Muscle Architecture and Neuromuscular Changes After High-Resistance Circuit Training in Hypoxia

The effects of normobaric hypoxic resistance training on muscle strength in healthy adults

Normobaric hypoxic resistance training (NHRT) has emerged as a novel approach to enhancing muscle strength, potentially offering advantages over conventional resistance training. However, its efficacy in healthy adults remains uncertain. This systematic review and meta-analysis aimed to evaluate the effects of NHRT on muscle strength indicators, including one-repetition maximum (1RM), isometric strength, and isokinetic strength, in healthy adults.

METHODS

Following PRISMA 2020 guidelines, four databases (PubMed, Web of Science, SportDiscus, and CNKI) were searched from inception to October 25, 2024, for randomized controlled trials. Study quality was assessed using the Cochrane Risk of Bias tool. Effect sizes were calculated using Review Manager 5.4.

RESULTS

A total of 22 RCTs involving 487 healthy adults were included. The meta-analysis revealed a significant small-to-moderate improvement in 1RM (SMD = 0.22, 95% CI [0.06, 0.38]) but no statistically significant effects on isometric strength (SMD = 0.32, 95% CI [-0.05, 0.70]) and isokinetic strength (SMD = 0.25, 95% CI [-0.11, 0.62]). Subgroup analyses indicated that oxygen concentrations of 14-16% and training loads of 60-80% 1RM produced the most substantial gains, particularly among untrained participants.

CONCLUSIONS

NHRT is a promising strategy for enhancing 1RM in healthy adults, with its effectiveness influenced by hypoxic levels, training load, and baseline training status. Optimal outcomes were observed at oxygen concentrations of 14-16% and moderate training loads (60-80% 1RM), particularly in untrained individuals. Further high-quality studies are warranted to confirm these outcomes and explore the underlying mechanisms.Registration number on PROSPEROCRD42024547100.

Central and peripheral nervous system activity and muscle oxygenation in athletes during repeated-sprint exercise in normoxia and normobaric hypoxia

AIM

To investigate central and peripheral nervous system activity and muscle oxygenation in athletes during repeated-sprint exercise in normoxia and normobaric hypoxia.

METHODS

The effects on vastus lateralis muscle strength in a cross-over study were examined in 18 athletes (13 males, 5 females) completing 10 × 6-s cycle sprints. Immediately after and again 5 minutes post-exercise, electromyography (EMG), heart rate variability, maximal voluntary contraction (MVC), muscle oxygenation, peak power output, and arterial oxygen saturation were compared to 2 baseline sets named ("Baseline" and "Pre").

RESULTS

Post-exercise MVC was significantly lower (6.7 ± 10.0%) than Baseline, but root-mean-square amplitude during hypoxia (all-times) was significantly lower than normoxia (0.38 ± 0.19 vs 0.41 ± 0.17 mV). Comparative frequency analysis of the percentage change in pre- to post-exercise EMG area, at low 1-29 hz (type-1 fibre) and high 75-100 hz (type-2 fibre) areas, revealed a significant reduction in type-1 fibre activity relative to type-2, by 20-30% across time and by 10% in type-1 activity between conditions.

CONCLUSION

Exercise in hypoxia appeared to cause a temporary increase in central sympathetic nervous system activity and greater recruitment of type-2 muscle fibres, with accompanying reduction in type-1. Acute hypoxia may stimulate type-2 fibre conditioning.

Efficacy of resistance training in hypoxia on muscle hypertrophy and strength development: a systematic review with meta-analysis

A systematic review and meta-analysis was conducted to determine the effects of resistance training under hypoxic conditions (RTH) on muscle hypertrophy and strength development. Searches of PubMed-Medline, Web of Science, Sport Discus and the Cochrane Library were conducted comparing the effect of RTH versus normoxia (RTN) on muscle hypertrophy (cross sectional area (CSA), lean mass and muscle thickness) and strength development [1-repetition maximum (1RM)]. An overall meta-analysis and subanalyses of training load (low, moderate or high), inter-set rest interval (short, moderate or long) and severity of hypoxia (moderate or high) were conducted to explore the effects on RTH outcomes. Seventeen studies met inclusion criteria. The overall analyses showed similar improvements in CSA (SMD [CIs] = 0.17 [- 0.07; 0.42]) and 1RM (SMD = 0.13 [0.0; 0.27]) between RTH and RTN. Subanalyses indicated a medium effect on CSA for longer inter-set rest intervals and a small effect for moderate hypoxia and moderate loads favoring RTH. Moreover, a moderate effect for longer inter-set rest intervals and a trivial effect for severe hypoxia and moderate loads favoring RTH was found on 1RM. Evidence suggests that RTH employed with moderate loads (60-80% 1RM) and longer inter-set rest intervals (≥ 120 s) enhances muscle hypertrophy and strength compared to normoxia. The use of moderate hypoxia (14.3-16% FiO2) seems to be somewhat beneficial to hypertrophy but not strength. Further research is required with greater standardization of protocols to draw stronger conclusions on the topic.

The Effect of Normobaric Hypoxia on Resistance Training Adaptations in Older Adults

Allsopp, GL, Hoffmann, SM, Feros, SA, Pasco, JA, Russell, AP, and Wright, CR. The effect of normobaric hypoxia on resistance training adaptations in older adults. J Strength Cond Res XX(X): 000-000, 2020-The effect of normobaric hypoxia on strength, body composition, and cardiovascular fitness was investigated after a resistance training intervention in older adults. A single-blinded, randomized control trial recruited 20 healthy adults aged 60-75 years for an 8-week resistance training intervention in normoxia (n = 10) or normobaric hypoxia (14.4% O2; n = 10). Subjects performed 2 sessions per week of upper-body and lower-body exercises at 70% of 1 repetition maximum (1RM). Pretraining and post-training, maximal oxygen uptake (V[Combining Dot Above]O2max), muscular endurance (30 maximal knee flexions/extensions), and 5RM were assessed, with 5RM used to calculate 1RM. Subjects underwent whole-body dual-energy x-ray absorptiometry (DXA) at pretraining and post-training for fat and lean mass quantification. Significance was set at p < 0.05. Subjects in both groups substantially improved their calculated 1RM strength for leg extension, pectoral fly, row, and squat (normoxia; 30, 38, 27, and 29%, hypoxia; 43, 50, 28, and 64%, respectively); however, hypoxia did not augment this response. Hypoxia did not enhance V[Combining Dot Above]O2max or muscular endurance responses after the training intervention, with no improvements seen in either group. Fat mass and lean mass remained unchanged in both groups after the intervention. In summary, 8 weeks of resistance training in hypoxia was well tolerated in healthy older adults and increased upper-body and lower-body strength. However, the magnitude of strength and lean muscle improvements in hypoxia was no greater than normoxia; therefore, there is currently no evidence to support the use of hypoxic resistance training in older adults.