Many sports injuries result in surgery and prolonged periods of immobilization, which may lead to significant atrophy accompanied by loss of maximal strength and range of motion and, therefore, a weak-leg/strong-leg ratio (as an imbalance index ∆ ) lower than 1. Consequently, there are common rehabilitation programs that aim to enhance maximal strength, muscle thickness and flexibility; however, the literature demonstrates existing strength imbalances after weeks of rehabilitation. Since no study has previously been conducted to investigate the effects of long-duration static stretch training to treat muscular imbalances, the present research aims to determine the possibility of counteracting imbalances in maximal strength and range of motion. Thirty-nine athletic participants with significant calf muscle imbalances in maximal strength and range of motion were divided into an intervention group (one-hour daily plantar flexors static stretching of the weaker leg for six weeks) and a control group to evaluate the effects on maximal strength and range of motion with extended and bent knee joint. Results show significant increases in maximal strength (d = 0.84–1.61, p < 0.001–0.005) and range of motion (d = 0.92–1.49, p < 0.001–0.002) following six weeks of static stretching. Group * time effects (p < 0.001–0.004, η² = 0.22–0.55) revealed ∆ changes in the intervention group from 0.87 to 1.03 for maximal strength and from 0.92 to 1.11 in range of motion. The results provide evidence for the use of six weeks of daily, one hour stretching to counteract muscular imbalances. Related research in clinical settings after surgery is suggested.

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.

Speed strength performances are heavily dependent on maximum strength. However, various strength testing methods determined inconsistent relationships between absolute and relative strength and sprint and jump performances. The aim of the study was to calculate the one tailed correlation coefficients between both the One-Repetition Maximum (1RM) and 1RM in relation to body mass (1RM/BM) in parallel squats and different jump (squat jump and countermovement jump) and sprint performances (5-, 10-, 20-, and 30-m) in youth soccer players (n= 63, 17.9±2.1 years old). Relative strength showed significantly larger correlations with jump performances (r= 0.52 to 0.58) than absolute strength (r= 0.16 to 0.26, z= -1.81 to -1.90, p= 0.029 to 0.035). However, the r values between relative strength measures and sprint performances (r= -0.32 to -0.42) were of non-statistical difference to the correlations of absolute strength measurements with sprint performances (r= -0.19 to -0.3, z= 0.349 to 1.17, p= 0.121 to 0.363). The results of this study support findings in previous literature of enhanced speed strength performances by higher levels of maximal strength in youth soccer players, with faster and more powerful athletes being able to generate larger forces against their own body weight. The data suggests that strength expressed relative to body mass might be considered as a superior predictor of speed strength performance in general.

This study aimed to investigate the variables determining performance in a simulated on-snow Skimo sprint competition, and how their relationship with performance evolves from the individual time trial to the final. Fifteen national-level junior Skimo athletes (mean ± SD: age, 17.8 ± 2.5 years; maximal oxygen uptake, 66.8 mL·kg⁻¹·min⁻¹) underwent a comprehensive assessment, involving submaximal and maximal endurance tests, maximal strength assessments, and a maximal sprint to determine maximal glycolytic capacity. Subsequently, a simulated sprint competition, comprising an individual time-trial and three heats (quarterfinal, semifinal, final), was conducted. Whole-body and upper body aerobic power (r = 0.69–0.93), maximal speed and power (r = 0.82–0.85) during the maximal performance test, as well as fat-free mass (r = 0.62–0.77) and body fat (r = −0.67–−0.77), exhibited significant correlations with performance in the time-trial, quarterfinal and semifinal. Moreover, maximal strength (r = 0.39–0.95) and transition duration (r = 0.52–0.85) showed moderate to large correlations with sprint performance. Overall, aerobic power, maximal speed and power, as well as fat-free mass, and body fat emerged as crucial determinants of Skimo sprint performance, while dynamic strength and the ability to transition quickly between sections also proved to be relevant factors.

A total of nineteen elite youth cross-country skiers (16 ± 2 years) were divided into three groups: a low-load group (LL, n = 6) performing 15–20 repetitions per exercise, a high-load group (HL, n = 9) executing 6–12 repetitions, and a control group (CON, n = 4) exclusively engaging in endurance training. Testing included 1RM in upper-body exercises, VO_2max running tests, and double-poling (DP) ergometer exhaustion and sprint tests to determine peak oxygen uptake (VO_2peak-DP) and peak power (PP). The results indicated that HL and LL showed significant increases in VO_2peak-DP, both in absolute values (d = −1.2 and −1.2, p < 0.05) and when normalized to body mass (d = 0.93 and 1.3, p < 0.05). Although there were no group*time effects for VO_2peak-DP, PP during DP tests to exhaustion increased in both HL (d = −1.6, p < 0.05) and LL (d = 1.4, p < 0.05) compared to CON. Standardized to body mass, only HL showed significant improvements in PP during the sprint test (d = 1.7, p < 0.05). During the sprint test, both absolute and body-mass-normalized peak power increased only in the high-load group (d = −1.0 and 0.93, p < 0.05). In conclusion, high-load resistance training resulted in the greatest improvements in strength, DP performance, and VO_2peak-DP, indicating a dose–response relationship to the load magnitude of resistance training.

Introduction: Sex differences in the performance of elite cross-country skiers are reported enlarged in techniques with a higher upper-body contribution, such as double poling (DP) and G3 skating. However, little is known about the development of upper-body strength, aerobic power, and DP performance throughout adolescence and early adulthood. Therefore, this study examined age- and sex-related differences in upper-body characteristics and their associations with DP performance. Methods: Ninety-eight competitive cross-country skiers and biathletes (47 males, 51 females) from the U16 to U23 age categories completed a 15-s DP ergometer sprint test (DPP _15s ), and an incremental DP ergometer test to volitional exhaustion (DPP _peak ). Maximal strength was assessed using one-repetition maximum tests in the bench press (1RM _press ) and bench pull (1RM _pull ). Maximal oxygen uptake (VO _2max ) was determined during treadmill running with poles. Age- and sex-related differences were analyzed using two-way MANOVAs followed by separate two-way ANOVAs. Partial Pearson correlations were used to examine associations between maximal strength, DP performance, and aerobic power while controlling for age and sex. Results: Males demonstrated higher values across all performance measures, which generally increased with age. Significant age × sex interactions were observed for absolute 1RM _pull (η²=0.13, p=0.010), 1RM _press (η²=0.12, p=0.010), DPP _peak (η²=0.14, p=0.003), VO _2peak during DP (η²=0.14, p=0.003) and VO _2max (η²=0.11, p=0.018), indicating larger absolute sex differences in older age groups. No significant age × sex interactions were observed for DPP _15s (η²=0.05, p=0.228) or body-mass normalized measures, except for VO _2peak during DP (η²=0.11, p=0.019). 1RM was moderately to strongly associated with DPP _15s , DPP _peak and VO _2peak during DP (r =0.50–0.61), whereas associations were weaker after normalization to body mass (r =0.13–0.39). Aerobic power showed strong associations with DPP _peak (r=0.73–0.82). Conclusion: Absolute sex differences in upper-body performance increased across age-groups and were more pronounced than previously reported for lower-body performance. However, body-mass normalized differences were largely stable across age groups. Upper-body strength was consistently associated with DP performance after accounting for age and sex, highlighting its potential relevance for DP performance development in youth athletes. However, the cross-sectional design precludes attribution of these age-related differences to maturation, training, or other developmental factors.