unit reported with AT repair. Treatment and rehabilitation approaches that focus on increasing the material stiffness of the repaired AT may be clinically beneficial. Transmission-mode ultrasound would seem useful for quantifying tendon properties post AT rupture repair and may have the potential to individually guide rehabilitation programmes, thereby aiding safer return to physical activity. Level of evidence II.