Objectives: Interval block resistance training (IBRT) and circuit resistance training (CRT) are periodization models aimed at enhancing neuromuscular and metabolic adaptations. This study aims to evaluate the effects of a 12-week IBRT program compared to CRT on body composition, muscle strength, speed, functional capacity, and autonomic recovery in young Chilean adults. Methods: A randomized, parallel, double-blind study was conducted with 30 participants assigned to IBRT (n = 15) or CRT (n = 15). Assessments included body mass index (BMI), waist circumference, right-hand grip strength, the running anaerobic sprint test (RAST), the 6 min walk test (6 MWT), and heart rate variability (HRV) indices: low-frequency to high-frequency ratio (LF/HF) and root mean square of successive differences (RMSSD, a time-domain HRV metric reflecting parasympathetic activity). Statistical analyses included t-tests and ANCOVA. Results: Groups were similar in age (IBRT: 25.2 ± 3.19; CRT: 23.27 ± 3.69, p = 0.14) and BMI (IBRT: 21.56 ± 2.22; CRT: 22.36 ± 1.70 kg/m², p = 0.40). Both groups improved significantly in waist circumference (IBRT: −1.85%; CRT: −2.37%), grip strength (IBRT: +5.47%; CRT: +4.02%), RAST (IBRT: −2.67%; CRT: −1.04%), 6 MWT (IBRT: +4.53%; CRT: +2.17%), LF/HF (IBRT: −11.43%; CRT: −5.11%), and RMSSD (IBRT: +5.36%; CRT: +3.81%) (all p ≤ 0.01). IBRT produced significantly greater gains in 6 MWT (B = 19.51, 95% CI: 0.79 to 38.23, p = 0.04). Conclusions: Both IBRT and CRT effectively improved body composition, muscle strength, speed, functional capacity, and autonomic recovery. However, IBRT demonstrated a superior effect on aerobic capacity.

Background/Objectives: Low-cost portable load cell dynamometers allow for real-time assessment of muscular strength. This study evaluated the reliability and repeatability of the G-Force load cell system during isometric hip abduction and adduction in young physically active Chilean adults. Methods: In total, 24 participants (19 men, 5 women) performed two maximal voluntary contractions per movement, repeated after a 24 h interval. Measured variables included Peak Force, peak rate of force development (Peak RFD), RFD at 50, 100, and 200 ms (RFD50, RFD100, RFD200), and maximum jerk. Reliability was assessed using intraclass correlation coefficients (ICCs), standard error of measurement (SEM), coefficient of variation (CV%) and Bland–Altman plots. Results: Peak Force showed excellent within-day (ICC = 0.94–0.96) and high between-day reliability (ICC = 0.87–0.89; CV = 20–30%). Bland–Altman analysis indicated negligible bias for Peak Force in abduction (−6.54 N; 95% CI −19.55 to 6.47) and adduction (−17.57 N; 95% CI −37.24 to 2.09), confirming the absence of systematic error. Peak RFD, RFD50–200, and maximum Jerk showed moderate repeatability and lower between-day reliability (ICCs = 0.39–0.70; CVs = 34–57%), indicating higher variability in explosive force indices compared with maximal strength. Conclusions: The G-Force load cell reliably measures maximal isometric hip strength, while Peak RFD, RFD50–200, and maximum jerk should be interpreted cautiously. These findings support the device as a practical, low-cost tool for sports and rehabilitation, though future studies should validate dynamic indices in larger and more diverse populations.

Introduction: Heart rate variability (HRV) reflects autonomic modulation and is associated with physical fitness, particularly in aerobic endurance disciplines. However, the relationship between HRV and performance in aerobic capacity tests, such as the Yo-Yo test, has not been fully explored in university athletes. Objective: To analyze the relationship between resting HRV parameters and performance in the Yo-Yo test among university athletes. Methodologic: A cross-sectional correlational study was conducted with 32 male athletes (aged 18–26 years). HRV was assessed in supine and standing positions using time-domain and frequency-domain analyses (SDNN, RMSSD, pNN50%, HF, LF, VLF, and the LF/HF ratio). Subsequently, the maximum velocity reached in the Yo-Yo Intermittent Recovery Test Level 2 (YIRT2) was recorded. Pearson’s correlation test (p < .05) was used to examine the relationships. Results: A moderate positive correlation was observed between the final velocity in the Yo-Yo test and the resting parameters SDNN (r = 0.42; p = .02), HF (r = 0.4; p = .03), and pNN50%. Additionally, a negative correlation was found between the postural change-induced variation in pNN50% and performance (r = -0.46; p = .01). Conclusions: HRV parameters associated with parasympathetic activity show a significant relationship with performance in the Yo-Yo test among university athletes. These findings suggest that HRV could be a useful tool for predicting aerobic capacity and optimizing training planning.

Objectives: To evaluate the validity, accuracy, and reliability of the G-Force force platform during isometric tests, through comparison with a gold-standard force platform in physically active young adults. Methods: Nine physically active participants (23.67 ± 4.97 years; body mass index: 25.79 ± 3.02 kg/m²) performed isometric posterior lower limb muscle tests per leg, following a standardized warm-up and familiarization protocol. The G-Force platform and compared against a gold-standard device, the Valkyria Trainer Balance (VTB) force platform. The measured variables included Peak Force and peak rate of force development (RFD) at 50, 100 and 150 ms (RFD50, RFD100 and RFD150). Intra- and inter-platform reliability were assessed using intraclass correlation coefficients (ICC), standard error of measurement (SEM), coefficient of variation (CV%), Bland–Altman analysis and Pearson’s correlation coefficients between both platforms. Results: Peak Force showed excellent intra-platform repeatability (ICC = 0.86–0.91) and moderate-to-good inter-platform reliability (ICC = 0.75–0.77), with the G-Force platform generally reporting slightly lower absolute values than VTB. RFD measures demonstrated moderate reproducibility (ICC = 0.75–0.87) and higher variability (CV = 47–57%). Bland–Altman analyses revealed minimal bias for Peak Force, while regression analyses indicated strong, significant associations between G-Force and VTB measurements (R² = 0.55–0.77; β = 0.74–0.88; p < 0.05). Conclusions: The G-Force force platform is a valid, reliable, and low-cost tool for assessing isometric strength in physically active young adults.

Objectives: We aimed to compare the agreement and reliability of the G-Force system (force plate and load cell) for the mid-thigh isometric pull test in physically active adults. Methods: Eighteen participants (age: 27.6 years; BMI: 27.8 kg/m²) performed three maximal IMTP trials under standardized conditions. Force-time data were collected simultaneously from both systems. Peak force and rate of force development (RFD) were analyzed. Agreement was assessed using intraclass correlation coefficients (ICC) and Bland–Altman analysis. Paired t-tests and Cohen’s d evaluated systematic differences. Linear regression analyses were conducted to assess proportional bias. Results: Peak force showed excellent agreement (ICC = 0.999; 95% CI: 0.998–1.000) but with a positive fixed bias (84.48 N) and narrow limits of agreement (42.10 to 126.90 N), indicating consistently higher values from the force plate. A statistically significant difference was observed (p < 0.001; d = 3.90). Although the standardized effect size was large, the absolute bias between devices was relatively small (84.48 N; ~6% of mean peak force). The elevated d value reflects the low variability of the inter-system differences rather than a substantial absolute discrepancy. Accordingly, the results indicate a consistent systematic bias that may affect direct interchangeability of absolute values despite the excellent agreement observed between systems. No proportional bias was detected (p = 0.159). In contrast, RFD max showed lower agreement (ICC = 0.887; 95% CI: 0.699–0.958), with a negative bias (−1361 N/s) and wide limits of agreement (−6168 to 3445 N/s). Differences were significant (p = 0.031; d = 0.55), with no proportional bias (p = 0.268). Conclusions: The G-Force system demonstrates acceptable agreement for peak force but cannot be considered interchangeable with force plate measurements due to the presence of substantial systematic bias. In contrast, agreement for RFD is reduced, indicating greater sensitivity of early-phase force-time measures to methodological differences between systems.

Background and Objectives: This cross-sectional study examined associations between kinanthropometric indicators and lower-body force-time performance in 34 professional male boxers. Athletes were recruited by convenience sampling; no a priori sample-size calculation was performed. Methods: CMJ, countermovement rebound jump, and isometric-squat outcomes were assessed using a force platform. Three missing derived anthropometric estimates were recalculated from recorded measurements, and site-specific corrected-girth calculations, the Rocha bone-mass equation, and the Kerr adipose-mass input were corrected following confirmation by the study anthropometry investigator. A matrix of 117 Spearman correlations was evaluated with Benjamini–Hochberg false-discovery-rate adjustment. Results: CMJ peak force was correlated with BMI (rho = 0.697), body mass (rho = 0.663), corrected girths (rho = 0.622), estimated muscle mass (rho = 0.573), and adipose mass (rho = 0.508). Isometric-squat peak force was correlated with corrected girths (rho = 0.651), estimated muscle mass (rho = 0.638), sitting height (rho = 0.547), and estimated bone mass (rho = 0.521). Thirteen of 117 exploratory associations survived FDR correction. Conclusions: In this exploratory sample, observed relationships with absolute force tended to be larger than those with relative, ballistic, or time-dependent outputs, but formal differences between dependent correlations were not tested. The observed associations, including those based on anthropometric tissue-mass estimates, do not establish causal or body-mass-independent effects.