Resistance Training Using Different Hypoxic Training Strategies: a Basis for Hypertrophy and Muscle Power Development

Effect of Low-Load Resistance Training on Hypertrophy in Conditions of Hypoxia

Several interesting studies have been analyzed in order to highlight the effect of low-load resistance training in hypoxia on strength and muscle growth (see Table 1). Additional muscle size gains of 3.2% were detected by Manimmanakorn et al. [21, 22] after 5 weeks of low-intensity resistance training under intermittent normobaric hypoxia (SaO₂ of 80%, ~4000 m asl) when compared with normoxia (3 sets of the repetitions to failure at 20% 1RM, 30 s of rest between sets). Traditionally, multiple sets of loads over 65–70% of 1RM were considered necessary for significant hypertrophy [8]. However, current trends suggest that low-load resistance training is also able to induce muscular hypertrophy through mechanisms not related to mechanical stress. In this case, muscle growth is highly dependent on metabolic factors, and training sets should be conducted to failure [27]. These two points must be considered in both normoxic and hypoxic conditions despite the fact that there is an accelerated build-up of metabolites observed in hypoxia during resistance training with moderate loads [17, 27–30], but not with low loads [51]. For instance, similar lactate and anabolic hormone responses were observed for squat exercises (5 sets of 15 repetitions at 30% 1RM, 90 s of rest) in conditions of normoxia, as those for systemic hypoxia (15% of the inspired oxygen fraction [FiO₂]) [51]. For this reason, performing a high number of repetitions in each training set is an important detail that could explain the positive results of Manimmanakorn et al. [21, 22] given the absence of strength and hypertrophic changes in other studies under similar conditions [14]. Manimmanakorn et al. [21, 22] compared three experimental situations: normoxia, normobaric hypoxia, and BFR, and participants were instructed to match the repetitions performed by the BFR group to ensure equal training loads between groups (~28 and 36 repetitions in knee flexion and extension respectively). However, Friedmann et al. [14], when comparing the effect of 4 weeks of resistance training in normoxia and normobaric hypoxia (FiO₂ = 12%; ~4500 m asl), used a fixed number of sets of repetitions in the same exercises (6 sets of 25 repetitions at 30% 1RM, 60 s of rest between sets). Additionally, the 30-s difference in rest periods between these studies could also have had a favorable influence on the Manimmanakorn et al. exercise routine as shorter recovery periods may heighten the metabolic stimulus to enhance the anabolic response [13]. Conversely, in these three studies [14, 21, 22], endurance-force was improved in hypoxia despite the exposure to lower SaO₂ than in hypoxic dose recommendations (<3500 m or SaO₂ >80%) [50], probably due to improved metabolic efficiency [22].

The Effect of Moderate-Load Resistance Training on Hypertrophy in Conditions of Hypoxia

When analyzing research conducted by using moderate resistance training programs (65–80% 1RM), three of the four available studies (see Table 1) display results which do not reveal a clear effect of systemic hypoxia on muscle strength and hypertrophy with respect to the same training in normoxic conditions. Accordingly, the influence of acute exercise-induced endocrine responses in muscle growth has recently been questioned, and may not have the expected anabolic effects in healthy subjects [52]. This contrasts with the evidence described in other studies in which a hypertrophic resistance training program produced strength gains [31], and muscle cross-sectional area (CSA) increases driven by the growth hormone response [26].

Compared to normoxic conditions, Nishimura et al. [31] observed that 6 weeks of resistance training (4 sets × 10 repetitions at 70% 1RM; 1 min rest) under normobaric hypoxic conditions (16% FiO₂) improved arm strength levels (66 vs 48%, non-significant), and a change in CSA in hypoxia was shown at early stage throughout the training period (1.3–1.9% increase in CSA) in untrained subjects. Similar results were reported by Kurobe et al. [17], who associated the better results of 8 weeks of resistance training of the arms (3 sets of 10 1RM; 1 min of rest) while breathing a reduced O₂ air (13% FiO₂) with higher growth hormone secretion. This is consistent with the greater blood lactate and anabolic hormone responses observed by Kon et al. [28] after a bench press and leg press resistance training session (5 sets of 14 repetitions at 50% 1RM; 1 min rest) in NH conditions of 13% of FiO₂ despite the fact that the load used was just below the low-limit threshold of what is considered a “moderate-load” (but nevertheless above what is considered a lower load). No differences between conditions in the theoretical maximal strength were reported by Kurobe’s team, but 1RM estimates in high repetition tests to failure (10RM, ~75% 1RM) may be unreliable due to fatigue and mechanical stress [53, 54]. Contrary to these findings, Ho et al. [15] concluded that 6 weeks of short-term resistance training (3 sets of 10RM; 2 min rest) under NH (15% FiO₂) offered no additional benefit to muscular performance or body composition. Likewise, Kon et al. [16] observed no strength or muscle size gains in response to bench press and leg press (16 sessions of 5 sets × 10 repetitions at 70% 1RM; 90 s of rest) though they did detect enhanced skeletal muscle endurance and angiogenesis.

Inconsistencies Among Studies of Hypertrophic Resistance Training Effects Mediated by Hypoxia

The lack of consensus among reported studies may be attributed to differences in protocols such as training intensity (light or moderate loads), number of sets (3 to 6), rest period between sets (30 to 120 s), muscles worked (arms, legs, chest), training program duration (4 to 8 weeks), and severity of hypoxia (from 12 to 16% FiO₂). Additionally, all studies were performed in untrained subjects and the muscle stimulus employed was lower than recommended for hypertrophy [8, 55]. In untrained subjects, neural modifications start during the early stages of training [14, 56] with an individual long-phase that could, in part, explain the different strength and/or muscle size gains observed among studies.

In the reviewed studies, hypoxia levels were performed at simulated hypoxia which ranged from 12 to 16% FiO₂, which although being at or slightly above the limit (~12% FiO₂) is not likely to produce discrepancies. During 3.5 h following moderate-intensity resistance training (6 sets of 8 repetitions at 70% 1RM) under acute severe hypoxia (~4300 m asl, 12% FiO₂), a relationship exists between protein synthesis rate and arterial oxygen saturation (SaO₂) (r ² = 0.49, p = 0.04). This means that for SaO₂ <80% or altitudes >3500 m, hypoxia may delay the anabolic response to resistance training compared to normoxia, although after 3.5 h, responses should be comparable [50]. No data concerning the effect of hypertrophic resistance training at terrestrial altitude are available, so differences between simulated and terrestrial hypoxia cannot be reported. At high or moderate altitude, other factors could also promote muscular changes over longer periods, such as the following: (1) reduced food intake (10–50%) linked to a loss of appetite [57] and/or change in diet; (2) increased energy expenditure due to a higher basal metabolic rate [58] and/or physical activity not matching energy intake; (3) dehydration [59]; and (4) absence of load adjustment during hypertrophic resistance training at altitude. Indeed, the reduction of the training load stimulus to the muscle during altitude training can be considered as one of the mediating factors related to the loss of muscle mass traditionally linked to an altitude camp. This can occur when the training load used in normoxia is maintained during the altitude stage. Therefore, if the 1RM absolute load improves by HH and the resistance training load is not adjusted accordingly, the stimulus to the muscle during the resistance altitude training will be reduced. In this context, Feriche et al. [40] describe a concentric bench press 1RM improvement at acute terrestrial altitude (~5.6%; ES = 1.1) with respect to the change observed in normoxia and NH. Similar results have also been described in the half squat [41]. However, there are no data from any longitudinal study at terrestrial altitude.

Finally, another point of consideration is that the minimum time needed to detect significant hypertrophic muscular changes in athletes is around 8 weeks [9]. The research revised deals with study periods ranging from 4 to 8 weeks, which could contribute to the discrepancies observed among the results, as well as limiting the use of terrestrial altitude. Changes in muscle size and strength resulting from resistance training during an extended period at real moderate altitude still require clarification. Moreover, real moderate altitude training camps normally last 3 weeks, which is an insufficient time to achieve the target, although the hypertrophic training must be carefully adapted to avoid the undesired results previously mentioned. For this reason, when pursuing hypertrophy, longer hypoxic training programs which simulated normobaric hypoxia is generally selected, usually involving IHRT sessions.

According to Scott et al. [13], more control research is needed to evaluate the real influence that hypoxia constitutes in encouraging hypertrophy and strength gains. The potential effect of hypoxia on growing muscle mass, specially by the marked metabolic stress that the exercise performed under hypoxic conditions causes, is well documented [27, 28, 30] although questioned in some contexts by the lack of positive results [14–16]. This information should be considered in future investigations conducted in hypoxia and take into account previous conclusions about how to apply hypertrophic methods [8, 9], type and severity of the hypoxia [11, 12, 50], influence of the training level on the sample [14, 56], and recommended length of the intervention [9]. Moreover, the ideal strategies for resistance training during altitude camps and other types of interventions combining terrestrial altitudes should also be explored.

Terrestrial or Simulated Altitude: Effect on Explosive Muscle Performance

Despite the reduced O₂ content of air during a terrestrial or simulated exposure to hypoxia, differences in barometric pressure can also affect performance in repeated (IHRT) or isolated high-speed explosive actions. In 28 combat sport athletes divided into two homogeneous groups, F-V curves in the bench press were compared between N, terrestrial moderate altitude (HH 2320 m asl), and normobaric hypoxia (NH 15.7% FiO₂) [40]. Results indicated a marked effect on the F-V curve of acute HH compared to negligible effects of N and NH. Acute HH led to a 3.2% mean increase in the load linked to mean maximal power, along with clear improvements in mean power, peak power, and peak velocity for the same absolute load. Hypobaric hypoxia also accounted for a 6% increase in 1RM after the ascent. This could be considered unsurprising given the confirmed relationship between mean velocity and weight lifted according to percentage 1RM [66–68], and the fact that this velocity is improved at terrestrial altitude [40, 42]. Using as a reference the 1RM recorded for N, the mean power curve for HH was shifted upwards and to the right, indicating that mean power would be overestimated for loads ≥60% of 1RM, compared to the curve obtained using as reference the corresponding 1RM recorded for HH [40]. The lack of power output change in NH concurs with the findings of Scott et al. [44]. Power and force trends over 5 sets of 5 repetitions at 80% of 1RM for acute moderate and high NH (13 and 16% FiO₂) failed to vary from trends recorded in normoxic conditions.

According to other findings, and with evidence of physiological and metabolic responses induced by acute NH exercise (i.e., cardiovascular and hormonal [17, 28, 30, 44]), velocity, power, and maximum dynamic strength after basic strength exercises show benefits from HH which are not found in N or NH [40–42]. A relationship has also been identified between metabolic stress induced by H⁺ elevation because of low SaO₂ and the recruitment of fast twitch muscle fibers [27]. While abnormalities in muscle electromyographic activity have been observed in conditions from acute hypoxia (~3500 m asl; 13% FiO₂; [69]), moderate altitudes do not lead to these detrimental effects [70]. Additionally, electromyographic activity at NH has been shown to be similar to that at normoxia during maximal voluntary contractions and power output [22]. For isolated short-burst actions involved in F-V curves (~5 s plus 3–5 min rest), hypoxic benefits to performance were not observed in moderate or high NH (16–13% FiO2) [40, 44], and only improved at terrestrial altitude [40]. This challenges the idea that the breathing of air impoverished in O₂ is solely responsible for inducing a switch from type I to II fibers [22] making the movement faster due the intrinsic capacity that larger motor neurons have to drive the impulses at higher speeds. It is likely that differences between results obtained at NH and HH seem mediated by other factors and/or interactions not yet investigated.

Álvarez-Herms et al. [71] did not observe any change in the height achieved during an isolated free squat jump (SJ) and countermovement jump (CMJ) after 4 weeks of endurance resistance training in simulated hypobaric hypoxia of 2500 m asl (n = 6 men and 1 women) and normoxic conditions (n = 3 men and 2 woman). This result is not surprising since the training was oriented to endurance. In contrast, a recent study conducted on 18 young male swimmers of a junior national team found mean peak power and peak velocity improvements of 12.1 ± 1.8% and 6.6 ± 1.2%, respectively, for loaded SJ after ascending to a terrestrial moderate altitude [42]. This study also demonstrated the persistence of altitude-induced improvements in jump performance after 2 weeks of exposure to real moderate hypoxia, showing mean improvements in both variables of 7.8 and 4.4%, respectively. Moreover, significant correlations between the percent change in jump height and the percent change in swimming start performance were also obtained following a short-term training program of 17 days [72]. These three studies show that moderate exposure to real or simulated hypobaric hypoxia does not impair the ability to apply force rapidly [42, 71, 72], and this capacity is likely to improve with specifically target-oriented training [42, 72]. In accordance with this hypothesis, it has been recently observed that 3 weeks of training high-living high at 2320 m asl does not produce adverse effects on muscular function in elite swimmers, even if the training is not solely focus on improving force and power [73]. In this study, the same group of swimmers were compared before and after 3 weeks of training at sea level at moderate altitude. Evaluations were separated by a 1-year period, although the intervention period, training targets, and relative loads were maintained. No changes in swimming start times were observed after the altitude period, while an impairment was registered at sea level. No differences between conditions were obtained during the loaded SJ performance despite a slight improvement in peak velocity after both training periods (Table 3).

The approximate 22.9% difference in air density at moderate altitude (~3% reduction for each 305 m rise; [5]) could contribute to making the movement faster than at normal altitudes. More studies are needed in order to analyze if an interaction exists between air pressure and composition, as well as examining the effects of longer training periods in these conditions. Researchers have identified [14, 74–76] and ruled out [77–79] differences in the physiological responses to exercise when comparing NH and HH. Millet et al. [76] reported power output improvements of 4.0 and 4.2% for elite and non-elite athletes, respectively, in conditions of HH vs 0.6 and 1.4% for NH. Bonetti and Hopkins [80] described increased ventilatory responses, changes in fluid balance, and nitric oxide metabolism, along with changes in the severity of acute mountain sickness and altered performance for HH compared to those for NH. However, these studies assessed the effects of chronic hypoxia, and no explanation has yet been offered for muscle power differences related to acute and longer periods spent at altitude.

Inconsistencies Among Studies Conducted into the Effect of Hypoxia on Muscle Power Resistance Training

The influences of whatever type of hypoxia on muscle-specific performance in sport have not been thoroughly examined. While its causes remain unclear, we have identified a mechanism which could positively affect the performance of and training for isolated explosive actions at terrestrial moderate altitude. Muscle force or power development in hypoxic conditions is not a variable that is commonly assessed in scientific publications. From the nine studies included in this part of the review, five analyze the effect of acute exposure and contain measurements at different severities (FiO₂ from 16 to 13%) and for different types of hypoxia (systemic vs terrestrial altitude). They examine the effect of various types of exercises (jumps, squat, deadlift, or bench press) and include subjects of differing sporting ability (elite vs non-trained). The remaining studies are conducted at real or simulated hypobaric hypoxia of around 2400 asl and analyze the effect of different types of resistance training on free or loaded jumps (SJ and CMJ). The presence of a control group, group size, sex distribution in groups, training level, or description of the training and assessment process are some of the differences observed among the abovementioned longitudinal studies. Finally, training orientation is also different among the studies. While Álvarez-Herms et al. [71] described resistance endurance training to improve anaerobic power during multiple jumps, García-Ramos et al. [42, 72] did not implement the study with specific tasks. The authors did however indicate that six of the ten resistance training exercises were oriented to strength-power training in the legs [42, 72]. Additionally, the concurrent strength and endurance training used in the García-Ramos et al. [73] study corroborate that an excessively oriented training aiming to improve endurance capacity attenuates strength training responses [81], even after an altitude training camp of 3 weeks [73, 82].

Controlled and power-oriented resistance training studies are clearly needed to analyze the effect of intermittent or sustained altitude exposure on power training.

In Hypertrophy

The evidence for greater muscle strength gains and structural physiological changes in response to resistance training under conditions of hypoxia is not conclusive. This is because although the balance of results tends to favor training in hypoxia, only one study revealed significant differences in performance between resistance training in normoxia and training in hypoxia.

Currently, the definitive mechanisms that may augment muscular responses to hypertrophy resistance training under hypoxic conditions are not yet fully understood. However, despite a need for further research, it may be reasonably suggested that (1) metabolite build-up during low-intensity (≤30% 1RM) resistance exercise may be intensified in hypoxia; (2) greater and faster changes may occur in hypoxia when multiple sets of 6–12 repetitions at moderate load (≥65% 1RM) are performed; and (3) the recommended simulated hypoxia level for all training modalities is moderate (13–16% FiO₂).

Preliminary studies seem to indicate that hypertrophy-oriented training conducted under conditions of intermittent hypoxia could promote more favorable physiological and functional changes than under chronic exposure. Terrestrial or simulated living low-training high strategies seem to benefit anabolic responses.

Issues related to nutrition and hydration, as well as the adjustment of the training load due to the possible influence of an ascent in altitude on the 1RM estimation (to avoid reducing muscle stimulus and muscle mass), should also be taken into account when spending long periods at altitude.

In Muscle Power Development

Ascent to altitude leads to velocity and power improvements although the mechanisms that promote the benefit of this type of hypoxia with respect to the NH still require clarification. Athletes should not be excessively concerned about the deterioration of muscular function when they take part of a 2–3-week training period at moderate altitude, even if the training is not strongly oriented to force and power development.

The following points should be considered in an altitude power-oriented training program: (1) loads used for power training under normal conditions should not be literally translated to training programs performed at higher altitudes. This is especially relevant because of the importance of locating and assessing the optimal muscular load for power training programs; (2) load adjustments during resistance training sessions at terrestrial altitude (according to the altitude 1RM) avoid reducing the muscle stimulus and/or inter- and intra-muscle coordination that commonly occurs after periods of altitude training; (3) F-V curves emerging from the different studies, despite involving different resistances, correspond to exercise volumes that do not induce local metabolic fatigue and could thus compromise muscle contractile properties. Protocols with inter-repetition or intra-set rest periods (cluster training) might therefore be more suitable for hypoxic resistance training focus on this topic.

Unlike simulated hypoxia, terrestrial altitude conditions seem to improve the ability to perform high-speed actions with moderate loads. Thus, training under these conditions could serve to improve velocity and technical skills in power-related sports.