Resistance Training Using Different Hypoxic Training Strategies: a Basis for Hypertrophy and Muscle Power Development
Feriche et al. Sports Medicine - Open
Resistance Training Using Different Hypoxic Training Strategies: a Basis for Hypertrophy and Muscle Power Development
Belén Feriche 0
Amador García-Ramos 0
Antonio J. Morales-Artacho 0
Paulino Padial 0
Key Points 0
0 Department of Physical Education and Sport, Faculty of Sport Sciences, University of Granada , Crta Alfacar sn, 18011 Granada , Spain
The possible muscular strength, hypertrophy, and muscle power benefits of resistance training under environmental conditions of hypoxia are currently being investigated. Nowadays, resistance training in hypoxia constitutes a promising new training strategy for strength and muscle gains. The main mechanisms responsible for these effects seem to be related to increased metabolite accumulation due to hypoxia. However, no data are reported in the literature to describe and compare the efficacy of the different hypertrophic resistance training strategies in hypoxia. Moreover, improvements in sprinting, jumping, or throwing performance have also been described at terrestrial altitude, encouraging research into the speed of explosive movements at altitude. It has been suggested that the reduction in the aerodynamic resistance and/or the increase in the anaerobic metabolism at higher altitudes can influence the metabolic cost, increase the take-off velocities, or improve the motor unit recruitment patterns, which may explain these improvements. Despite these findings, the applicability of altitude conditions in improving muscle power by resistance training remains to be clarified. This review examines current knowledge regarding resistance training in different types of hypoxia, focusing on strategies designed to improve muscle hypertrophy as well as power for explosive movements.
Altitude training is frequently part of an elite athlete’s
exercise program. By inducing tissue hypoxia due to a
lower arterial partial pressure of oxygen (PO2), altitude
training causes a physiological response that affects
performance. Traditionally, the ascent to a higher altitude
has been associated with impaired endurance
performance [1, 2]. However, when remaining at altitude,
changes in the body systems involved in aerobic energy
supply seem to elicit beneficial chronic adaptations
improving performance [3, 4]. Conversely, non-oxidative
metabolism-dependent, short-lasting activities (under
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1 min) seem to offer immediate benefits when
performed in altitude conditions [1, 5].
A hypoxic environment appears to create advantageous
responses in the development of muscle performance with
increased hypertrophy and gains in both muscle strength
and speed of explosive movements. Despite the importance
of resistance training to performance, the muscle response
under conditions of hypoxia has not been studied in detail.
Very few studies have evaluated the effect of induced
hypoxia on anaerobic and aerobic metabolism or the capacity
for recovery from different work/rest ratios in repeated
exercise training [6, 7], as used during resistance training.
The combination of the load, sets, repetition of sets, rest,
and speed of movement are key factors in understanding
the physical and functional muscle changes derived from
specific resistance training, as well as the influence that
“hypoxic conditions” could have on the results. For
hypertrophy and strength gains, when looking for increased
muscular cell swelling and metabolite accumulation,
traditional methodology usually combines 6–12 sets of 8–12
repetitions at low velocity with loads of 65–80% of 1
repetition maximum (1RM) and 1–3 min of rest in between sets
[8, 9]. For muscle power resistance training, geared to a
neuromuscular goal and avoiding metabolic fatigue,
sessions usually combine 4–6 sets of 4–6 repetitions with
loads of 0–50% 1RM for ballistic exercises  and 3–
5 min of rest (sometimes this method includes
interrepetition rests of 10–60 s, cluster training).
Given different both resistance training purposes, and
considering the limited number of studies that have
examined muscular adaptation and performance under hypoxic
conditions, it is necessary to analyze the differences in the
experimental designs, participant training levels, and the
type and severity of hypoxia before drawing conclusions.
In the following section, muscle hypertrophy and power
trainability under hypoxic conditions will be reviewed in
order to clarify the consistency of the results available.
Throughout the different sections of this review,
different types of hypoxia and hypoxic training strategies will be
referred to. Current training trends in hypoxia combine
different types and dosages of hypoxia (H) resulting in
numerous possible combinations [11, 12]:
Hypobaric hypoxia (HH), or altitude, produces the
hypoxic effect (decrease the availability of oxygen in
the tissues) by reducing the barometric pressure,
mainly by ascent to altitude or by using hypobaric
chambers. The reduction in barometric pressure
reduces air resistance to the movement as a result of
lower air density. An ascent to altitude also results
in reduced air temperature and humidity.
Normobaric or systemic hypoxia (NH) provides the
hypoxic effect by reducing the oxygen pressure in the
inspired air (nitrogen dilution or oxygen filtration).
In this sense, natural or artificial methods of hypoxia can
be used for training, resulting in strategies such as live
high-train high (LHTH), live high-train low (LHTL), or live
low-train high (LLTH) among others. Intermittent hypoxic
devices are also used during training sessions (IHT) or
resting periods. Training at low altitude allows the athlete
to maximize performance by maintenance of sea-level
training intensity and oxygen flux . Extensive literature
relating to the use of these combinations in endurance
training exists, although its application in resistance
training is mainly limited to the use of intermittent artificial
hypoxic exposure while resting under normoxic conditions
(LLTH) to increase hypertrophy, also called intermittent
hypoxic resistance training (IHRT) .
A Description of the Mechanisms and Metabolic
Factors Related to the Hypertrophic and
Functional Muscle Response in Acute and Chronic
Several studies have tested the degree to which hypoxic
conditions are able to produce beneficial changes to
muscle strength and hypertrophy [14–17]. These studies
follow on from previous research into the apparent
beneficial effects on muscle hypertrophy and strength gains of
low resistance training (20–50% 1RM) when combined
with blood flow restriction (BFR) in different subject
populations [18–24]. The main mechanisms proposed for these
improvements relate to responses to the metabolite
buildup [25–27] (Fig. 1). Then, moderate intensity resistance
training under hypoxic conditions enhances
exerciseinduced metabolic stress mechanisms (anabolic hormones,
cytokines, reactive oxygen species, and oxidative stress
factor, among others), which clearly have an important role in
muscle growth [17, 27–30]). Accordingly, several studies
have been conducted in hypoxia aiming to achieve strength
and hypertrophic benefits [14, 17, 21, 22, 30, 31].
On the other hand, improvements in sprinting, throwing,
and jumping at altitude have been reported [1, 2, 5, 32]. To
date, researchers have mainly used the hypothesis of
reduced energy costs to account for improved isolated
high-speed actions  through a reduction in
aerodynamic resistance in approximate proportion to the
square of the velocity (e.g., when cycling, running, or
throwing objects) [1, 5, 32]. However, although the
underlying mechanisms are not completely defined, modified
motor unit recruitment patterns due to increased anaerobic
metabolism release [34, 35] could also partly explain these
improvements. Furthermore, it could be considered that if
this occurs in isolated movements, some benefit could also
be obtained during a prolonged period of specific explosive
resistance training at altitude, opening up a new line of
investigation that considers the effects of both air
composition and its resistance.
Fig. 1 Hypertrophy mechanisms mediated by metabolites
Up until now, the impact of whatever type of hypoxia on
muscle performance and the biomechanics of specific
movements in sport has not been examined in depth.
Alterations in the biomechanical and neuromuscular
components associated with force production are some of the
factors recently suggested by Chapman et al.  that
influence changes in performance following altitude training.
According to this, the improvement in speed movement
can also be attributed to an enhanced firing frequency of
motoneurons and spinal reflexes. Acute simulated hypoxia
has been related to the increase of the spinal excitability
. Additionally, Tomazin et al.  recently observed a
greater increase in the H-reflex amplitude of the soleus
muscle at a terrestrial altitude of 2320 m when compared
to that at sea level (~35%; p < 0.05), a response that could
be linked to a direct effect of hypoxemia on the
supraspinal structures. There may, however, be further
contributory mechanisms, such as increased anaerobic metabolism
[27, 34, 35] and/or reduced air density, as mentioned
above , that influence muscle contraction properties
and thus improve explosive speed [1, 5] (Fig. 2). In fact,
breathing hypoxic gas mixtures while training seems to
display a tendency for fast fiber areas to increase in size
following training compared to breathing normoxic air
 that could be also linked to a greater fast fiber
recruitment. Accordingly, other studies have revealed maximal
power gains during a force-velocity (F-V) curve [40–43],
as well as 1RM gains [40–42], after acute exposure to real
moderate altitude in bench press, back half squat, and
squat jump exercises. However, Scott et al.  detected
no changes in force and power performance during back
squat and dead lift exercises (80% 1RM) at simulated
moderate and high altitudes.
Hypertrophy Trainability in Conditions of Hypoxia
Scott et al.  reviewed the hypothetical benefits on
muscle strength and muscle size of intermittent hypoxic
resistance training compared to BFR. Resistance training
under conditions of systemic hypoxia (NH; achieved by
nitrogen dilution, oxygen extraction in an environmental
chamber or wearing a face mask) could be considered as
an alternative to BFR while avoiding problems such as
restricting training to only the limbs, pain, petechial
hemorrhage, and dizziness related to the use of
compression cuffs . As has been mentioned, moderate
intensity resistance training under hypoxic conditions
enhances the build-up of metabolites, which clearly have
an important role in muscle growth [17, 27–30].
However, the literature does not provide data addressing the
effect of a resistance training period at terrestrial
altitude, and conclusions obtained during IHRT in
normobaric hypoxia are not clear. Gains in muscle mass and
strength after a hypoxic training period display no
consistent differences when compared with those observed
in normoxia (Table 1).
The effect of the hypertrophic resistance training at
terrestrial altitude on muscle mass has not been studied
in detail. Early studies associated the ascent and
extended periods spent at high altitude (>5500 m) with up
to a 15% muscle mass loss  and reduced strength
gains (−6.4%) compared to normoxia . Explanations
for this have included altitude-induced protein synthesis
rate reduction [47, 48] or increased protein degradation
during exercise , leading to a negative
synthesis/degradation of protein balance. Nevertheless, training camps
are usually held at moderate altitudes (1800–2500 m),
while no data are reported in scientific literature about
hypertrophic resistance training during intermittent or
chronic periods at terrestrial altitude; protein
metabolism seems to be unaffected by O2 availability at
moderate simulated altitudes in acute NH .
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
lowintensity 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 . 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 . 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 . 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]) . 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 . 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. , 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 .
Conversely, in these three studies [14, 21, 22],
enduranceforce was improved in hypoxia despite the exposure to
lower SaO2 than in hypoxic dose recommendations
(<3500 m or SaO2 >80%) , probably due to improved
metabolic efficiency .
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 . This contrasts with the evidence described
in other studies in which a hypertrophic resistance
training program produced strength gains , and
muscle cross-sectional area (CSA) increases driven by
the growth hormone response .
Compared to normoxic conditions, Nishimura et al.
 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. , 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.  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.  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.  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) (r2 = 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 . 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 
and/or change in diet; (2) increased energy expenditure
due to a higher basal metabolic rate  and/or physical
activity not matching energy intake; (3) dehydration
; 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.  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 . 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 . 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
According to Scott et al. , 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 . Moreover,
the ideal strategies for resistance training during altitude
camps and other types of interventions combining
terrestrial altitudes should also be explored.
Muscle Power Trainability in Conditions of
The influence of altitude or hypoxic conditions on
muscular function during muscle power resistance exercises has
not been examined in detail. Exercise-induced fatigue may
have a central element which may or may not include a
peripheral cause. Although the direct impact of hypoxia
on the brain cannot be ruled out , sensory feedback of
metabolite accumulation due to lower O2 availability (such
as H+ or Pi) may explain why the central command and
power output in hypoxia is reduced . Literature
describes a direct but moderate influence of the inspired O2
fraction on the central nervous system. This conclusion
was reached by Millet et al.  after studying the
response of intermittent isometric unilateral knee extensions
to failure with and without blood flow restriction (BFR,
via a cuff ), in N and while breathing a reduced O2 air (NH
of 11% FiO2 and 84% SaO2). Both, hypoxia and the
occlusion cuff, affected the number of repetitions. However,
considering the muscle similarly affected in the two
conditions, performance was slightly but significantly lower
during NH than in N with cuff on. The design used in this
study leads us to conclude that systemic hypoxia has a
direct influence on the central drive, independent of the
factors developed within the working muscles. During severe
levels of hypoxia, the type of muscle contraction or the
total muscle mass involved in exercise can also limit the
influence of this mechanism .
Perrey and Rupp  reviewed studies analyzing the
effects of acute or prolonged exposure to terrestrial high
altitude (≥3700 m) on the contractile properties of the
muscle. Under conditions of acute altitude, muscle
function was altered after intermittent contractions, while
modifications to chronic hypoxia seemed to minimize
the effect on skeletal muscle function. Impaired muscle
function in response to high altitude was especially
appreciable during exercise protocols involving prolonged
isometric muscle contractions (<30% of the maximal
voluntary contraction), and during repeated submaximal
intermittent contractions, both of which depend
principally on systemic O2 transport . Besides muscle
deterioration due to high altitude, reduced muscular power
has also been described .
In contrast, terrestrial moderate altitude throws up
different results. Two previous studies reviewed the effect of
altitude on elite athletes’ performance in sprints from 100
to 400 m [1, 2], and throwing and jumping performance
. As was predicted by mathematical models ,
enhanced sprint performances (0.2–0.7%), the hammer throw
and triple and long jump (~1%) are described at altitudes
above 1500 m . Nowadays, there is no doubt about the
benefit of terrestrial altitude on these athletic disciplines.
Nevertheless, some authors suspect that the slower
takeoff speeds in high jump, or the slower velocity in the
hurdle race (i.e., from 10.3 to 8.5 m s−1 in 100 m sprint and
110 hurdles, respectively, in men), reduces the influence of
aerodynamic drag and it is this that accounts for the
minimal effect on performance between altitudes . However,
despite the greater difference in speed compared with
sprints, jumps, and throws, the changes described in
muscle power and velocity during isolated resistance
exercises (back squat, bench press or squat jump) from the first
hours following ascent [40–43] indicate that some kind of
relationship between HH and muscular function must
exist, independently of the changes in speed linked to air
density (Tables 2 and 3).
Very frequently, power and velocity assessed during
the F-V curves are measured by means of linear
transducers attached to the bar, making it difficult to identify
if athletes applied more force at altitude or if the results
are produced by lower resistance to movement. To analyze
this, García-Ramos et al.  examined the leg extensor
muscle response using a F-V curve and unloaded jumps
H H ca
2 R G G (1
d ,y ch
(CMJ and SJ) in 17 elite swimmers before and just after an
ascent to terrestrial moderate altitude. The novel aspect of
this study is that during the F-V curve, the maximum
values of force and velocity at each load were recorded by
a force platform and a linear velocity transducer
respectively. Thus, these variables were modeled by a linear
regression [F(V) = F0 aV], where F0 (force intercept at zero
V), V0 (velocity intercept at zero force), and maximum
power output (P0 = F0V0/4) were considered as the maximal
mechanical capabilities of the neuromuscular system to
generate force, velocity, and power, respectively . The
results revealed higher magnitudes in P0 (+6.79%; p < 0.01)
of the leg extensors at altitude, which were linked to an
increase in the V0 (+7.60%; p < 0.05), while no changes for
the F0 (+0.02%) were achieved. In addition, the results for
unloaded jumps performed on a force platform showed a
clear tendency towards improvements in the amount of
force applied when performed at altitude, with jump height
increasing by an average of 3.4%. These results highlight
the influence that the aerodynamic drag forces could have
on velocity and show a clear altitude effect on the F-V
relationship at the same absolute load [40, 42, 43], sustaining
the hypothesis that the hypobaric hypoxia and muscular
function relationship must, at some point, converge in
addition with the additional benefit produced by the lower
aerodynamic resistance on isolated explosive movements.
Terrestrial or Simulated Altitude: Effect on Explosive
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) . 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 . The lack of power output
change in NH concurs with the findings of Scott et al.
. 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
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 . While abnormalities in muscle
electromyographic activity have been observed in conditions from
acute hypoxia (~3500 m asl; 13% FiO2; ), moderate
altitudes do not lead to these detrimental effects .
Additionally, electromyographic activity at NH has been shown
to be similar to that at normoxia during maximal voluntary
contractions and power output . For isolated
shortburst 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 . 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 
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.  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 .
This study also demonstrated the persistence of
altitudeinduced 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
. 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
targetoriented 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 . 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;
) 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.  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 
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
musclespecific 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.  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.
 study corroborate that an excessively oriented
training aiming to improve endurance capacity attenuates
strength training responses , 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.
Current evidences suggest potentially promising
applications of hypoxia for muscle hypertrophy and power
training. Nevertheless, there is still insufficient data on
which to base training programs. To help design altitude
training protocols, data from more specific controlled
studies are needed.
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
hypertrophyoriented 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
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
intramuscle 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
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.
This review was supported by the Spanish Ministry of Science and
Innovation (DEP2012-35774; DEP2015-64350-P [MINECO/FEDER]), Ministry of
Education, Culture and Sport.
BF conceived, designed, drafted, and revised the manuscript; contacted the
authors; and interpreted the findings; BF, AGR, AMA, and PP executed the
review, interpreted the findings, and contributed to the writing of the paper,
tables, and figures. All authors read and approved the final manuscript.
Belén Feriche, Antonio Jesús Morales-Artacho, Amador García-Ramos and
Paulino Padial declare that they have no competing interests.
Consent for Publication
Ethics Approval and Consent to Participate
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