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Recent studies have shown that low-intensity resistance training with vascular occlusion (kaatsu training) induces muscle hypertrophy. A local hypoxic environment facilitates muscle hypertrophy during kaatsu training. We postulated that muscle hypertrophy can be more efficiently induced by placing the entire body in a hypoxic environment to induce muscle hypoxia followed by resistance training. Fourteen male university students were randomly assigned to hypoxia (Hyp) and normoxia (Norm) groups (n = 7 per group). Each training session proceeded at an exercise intensity of 70% of 1 repetition maximum (RM), and comprised four sets of 10 repetitions of elbow extension and flexion. Students exercised twice weekly for 6 wk and then muscle hypertrophy was assessed by magnetic resonance imaging and muscle strength was evaluated based on 1RM. Muscle hypertrophy was significantly greater for the Hyp-Ex (exercised flexor of the hypoxia group) than for the Hyp-N (nonexercised flexor of the hypoxia group) or Norm-Ex flexor (P < .05, Bonferroni correction). Muscle hypertrophy was significantly greater for the Hyp-Ex than the Hyp-N extensor. Muscle strength was significantly increased early (by week 3) in the Hyp-Ex, but not in the Norm-Ex group. This study suggests that resistance training under hypoxic conditions improves muscle strength and induces muscle hypertrophy faster than under normoxic conditions, thus representing a promising new training technique.
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Akinobu Nishimura is with the Department of Orthopaedic Surgery, Mie University Graduate School of
Medicine, Tsu City, Mie Prefecture, Japan. Masaaki Sugita is with the Department of Health and
Physical Education, Mie University Faculty of Education, Tsu City, Mie Prefecture, Japan. Ko Kato is
with the Department of Orthopaedic Surgery, Mie University Graduate School of Medicine, Tsu City,
Mie Prefecture, Japan. Aki Fukuda is with the Department of Orthopaedic Surgery, Suzuka Kaisei
Hospital, Suzuka, Japan. Akihiro Sudo is with the Department of Orthopaedic Surgery, Mie University
Graduate School of Medicine, Tsu City, Mie Prefecture, Japan, and Atsumasa Uchida is with the
Department of Orthopaedic Surgery, Mie University Graduate School of Medicine, Tsu City, Mie
Prefecture, Japan.
Nishimura et al.
Resistance Training Under Hypoxic Conditions
Hypoxia Increases Muscle Hypertrophy
Induced by Resistance Training
Akinobu Nishimura, Masaaki Sugita, Ko Kato, Aki Fukuda,
Akihiro Sudo, and Atsumasa Uchida
Purpose: Recent studies have shown that low-intensity resistance training with
vascular occlusion (kaatsu training) induces muscle hypertrophy. A local hypoxic
environment facilitates muscle hypertrophy during kaatsu training. We postulated
that muscle hypertrophy can be more efficiently induced by placing the entire body
in a hypoxic environment to induce muscle hypoxia followed by resistance training.
Methods: Fourteen male university students were randomly assigned to hypoxia
(Hyp) and normoxia (Norm) groups (n = 7 per group). Each training session
proceeded at an exercise intensity of 70% of 1 repetition maximum (RM), and
comprised four sets of 10 repetitions of elbow extension and flexion. Students
exercised twice weekly for 6 wk and then muscle hypertrophy was assessed by
magnetic resonance imaging and muscle strength was evaluated based on 1RM.
Results: Muscle hypertrophy was significantly greater for the Hyp-Ex (exercised
flexor of the hypoxia group) than for the Hyp-N (nonexercised flexor of the hypoxia
group) or Norm-Ex flexor (P < .05, Bonferroni correction). Muscle hypertrophy
was significantly greater for the Hyp-Ex than the Hyp-N extensor. Muscle strength
was significantly increased early (by week 3) in the Hyp-Ex, but not in the
Norm-Ex group. Conclusion: This study suggests that resistance training under
hypoxic conditions improves muscle strength and induces muscle hypertrophy
faster than under normoxic conditions, thus representing a promising new training
Keywords: hypoxic conditions, resistance training, magnetic resonance imaging,
MRI, rating of perceived exertion, RPE, muscle hypertrophy
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Training under hypoxic conditions is believed to be generally useful for
improving aerobic performance by increasing erythropoietin and maximum oxygen
intake (VO2max).1 The altitude house,2 where air inside the house is aspirated to
increase the concentration of nitrogen and thus create a hypoxic state at low altitude, is
more convenient for training under hypoxic conditions than at high altitude. Training
under hypoxic conditions is currently under investigation using such facilities. On the
other hand, chronic exposure to high altitude leads to a reduction in muscle
cross-sectional area (CSA) and a decrease in the size of muscle fibers in humans.3
Hypoxia per se might be responsible for this atrophy, but malnutrition and reduced
activity levels are also possible causes of muscle loss.4–6 We assume that such muscle
loss has not been identified by studies using intermittent hypoxia in which subjects
exercise under hypoxic conditions but live in a normoxic environment between
training sessions.
The effects of resistance training under hypoxic conditions with recovery under
normoxia are mostly unknown. Takarada et al7 reported that low-intensity training
(30–50% 1-repetition maximum [1RM]) with vascular occlusion not only increases
electrophysiological activities during exercise, but also significantly increases the
CSA of the biceps brachii, brachialis and triceps brachii muscles after 16 weeks of
training. Their study showed that low-intensity resistance training with restricted
muscular venous blood flow causes muscle hypertrophy and strength gain, and they
described this process as kaatsu training. These findings suggest that restricted blood
circulation causes an intermittent hypoxic and acidic muscular environment, which
induces the recruitment of additional motor units and leads to increased muscle
hypertrophy. We postulated that muscle hypertrophy could be efficiently achieved by
resistance training under intermittent systemic hypoxia. Kaatsu training is useful for
resistance training of the extremities, but not for those of the trunk muscles because
tourniquets are required. If resistance training under hypoxic conditions leads to
hypertrophy, it would be very useful because all muscles including those of the trunk
could benefit.
We compared the induction of muscle hypertrophy by resistance training under
hypoxic and normoxic conditions using magnetic resonance imaging (MRI). Muscle
strength was also assessed based on 1RM (the maximum weight that can be lifted once
over the entire range of motion). Levels of exertion were compared between training
under hypoxic and normoxic conditions using the subjective rating of perceived
exertion (RPE) scale.
Materials and methods
Fourteen untrained male university students (age [mean ± standard deviation], 21.4 ±
1.1 y; height, 173.0 ± 5.4 cm; body mass, 65.9 ± 8.1 kg; body fat, 12.6 ± 3.7%)
provided written, informed consent to participate in the study after reviewing a
detailed explanation of its purposes and procedures. Body mass and percentage fat
were measured using a body composition analyzer (BC-118E, Tanita, Tokyo, Japan).
The students were randomly assigned to hypoxia (Hyp) and normoxia (Norm) groups
(n = 7 per group). Table 1 summarizes the physical characteristics of the participants.
Training intensity was established based on the maximum weight that could be lifted
over the entire range of motion (1RM) under normoxia. We referred to a published
protocol8 to determine 1RM from the 10RM test. Exercise intensity was set before the
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first training session and then revised 3 wk later. The participants continued with their
normal lives and were instructed not to start any new exercise during the course of the
study. The ethics review board of Mie University approved the study protocol.
\ insert Table 1 \
Constant Hypoxic Room
The constant hypoxic room (Alticube, USK, Nara, Japan) consists of a compressor, a
hypoxia generator with a polymer membrane and a hypoxic chamber covered with a
membrane separation-type vinyl tent to block air (width × length × height: 2900 ×
2000 × 2200 mm). Ambient air was compressed using the hypoxia generator and
hyperoxic air separated by the polymer membrane from hypoxic air was sent to the
hypoxic chamber to create a hypoxic environment. The oxygen concentration was
maintained at 16.0% (normal oxygen concentration, approx. 21%) during training and
room temperature was maintained at 27°C using an air conditioner for both groups.
Each participant stood holding a dumbbell in the nondominant hand and performed
standing French presses and arm curls. The Hyp and Norm groups sat inside the
hypoxic chamber under hypoxic condition or outside the chamber under normoxic
conditions, respectively, for 30 min before training. Both groups warmed up by
performing 10 repetitions each of standing French presses and arm curls at 30% 1RM
before the 30-min acclimation period. The exercise intensity during training was 70%
1RM for both standing French presses and arm curls. Each training session consisted
of four sets of 10 repetitions each, with a 1-min break between sets. If 10 repetitions
could not be finished, the set was ended at that point. A 3-min break was provided
between sets of both exercises. The Hyp group rested in the hypoxic chamber for 30
min after the end of each training session and then left the chamber for the next bout of
exercise. At 3 weeks after starting the training regime, muscle strength was measured
to adjust the intensity level of exercise as needed. The groups trained twice each week
for 6 consecutive weeks and all 14 participants completed the total of 12 sessions. The
dumbbell was raised or lowered at comparable rates within about 3 s (1 and 2 s for
concentric and eccentric movement). The mean numbers of standing French press
repetitions performed by the exercised upper arms of the Hyp (Hyp-Ex) and Norm
(Norm-Ex) groups were 36.2 ± 3.3 and 36.1 ± 2.9, and the mean numbers of arm curl
repetitions performed by the Hyp-Ex and Norm-Ex group were 35.6 ± 4.0 and 35.4 ±
3.2, respectively. The numbers of repetitions did not significantly differ between the
Hyp-Ex and Norm-Ex groups. Table 2 summarizes the exercise intensity and
repetitions actually applied by the two groups. The mean duration of each training
session was about 13 min.
\ insert Table 2 \
Magnetic Resonance Imaging
The cross-sectional areas of the Norm-Ex and Hyp-Ex groups, and of the
nonexercised arm of the Hyp (Hyp-N) group were measured using a 1.5-T super
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coiling system (Excel ART, Toshiba, Tokyo, Japan), of which the coil covered the
whole upper arm. The center of the upper arm was defined as the point equidistant
between the head of the humerus and the olecranon. Multislice sequences at 6-mm
intervals were obtained under the following conditions: 1.0 mm interception gap, 20 ×
20 cm field of view (FOV), spin echo T1-weighted pulse sequence, 500-ms repetition
time and 15-ms echo time. The scan matrix was 208 × 320. Scanning was immediately
initiated after supine participants relaxed while extending the arms to minimize
possible effects of a gravitational fluid shift. The entire sequence was completed
within 3–5 min. Images were captured along the long axis of the upper arm and the
range of acceptable slices was carefully determined. Of 11 slices, three images at the
center of the upper arm were selected to measure muscle CSA. Tissue outlines and the
CSA of muscles and other tissues were determined using Image J (version 1.37v)
software. The CSA value was the mean of three determinations. The standard
deviation of these three sets of measurements was 1.0%. The percent increase in CSA
relative to that before training was then calculated.
Muscle Strength
We determined muscle strength of each participant as 1RM from 10 RM tests8 at 1 wk
before, and at 3 and 6 wk after starting training. We referred to a published protocol8
to determine 1RM from the 10RM test (for example, 10 repetition maximum equals
75% 1RM). The intensity of subsequent training sessions was adjusted based on 1RM
measured 3 weeks after start of training.
Saturation (SpO2)
We measured SpO2 at 10 min before, and at 0 and 30 min and 24 h after each training
session using a PULSOX-3i (Minolta, Osaka, Japan) placed on the index finger of the
dominant hand (nonexercise side). The SpO2 after 24 h was measured in both groups
under normoxic conditions.
Rating of Perceived Exertion
The level of perceived exertion was assessed after training using the Borg 6-20 RPE
scale.9 After each training session, the examiner showed the board with RPE scores to
the participant. The RPE scores of the participants were determined based on their
descriptions of difficulty in lifting the weight ranging from “very, very light” (6
points) to “very, very heavy” (20 points).
Statistical Analysis
Unless otherwise stated, standard deviations were calculated for variables. The
statistical significance of differences among the degrees of change in the CSA of
Hyp-Ex, Hyp-N and Norm-Ex was determined using the one-way analysis of variance
(ANOVA). The statistical significance of differences among the CSA of Hyp-Ex,
Hyp-N, and Norm-Ex was determined using a two-way (group × time) ANOVA. The
statistical significance of differences between the muscle strength of Hyp-Ex and
Norm-Ex was determined using a two-way (group × time) repeated-measures
ANOVA. The significance of individual differences was evaluated using the
Bonferroni post hoc test. Differences between two variables within the same
individual were examined using Student’s paired t test. Differences in physical data,
characterization of exercise training, SpO2 and RPE between Hyp and Norm were
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examined using Student’s unpaired t test. A value of P < .05 with the Bonferroni
correction was regarded as significant for all statistical analyses.
Changes in CSA After Training
Table 3 shows the CSAs of all groups. The percentage increases of the elbow extensor
and flexor of the Hyp-Ex group were significantly larger (P < .05) compared with
before training (7.3 and 9.6%, respectively at week 6; Figure 1). In contrast, the CSA
values of the Norm-Ex and Hyp-N groups did not significantly increase.
\ insert Table 3 \
Muscle Strength (1RM)
Figure 2 shows the 1RM for all groups. This value significantly increased in both the
Hyp-Ex and Norm-Ex groups. Muscle strength following the standing French presses
and arm curls in the Hyp-Ex group increased by 71.1 and 61.6%, respectively, and in
the Norm-Ex group by 56.4 and 38.9%, respectively, with no significant differences
between the groups. Muscle strength significantly increased after 6 wk and between 3
and 6 wk of training in both groups, but at 3 wk of training only in the Hyp-Ex group.
\ insert Figure 1 thru 4 \
Figure 3 shows the effect of training under hypoxic and normoxic conditions on SpO2.
The SpO2 value of the Hyp group was significantly lower at 10 min before and at 0
and 30 min after training. At 24 h after training, no significant intergroup differences
were apparent, because SpO2 was measured in both groups under normoxic
Rating of Perceived Exertion
Figure 4 shows that the RPE did not significantly differ after either the standing
French presses or arm curls.
Compared with normoxic conditions, resistance training under hypoxic conditions
induced more muscle hypertrophy without differences in RPE during training. Muscle
strength (defined as 1RM) was significantly increased at week 6 for both groups, but
only for the Hyp-Ex group at week 3, suggesting that exposure to hypoxia accelerates
increases in muscle strength. Muscle hypertrophy was not significant in the Hyp-N
group (non-exercised-arm under hypoxic conditions), indicating that hypoxia alone is
insufficient to increase muscle strength (1RM) or induce muscle hypertrophy and that
it must be combined with exercise to achieve these results.
The time course of gains in muscle strength is affected by neural factors and by
muscle hypertrophy.10 We found here that 1RM had significantly increased in the
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Hyp-Ex group by week 3, whereas 6 wk were required to elicit an increase in CSA.
We also found a significant increase in 1RM of the Hyp-Ex group by week 6, although
CSA in this group did not increased significantly until that time. Dissociation between
increases in 1RM and in CSA would be expected since neuromuscular adaptations
occur before hypertrophy.11–13 The role of neural factors is particularly critical during
the early phase of strength training. Although protein synthesis is noticeable after a
single strength training session,14 overt changes in muscle hypertrophy are not evident
until 8 wk of exercise training in beginners.13,15–17 This delay, which is concurrent with
a substantial gain in muscle strength, has led some to suggest that neural factors are
important.12 Surface electromyography (SEMG) findings have revealed that strength
gains during the early phase of a training regimen without noticeable muscle
hypertrophy are associated with an increase in the amplitude of SEMG activity. This
has been interpreted as an increase in neural drive, which denotes the magnitude of
efferent neural output from the CNS to active muscle fibers. Moritani et al12 concluded
that increases in the amplitude of the SEMG signal appear well before muscle
hypertrophy, and thus neural factors account for the larger proportion of the initial
increase in strength. The present study found that 1RM significantly increased only in
the Hyp-Ex group after 3 wk. Although neural factors were involved in both groups,
the effect of early muscle hypertrophy might have increased muscle strength after 3
wk in the Hyp-Ex group.
Takarada et al7 reported that vascular occlusion combined with low-intensity
resistance training (Kaatsu training) effectively increases cross-sectional area and
muscle strength. Abe et al18 reported that muscle hypertrophy and muscle strength
could be increased by walking slowly while restricting blood flow. They also reported
that the combination of vascular occlusion and resistance training achieves muscle
hypertrophy because of hypoxia. They suggested that restricted blood circulation
causes a hypoxic and acidic muscular environment, which leads to greater muscle
hypertrophy. We believe that this mechanism functioned in the present study. The
effects of exercise training regimens under hypoxia are likely mediated by moderate
production of reactive oxygen species (ROS) promoting tissue growth,7 growth
hormone secretion stimulated by the intramuscular accumulation of metabolic
subproducts, such as lactate,19 testosterone secretion in Leydig cells20 and additional
recruitment of type ІІ fibers.7 These studies suggest that the ability of resistance
exercise to elicit muscle hypertrophy involves not only mechanical stress but also
metabolic, hormonal and neuronal factors. However, the precise mechanism of
muscle hypertrophy in resistance training under hypoxia (hypoxic training) requires
further studies for clarification.
Narici et al3 reported that subjects exposed to chronic hypoxia show less muscle
hypertrophy than the same subjects exposed to normoxia and Mizuno et al21 also
reported that general chronic exposure to hypoxia leads to muscle atrophy rather than
hypertrophy. However, these studies had a different design from our study in that the
conditions comprised chronic hypoxia, whereas our conditions consisted of training
under hypoxia with recovery under normoxia. Some authors4–6 have reported that
chronic hypoxia leads to malnutrition and reduced activity levels; thus, chronic
hypoxia is considered a possible cause of muscle loss. Friedmann et al22 reported that
strength training under hypoxia with recovery under normoxia could not promote
muscle hypertrophy. However, their study conditions comprised low-resistance (30%
1RM)/high repetition whereas ours comprised moderate resistance (70% 1RM)/low
repetition. Considering these findings, hypoxic training cannot lead to muscle
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hypertrophy simply by low resistance training such as kaatsu training. The induction
of muscle hypertrophy under hypoxia requires high resistance training.
This study suggests that exercise combined with exposure to hypoxia effectively
induces muscle hypertrophy and increases muscle strength. However, with kaatsu
training, tourniquets can only be placed on the extremities, whereas training under
hypoxic conditions can exercise all muscles, including those of the trunk. Thus, more
balanced muscle training programs can be designed, as this method is applicable to the
whole body.
One limitation of this study is that it was not blinded (the Norm group trained
outside the chamber, so the subjects knew the group [Hyp or Norm] to which they had
been assigned), so some biases may affect the results. Another is that we used only
one O2 level (16%), so how the level of O2 affects muscle hypertrophy is unclear.
Further studies (several levels of hypoxia) are required to determine a dose-response
Because hypoxia leads to hypoxic pulmonary hypertension,23,24 adaptation must
be carefully controlled. Although further investigation is needed, high resistance
training under hypoxia appears to offer a practical and effective method of increasing
strength and inducing muscle hypertrophy.
Compared with normoxic conditions, resistance training under hypoxic conditions
efficiently improves muscle strength and rapidly induces muscle hypertrophy without
an obvious increase in the rate of perceived exertion (RPE) since it occurred at 6 rather
than at 8 wk in this group of beginners. While further studies are currently
investigating whether harmful effects can be induced during training, resistance
training under hypoxic conditions might be a novel method of increasing strength and
muscle size.
We are grateful to Takafumi Kawai and Tsubasa Sakurai for exceptional technical assistance, to
Tomomi Yamada for statistical analysis, and to the radiological technologists of Suzuka Kaisei
Hospital for the MRI measurements. This study was sponsored by grants from the Ministry of
Education, Culture, Sports, Science and Technology (Japan).
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Figure 1 — Degree of change in cross-sectional area (CSA) of extensors (top) and flexors
(bottom) after 6 wk of training. *Significant differences between groups (P < .05). Hyp-Ex,
exercised arm of hypoxia group; Hyp-N, nonexercised arm of hypoxia group; Norm-Ex,
exercised arm of normoxia group.
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Figure 2 — Degree of change in muscle strength (1 repetition maximum [RM]) of the extensors (top)
and flexor (bottom) after 6 wk of training. The Hyp-Ex and Norm-Ex groups did not significantly differ.
Significant differences from pretraining values, *P < .05 and **P < .01, respectively; significant
differences from 3-wk values, #P < .05 and ##P < .01, respectively. Hyp-Ex, exercised arm of hypoxia
group; Norm-Ex, exercised arm of normoxia group.
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Figure 3 — Mean SpO2 for hypoxia (Hyp) and normoxia (Norm) groups. All values of SpO2 were
measured at 24 h after training under normoxic conditions. *Significant differences between groups (P <
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Figure 4 — Changes in rating of perceived exertion (RPE) after each set of standing French presses
(top) and arm curls (bottom). Intergroup differences are not significantly different at any time point.
Hyp, hypoxia group; Norm, normoxia group.
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Table 1 Physical data of hypoxia (Hyp) and normoxia (Norm)
groups before and after 6 wk of training
Hyp (n = 7) Norm (n = 7)
Pretraining Posttraining Pretraining Posttraining
Age (y) 22.7 ± 2.7 21.6 ± 1.6
Height (cm) 174.6 ± 5.0 171.4 ± 5.2
Body mass (kg) 66.8 ± 6.0 66.9 ± 6.8 65.0 ± 8.1 65.1 ± 8.1
Body fat (%) 12.3 ± 3.0 13.2 ± 0.7 12.8 ± 4.5 12.7 ± 4.3
Note. Body mass and percentage fat were measured using a body composition analyzer (BC-118E,
Tanita, Tokyo, Japan). The Hyp and Norm groups did not significantly differ.
Table 2 Characterization of exercise training
Hyp-Ex Norm-Ex
French press Intensity % 1RM (%) 0–3 wk 70.7 ± 2.1 70.6 ± 1.7
4–6 wk 69.6 ± 1.4 71.0 ± 1.5
Repetitions 36.2 ± 3.3 36.1 ± 2.9
Arm curl Intensity % 1RM (%) 0–3 wk 70.3 ± 0.8 69.5 ± 1.3
4–6 wk 70.1 ± 0.7 69.7 ± 0.5
Repetitions 35.6 ± 4.0 35.4 ± 3.2
Note. The Hyp-Ex and Norm-Ex groups did not significantly differ. RM, repetition maximum;
Hyp-Ex, exercised arm of hypoxia group; Norm-Ex, exercised arm of normoxia group.
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Table 3 Cross-sectional areas (CSA) of elbow extensor and
flexor before and after 6 weeks of training
Extensors (cm2) Flexors (cm2)
Pretraining Posttraining ΔChange Pretraining Posttraining ΔChange
Hyp-Ex 25.6 ± 2.6 27.5 ± 3.9* 1.9 ± 1.1 12.9 ± 1.1 14.2 ± 1.5* 1.3 ± 0.9
Hyp-N 26.3 ± 3.1 26.2 ± 3.2 –0.1 ± 0.2 13.2 ± 1.5 13.1 ± 1.7 0.0 ± 0.3
Norm-Ex 25.8 ± 4.4 25.9 ± 4.4 0.3 ± 0.6 12.3 ± 1.9 12.5 ± 1.9 0.3 ± 0.3
Note. The Hyp-Ex, Hyp-N and Norm-Ex groups did not significantly differ. *P < .05, significant
differences from pretraining values. RM, repetition maximum; Hyp-Ex, exercised arm of hypoxia
group; Hyp-N, nonexercised arm of hypoxia group; Norm-Ex, exercised arm of normoxia group.
... The total sample comprised 348 participants (n = 164 for RTN and n = 184 for RTH). These 17 studies assessed changes in muscle hypertrophy (n = 83 for CSA 3,5,8 ; n = 184 for lean mass 7,40,41 and n = 60 for muscle thickness 6,38,42 ) and/or strength development (n = 232 for 1RM 15,39,43,44 ). ...
... Exercise program periods ranged from 3 43 38,43,45 ; the remainder of the studies employed moderate-load programs (60-80% 1RM) 15,41,44 . Six studies used short inter-set rest intervals (< 60 s) 3,6-8,44,47 , 3 used moderate inter-set rest intervals (> 60-< 120 s) 39,41,46 and 5 used long inter-set rest intervals (≥ 120 s) 5,15,21,40,42 . (Table 1). . ...
... The body of research that included training protocols specific to muscular strength improvements (e.g., higher loads with longer inter-set rests) did not show a benefit to conditions of systemic hypoxia 50 . Only 2 of the 4 included studies that employed long inter-set rest intervals 5,42 showed a clear benefit of hypoxia for strength development (Fig. 8C), which only would partially support this beneficial effect. Indeed, recovery periods ≥ 120 s seem to mitigate any additive benefit from the hypoxic stimulus 50 , while shorter inter-set rest periods could entail more challenging metabolic conditions for muscle development 56 . ...
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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.
... Methodological classification of studies used in the systematic review. A rating of 9-11 is considered excellent, a rating of 6-8 is good, a rating of 4-5 is acceptable, and a rating of ≤3 shows a lack of methodological quality [6,[15][16][17][18][19][20][21][22][23][24][25][26][27][28][29][30][31][32]. Methodological classification of studies used in the systematic review. ...
... Methodological classification of studies used in the systematic review. A rating of 9-11 is considered excellent, a rating of 6-8 is good, a rating of 4-5 is acceptable, and a rating of ≤3 shows a lack of methodological quality [6,[15][16][17][18][19][20][21][22][23][24][25][26][27][28][29][30][31][32]. Appl. ...
... The papers included healthy participants (n = 188, 35.67%) [15,16,19,21,25,28], trained or athletes (n = 183, 34.72%) [6,22,24,27,[29][30][31] and a sedentary or untrained population (n = 156, 29.60%) [17,18,20,23,26,32]. The studies analysed the effect of resistance exercise in hypoxia with a frequency of two to four times per week during 4 to 12 weeks, with a total of ten to 36 sessions. ...
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Background. Training muscle capacities in hypoxic conditions increases some manifestations, such as hypertrophy and muscle strength, due to a change in the muscle phenotype as a result of the activation of hypoxia-inducible factors (HIF). Despite the proven benefits of resistance training in hypoxic conditions that allow conjecture regarding the effectiveness in facilitating muscular capacities in different populations, there is still controversy regarding the difference between resistance training in hypoxia and normoxia. The objective of this review was to compile the present evidence and update the methods and effectiveness of resistance training in simulated hypoxia for the development of strength and muscle hypertrophy. Methodology. A systematic search for an integrative review was carried out based on the preferred reporting guidelines for systematic reviews and meta-analysis (PRISMA) in 4 stages: identification, data selection, data collection and extraction, and quality evolution. Results. Four studies (92 participants) reported benefits in strength when training in hypoxia, three (101 participants) benefits in hypertrophy, and twelve (327 participants) benefits in strength and hypertrophy. Conclusion. Based on the findings of this systematic review, it is concluded that there are positive effects on muscle size and ability to generate force after a hypoxic training programme. However, some studies did not show a statistically greater benefit than for the normoxia groups, but several methodologies have been identified that promote the benefits of hypoxia.
... Longterm or severe exposure to hypoxic conditions associated with physical activity or not, e.g., spending several weeks in the mountains while being active or just sojourning at 4000-5000 m altitude, improves oxygen transport and vascularization, but also induces muscle atrophy (Deldicque and Francaux 2013). Acute exercise sessions in hypoxia, e.g., in hypoxic rooms with recovery at sea level, is associated with improved VO 2 max, muscle hypertrophy and increased one-repetition maximum (Nishimura et al. 2010). Several mechanisms involved in those adaptations have already been uncovered such as the regulation of protein balance, the oxidative and glycolytic metabolism, the hypoxia-inducible factor (HIF)-1α pathway and targeted genes, mitochondrial biogenesis and modifications at the neuromuscular junction (Deldicque and Francaux 2013). ...
... This training strategy is quite popular in athletes as it is known to induce intramuscular adaptations related to both aerobic and anaerobic performance (Millet et al. 2010). Much less has been reported concerning the regulation of muscle mass (Manimmanakorn et al. 2013;Nishimura et al. 2010) and even less concerning the regulation of satellite cells. Only one in vivo study was found in thoroughbred horses. ...
Exercise modulates the circulating levels of the endocannabinoids ligands N-arachidonoylethanolamine (AEA) and 2-arachidonoylglycerol (2-AG) and possibly the levels of their receptors and downstream signaling in skeletal muscle. The aim of the present study was to investigate the regulation of the endocannabinoid system by several exercise paradigms in human skeletal muscle. A second aim was to compare endocannabinoid regulation in healthy and prediabetic people in response to an acute endurance exercise. Blood and muscle samples were taken before and after resistance and endurance exercise in normoxia and hypoxia to measure plasma endocannabinoid levels as well as muscle protein expression of CB1, CB2 and downstream signaling. We found that: 1) an acute resistance exercise session decreased plasma 2-AG and N-palmitoylethanolamine (PEA) levels in normoxia; 2) 4 weeks resistance training decreased plasma AEA, PEA and N-oleoylethanolamine (OEA) levels in both normoxia and hypoxia; 3) an acute moderate intensity endurance exercise increased plasma OEA levels in the healthy and prediabetic groups in normoxia and hypoxia while plasma 2-AG levels increased in the healthy group and AEA in the prediabetic group only in normoxia. The expression of the cannabinoid receptors was only marginally regulated by acute exercise, hypoxia and prediabetes and downstream signaling did not follow the changes detected in the endocannabinoid ligands. Altogether, our results suggest that resistance and endurance exercise regulate the levels of the endocannabinoid ligands and CB1 expression in opposite ways. The physiological impact of the changes observed in the endocannabinoid ligands in human skeletal muscle after exercise needs further investigation.
... New insight regarding muscle strength and hypertrophy responses with resistance training performed in different levels of hypoxia is starting to form with a postulation that the combination of lower aerodynamic resistance and/or increased anaerobic metabolism enhances motor unit recruitment patterns and metabolic cost at higher altitudes [143]. In this regard, the combination of moderate hypoxia and resistance exercise has been demonstrated to improve aspects of anabolic (muscular strength and hypertrophy) [144][145][146][147][148][149] and overall body composition [134] (Table 2). These findings agree with other studies incorporating upper and lower body resistance training (70% of 1-RM, 5 × 10 repetitions) between 5 and 8 weeks that have reported a 1-2% reduction in fat mass in severe hypoxia (~ 3000-3900 m), although no statistical significance was noted when compared to resistance training in normoxia [146,149]. ...
... Croymans and colleagues [169] reported improved insulin sensitivity, lean body mass, relative strength as well as GLUT4 expression in overweight individuals and/or individuals with obesity after 12 weeks of resistance training (3 times/week) at normoxia. Resistance training for 6 weeks (70% of 1-RM, 4 × 10 repetitions) aided by moderate hypoxia (~ 2100 m) elicited a greater muscle hypertrophy response (cross-sectional area) as compared with normoxia [148]. Such findings may be of potential clinical significance considering glucose tolerance and insulin sensitivity are positively correlated with muscle mass [170]. ...
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Obesity is a major global health issue and a primary risk factor for metabolic-related disorders. While physical inactivity is one of the main contributors to obesity, it is a modifiable risk factor with exercise training as an established non-pharmacological treatment to prevent the onset of metabolic-related disorders, including obesity. Exposure to hypoxia via normobaric hypoxia (simulated altitude via reduced inspired oxygen fraction), termed hypoxic conditioning, in combination with exercise has been increasingly shown in the last decade to enhance blood glucose regulation and decrease the body mass index, providing a feasible strategy to treat obesity. However, there is no current consensus in the literature regarding the optimal combination of exercise variables such as the mode, duration, and intensity of exercise, as well as the level of hypoxia to maximize fat loss and overall body compositional changes with hypoxic conditioning. In this narrative review, we discuss the effects of such diverse exercise and hypoxic variables on the systematic and myocellular mechanisms, along with physiological responses, implicated in the development of obesity. These include markers of appetite regulation and inflammation, body conformational changes, and blood glucose regulation. As such, we consolidate findings from human studies to provide greater clarity for implementing hypoxic conditioning with exercise as a safe, practical, and effective treatment strategy for obesity.
... It is worth noting that both normoxic (FiO2 = 20.9%) and hypoxic (FiO2 = 12.7%) conditions have demonstrated the ability to signi cantly elevate the thickness of the biceps brachii and triceps brachii muscles, underpinning the notion that resistance training under hypoxic conditions is adept at driving skeletal muscle hypertrophy 28 . These promising results also nd resonance in parallel studies 29,30 . This study further observed notable enhancements in the lean body weight of the lower limbs, trunk, and entire body within the hypoxic resistance training group of rats when contrasted with the hypoxic sedentary group. ...
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MicroRNAs (miRNAs) may play a crucial regulatory role in the process of muscle atrophy induced by high-altitude hypoxia and its amelioration through resistance training. However, research in this aspect is still lacking. Therefore, this study aimed to employ miRNA microarray analysis to investigate the expression profile of miRNAs in skeletal muscle from an animal model of hypoxia-induced muscle atrophy and resistance training aimed at mitigating muscle atrophy. The study utilized a simulated hypoxic environment (oxygen concentration at 11.2%) to induce muscle atrophy and established a rat model of resistance training using ladder climbing, with a total intervention period of 4 weeks. The miRNA expression profile revealed 9 differentially expressed miRNAs influenced by hypoxia (e.g., miR-341, miR-32-5p, miR-465-5p) and 14 differentially expressed miRNAs influenced by resistance training under hypoxic conditions (e.g., miR-338-5p, miR-203a-3p, miR-92b-3p) (∣Fold Change∣≥1.5, p༜0.05). The differentially expressed miRNAs were found to target genes involved in muscle protein synthesis and degradation (such as Utrn, mdm2, eIF4E), biological processes (such as negative regulation of transcription from RNA polymerase II promoter, regulation of transcription, DNA-dependent), and signaling pathways (such as Wnt signaling pathway, MAPK signaling pathway, ubiquitin-mediated proteolysis, mTOR signaling pathway). This study provides a foundation for understanding and further exploring the molecular mechanisms underlying hypoxia-induced muscle atrophy and the mitigation of atrophy through resistance training.
... Hypoxia conditioning was not superior than equivalent normoxic training to modify lean mass, while there were also no significant differences between middle-aged and older adults. These findings are inconsistent with previous observations made in younger adults [61][62][63][64]. These authors reported that resistance exercise at moderate intensity (70-85% 1RM) in hypoxia is more effective for improving lean body mass and fibre cross-sectional area than higher exercise intensities (> 85%1RM) in normoxia. ...
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Background The effects of hypoxia conditioning, which involves recurrent exposure to hypoxia combined with exercise training, on improving body composition in the ageing population have not been extensively investigated. Objective This meta-analysis aimed to determine if hypoxia conditioning, compared to similar training near sea level, maximizes body composition benefits in middle-aged and older adults. Methods A literature search of PubMed, EMBASE, Web of Science, Scopus and CNKI (China National Knowledge Infrastructure) databases (up to 27th November 2022) was performed, including the reference lists of relevant papers. Three independent reviewers extracted study characteristics and health outcome measures. Search results were limited to original studies of the effects of hypoxia conditioning on body composition in middle-aged and older adults. Results Twelve studies with a total of 335 participants were included. Hypoxia conditioning induced greater reductions in body mass index (MD = -0.92, 95%CI: -1.28 to -0.55, I² = 0%, p < 0.00001) and body fat (SMD = -0.38, 95%CI: -0.68 to -0.07, I² = 49%, p = 0.01) in middle-aged and older adults compared with normoxic conditioning. Hypoxia conditioning improved lean mass with this effect not being larger than equivalent normoxic interventions in either middle-aged or older adults (SMD = 0.07, 95%CI -0.12 to 0.25, I² = 0%, p = 0.48). Subgroup analysis showed that exercise in moderate hypoxia (FiO2 > 15%) had larger effects than more severe hypoxia (FiO2 ≤ 15%) for improving body mass index in middle-aged and older adults. Hypoxia exposure of at least 60 min per session resulted in larger benefits for both body mass index and body fat. Conclusion Hypoxia conditioning, compared to equivalent training in normoxia, induced greater body fat and body mass index improvements in middle-aged and older adults. Adding hypoxia exposure to exercise interventions is a viable therapeutic solution to effectively manage body composition in ageing population.
... Normobaric hypoxia (reduced inspired fraction of oxygen [F i O 2 ]) during resistance training can further increase muscle strength and hypertrophy compared to normoxia (Manimmanakorn et al., 2013;Nishimura et al., 2010). These adaptations likely occur due to a cascade of physiological events, prompted by an increased reliance on anaerobic metabolism and subsequent accumulation of metabolites , which may also cause heightened muscle fiber recruitment for a given submaximal workload (Scott et al., 2017). ...
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Purpose: To determine whether performing resistance exercise in hypoxia acutely reduces performance and increases markers of fatigue, and whether these responses are exaggerated if exercising at high versus low work rates (i.e., exercising to failure or volume matched non-failure). Methods: Following a within-subject design, 20 men completed two trials in hypoxia (13% oxygen) and two in normoxia (21% oxygen). The first session for hypoxic and normoxic conditions comprised six sets of bench press and shoulder press to failure (high work rate), while subsequent sessions involved the same volume distributed over 12 sets (low work rate). Physical performance (concentric velocity) and perceptual responses were measured during exercise and for 72 hr post-exercise. Neuromuscular performance (bench throw velocity) was assessed pre- and post-session. Results: Hypoxia did not affect physical performance, neuromuscular performance, and perceptual recovery when exercising at high or low work rates. Higher work rate exercise caused greater acute decrements in physical performance and post-exercise neuromuscular performance and increased perceived exertion and muscle soreness (p ≤ 0.006), irrespective of hypoxia. Conclusions: Hypoxia does not impact on resistance exercise performance or increase markers of physical and perceptual fatigue. Higher exercise work rates may impair physical performance, and exaggerate fatigue compared to low work rate exercise, irrespective of environmental condition. Practitioners can prescribe hypoxic resistance exercise without compromising physical performance or inducing greater levels of fatigue. For athletes who are required to train with high frequency, decreasing exercise work rate may reduce post-exercise markers of fatigue for the same training volume.
... Whereas our group and others have recently demonstrated that hypoxia during contractile activity impairs exercise-induced anabolic signaling, 10 and rates of muscle protein synthesis, 11,12 others have suggested that hypoxia during resistance exercise training is advantageous for muscle hypertrophy and strength development. 13,14 The discrepant findings warrant further investigations on the impact of hypoxia on mechanisms regulating skeletal muscle mass. ...
Cumulative evidence supports the hypothesis that hypoxia acts as a regulator of muscle mass. However, the underlying molecular mechanisms remain incompletely understood, particularly in human muscle. Here we examined the effect of hypoxia on signaling pathways related to ribosome biogenesis and myogenic activity following an acute bout of resistance exercise. We also investigated whether hypoxia influenced the satellite cell response to resistance exercise. Employing a randomized, crossover design, eight men performed resistance exercise in normoxia (FiO2 21%) or normobaric hypoxia (FiO2 12%). Muscle biopsies were collected in a time-course manner (before, 0, 90, 180 min and 24 h after exercise) and were analyzed with respect to cell signaling, gene expression and satellite cell content using immunoblotting, RT-qPCR and immunofluorescence, respectively. In normoxia, resistance exercise increased the phosphorylation of RPS6, TIF-1A and UBF above resting levels. Hypoxia reduced the phosphorylation of these targets by ~37%, ~43% and ~ 67% throughout the recovery period, respectively (p < .05 vs. normoxia). Resistance exercise also increased 45 S pre-rRNA expression and mRNA expression of c-Myc, Pol I and TAF-1A above resting levels, but no differences were observed between conditions. Similarly, resistance exercise increased mRNA expression of myogenic regulatory factors throughout the recovery period and Pax7+ cells were elevated 24 h following exercise in mixed and type II muscle fibers, with no differences observed between normoxia and hypoxia. In conclusion, acute hypoxia attenuates ribosome signaling, but does not impact satellite cell pool expansion and myogenic gene expression following a bout of resistance exercise in human skeletal muscle.
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Purpose Lactate has recently been the focus of research for its hypertrophic effects. The aim of this study was to introduce lacto-resistance exercise, emphasizing lactate production during exercise, and compare its hypertrophic effects with traditional resistance training in professional bodybuilders. Methods Twenty-four participants performed traditional and lacto-resistance exercises in two separate sessions. Blood lactate concentrations and metabolic stress, estimated by plasma ammonia levels (PAL) and muscle oxygen saturation (SmO2), were compared between two exercises. The participants were then matched based on their rectus femoris muscle cross-sectional area (RFCSA) and allocated to the control (C, n = 8), traditional resistance training (TRT, n = 8), and lacto-resistance training (La-RT, n = 8) groups. The TRT and La-RT groups completed their own four-week resistance training program and RFCSA and one-repetition maximum (1RM) changes were compared between the groups. Results The average changes in blood lactate concentrations (1.38-fold), SmO2 (0.86-fold), and PAL (1.16-fold) from rest to post-exercise were significantly higher in the lacto-resistance exercise compared to those obtained following traditional resistance exercise (all P < 0.05). After four weeks of resistance training, the values of 1RM squat (TRT: 1.14-fold, P < 0.05; La-RT: 1.2-fold induction, P < 0.05) and 1RM leg press (TRT: 1.11-fold, P < 0.05; La-RT: 1.20-fold, P < 0.05) were significantly higher than in the C group. Post-training values of RFCSA in La-RT group were significantly higher than in the C group (1.22-fold, P < 0.01), but not in the TRT group. Conclusion Lacto-resistance training, therefore, is a useful hypertrophy-oriented exercise, even for professional athletes who hardly experience muscle mass gains with traditional resistance training.
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In general, the concept of a mechanism in biology has three distinct meanings. It may refer to a philosophical thesis about the nature of life and biology, to the internal workings of a machine-like structure, or to the causal explanation of a particular phenomenon [1]. Understanding the biological mechanisms that justify acute and chronic physiological responses to exercise interventions determines the development of training principles and training methods. A strong understanding of the effects of exercise in humans may help researchers to identify what causes specific biological changes and to properly identify the most adequate processes for implementing a training stimulus [1]. Despite the significant body of knowledge regarding the physiological and physical effects of different training methods (based on load dimensions), some biological causes of those changes are still unknown. Additionally, few studies have focused on natural biological variability in humans and how specific human properties may underlie different responses to the same training intervention. Thus, more original research is needed to provide plausible biological mechanisms that may explain the physiological and physical effects of exercise and training in humans. In this Special Issue, we discuss/demonstrate the biological mechanisms that underlie the beneficial effects of physical fitness and sports performance, as well as their importance and their role in/influences on physical health. A total of 28 manuscripts are published here, of which 25 are original articles, two are reviews, and one is a systematic review. Two papers are on neuromuscular training programs (NMTs), training monotony (TM), and training strain (TS) in soccer players [2,3]; five articles provide innovative findings about testosterone and cortisol [4,5], gastrointestinal hormones [6], spirulina [7], and concentrations of erythroferrone (ERFE) [8]; another five papers analyze fitness and its association with other variables [7,9–12]; three papers examine body composition in elite female soccer players [2], adolescents [6], and obese women [7]; five articles examines the effects of high-intensity interval training (HIIT) [7,10,13–15]; one paper examines the acute effects of different levels of hypoxia on maximal strength, muscular endurance, and cognitive function [16]; another article evaluates the efficiency of using vibrating exercise equipment (VEE) compared with using sham-VEE in women with CLBP (chronic lowback pain) [17]; one article compares the effects of different exercise modes on autonomic modulation in patients with T2D (type 2 diabetes mellitus) [14]; and another paper analyzes the changes in ABB (acid–base balance) in the capillaries of kickboxers [18]. Other studies evaluate: the effects of resistance training on oxidative stress and muscle damage in spinal cord-injured rats [19]; the effects of muscle training on core muscle performance in rhythmic gymnasts [20]; the physiological profiles of road cyclist in different age categories [21]; changes in body composition during the COVID-19 [22]; a mathematical model capable of predicting 2000 m rowing performance using a maximum-effort 100 m indoor rowing ergometer [23]; the effects of ibuprofen on performance and oxidative stress [24]; the associations of vitamin D levels with various motor performance tests [12]; the level of knowledge on FM (Fibromyalgia) [25]; and the ability of a specific BIVA (bioelectrical impedance vector analysis) to identify changes in fat mass after a 16-week lifestyle program in former athletes [26]. Finally, one review evaluates evidence from published systematic reviews and meta-analyses about the efficacy of exercise on depressive symptoms in cancer patients [27]; another review presents the current state of knowledge on satellite cell dependent skeletal muscle regeneration [28]; and a systematic review evaluates the effects of exercise on depressive symptoms among women during the postpartum period [29]
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Four male subjects aged 23-34 years were studied during 60 days of unilateral strength training and 40 days of detraining. Training was carried out four times a week and consisted of six series of ten maximal isokinetic knee extensions at an angular velocity of 2.09 rad.s-1. At the start and at every 20th day of training and detraining, isometric maximal voluntary contraction (MVC), integrated electromyographic activity (iEMG) and quadriceps muscle cross-sectional area (CSA) assessed at seven fractions of femur length (Lf), by nuclear magnetic resonance imaging, were measured on both trained (T) and untrained (UT) legs. Isokinetic torques at 30 degrees before full knee extension were measured before and at the end of training at: 0, 1.05, 2.09, 3.14, 4.19, 5.24 rad.s-1. After 60 days T leg CSA had increased by 8.5% +/- 1.4% (mean +/- SEM, n = 4, p less than 0.001), iEMG by 42.4% +/- 16.5% (p less than 0.01) and MVC by 20.8% +/- 5.4% (p less than 0.01). Changes during detraining had a similar time course to those of training. No changes in UT leg CSA were observed while iEMG and MVC increased by 24.8% +/- 10% (N.S.) and 8.7% +/- 4.3% (N.S.), respectively. The increase in quadriceps muscle CSA was maximal at 2/10 Lf (12.0% +/- 1.5%, p less than 0.01) and minimal, proximally to the knee, at 8/10 Lf (3.5% +/- 1.2%, N.S.). Preferential hypertrophy of the vastus medialis and intermedius muscles compared to those of the rectus femoris and lateralis muscles was observed.(ABSTRACT TRUNCATED AT 250 WORDS)
The aim of this study was to explore the effect and action mechanisms of intermittent hypoxia on the production of testosterone both in vivo and in vitro. Male rats were housed in a hypoxic chamber (12% O(2) + 88% N(2), 1.5 l/ml) 8 h/day for 4 days. Normoxic rats were used as control. In an in vivo experiment, hypoxic and normoxic rats were euthanized and the blood samples collected. In the in vitro experiment, the enzymatically dispersed rat Leydig cells were prepared and challenged with forskolin (an adenylyl cyclase activator, 10(-4) M), 8-Br-cAMP (a membrane-permeable analog of cAMP, 10(-4) M), hCG (0.05 IU), the precursors of the biosynthesis testosterone, including 25-OH-C (10(-5) M), pregnenolone (10(-7) M), progesterone (10(-7) M), 17-OH-progesterone (10(-7) M), and androstendione (10(-7)-10(-5) M), nifedipine (L-type Ca(2+) channel blocker, 10(-6)-10(-4) M), nimodipine (L-type Ca(2+) channel blocker, 10(-5) M), tetrandrine (L-type Ca(2+) channel blocker, 10(-5) M), and NAADP (calcium-signaling messenger causing release of calcium from intracellular stores, 10(-6)-10(-4) M). The concentrations of testosterone in plasma and medium were measured by radioimmunoassay. The level of plasma testosterone in hypoxic rats was higher than that in normoxic rats. Enhanced testosterone production was observed in rat Leydig cells treated with hCG, 8-Br-cAMP, or forskolin in both normoxic and hypoxic conditions. Intermittent hypoxia resulted in a further increase of testosterone production in response to the testosterone precursors. The activity of 17β-hydroxysteroid dehydrogenase was stimulated by the treatment of intermittent hypoxia in vitro. The intermittent hypoxia-induced higher production of testosterone was accompanied with the influx of calcium via L-type calcium channel and the increase of intracellular calcium via the mechanism of calcium mobilization. These results suggested that the intermittent hypoxia stimulated the secretion of testosterone at least in part via stimulatory actions on the activities of adenylyl cyclase, cAMP, L-type calcium channel, and steroidogenic enzymes.
This study investigated skeletal muscle adaptations to high altitude and a possible role of physical activity levels. Biopsies were obtained from the m. quadriceps femoris (vastus) and m. biceps brachii (biceps) in 15 male subjects, 7 active and 8 less active. Samples were obtained at sea level and after 75 days altitude exposure at 5250 m or higher. The muscle fiber size decreased at an average of 15% in the vastus and biceps, respectively, and to the same extent in both groups. In both muscles, the mean number of capillaries was 2.1-2.2 cap.fiber(-1) before and after the exposure. As mean fiber area was reduced, the mean number of capillaries per unit area increased in all subjects (from 320 to 405 cap/mm2) with no difference between the active and less active groups. The two enzymes selected to reflect mitochondrial capacity, citrate synthase (CS) and 3-hydroxyl-CoA-dehydrogenase (HAD), did not change in the leg muscles with altitude exposure, CS: 28.7 (20.7-37.8) vs. 27.8 (23.8-29.4); HAD: 35.2 (20.3-43.1) vs. 30.6 (20.7-39.7) micromol.min(-1).g(-1) d.w, pre- and post-altitude, respectively. The muscle buffer capacity was elevated in both the vastus; 220 (194-240) vs. 232 (200-277) and the biceps muscles; 233 (190-301) vs. 253 (193-320) after the acclimatization period. In conclusion, mean fiber area was reduced in response to altitude exposure regardless of physical activity which in turn meant that with an unaltered capillary to fiber ratio there was an elevation in capillaries per unit of muscle area. Muscle enzyme activity was unaffected with altitude exposure in both groups, whereas muscle buffer capacity was increased.
The time course of strength gain with respect to the contributions of neural factors and hypertrophy was studied in seven young males and eight females during the course of an 8 week regimen of isotonic strength training. The results indicated that neural factors accounted for the larger proportion of the initial strength increment and thereafter both neural factors and hypertrophy took part in the further increase in strength, with hypertrophy becoming the dominant factor after the first 3 to 5 weeks. Our data regarding the untrained contralateral arm flexors provide further support for the concept of cross education. It was suggested that the nature of this cross education effect may entirely rest on the neural factors presumably acting at various levels of the nervous system which could result in increasing the maximal level of muscle activation.
The effects of prolonged severe hypoxia on human performance capacity and muscle structure and function have recently been studied during real and simulated ascents to Mt. Everest. The results of several independent research teams, using different techniques, are broadly compatible. It is found that body and muscle mass is significantly reduced after exposure to hypoxia. As a consequence, muscle fiber size is also reduced. The capillary density of muscle tissue is increased, not because of capillary neoformation, but because of the reduction in muscle fiber size. The activities of enzymes of the oxidative pathways are decreased in skeletal muscle tissue. A loss of mitochondria is the structural evidence of the diminished potential for muscle oxidative metabolism. In contrast to these results, recent experimentations with hypoxia in human exercise settings have demonstrated that if hypoxia is only present during a limited daily period of an endurance training session, hypoxia has a different effect on muscle tissue. It is found that muscle fiber size, capillarity, myoglobin concentration and muscle oxidative capacity are all enhanced with training in hypoxia. These controversial findings raise questions regarding the nature of the adaptational mechanisms triggered by the different hypoxic stimuli to which subjects had been subjected and thus offer important new venues for further studies on the control of protein metabolism in muscle tissue.
The maximal muscular power (both instantaneous, w, and average, w-.) and the cross-sectional area of the left thigh (CSA) were measured on six subjects before (B) and after (A) prolonged exposure to high altitude (above 5000 m asl). w and w were determined during a standing high jump off both feet on a force platform, and CSA by computed tomography. It was observed that: (1) in B, body weight (BW) = 74.1 +/- 5.8 kg, w = 3330 +/- 460 W (44.8 +/- 3.4 w-. = 1795 +/- 395 W (24.6 +/- 4.3, and CSA = 184.5 +/- 23.1 cm2; 2) in A, BW = 70.4 +/- 6.6 kg, w = 3005 +/- 472 W (42.5 +/- 3.6, w = 1531 +/- 267 W (21.9 +/- 3.1, and CSA = 163.5 +/- 23.1 cm2. Thus, w and w-. were decreased both in absolute terms (-9.8% and -14.7%, respectively) and per unit BW (-5.1% and -11.0%). However, because of the concomitant decrease in CSA, when expressed per unit cross-sectional area of the muscle, w (9.04 +/- 0.71 and 9.20 +/- 0.72 W +/- cm2) and w (4.87 +/- 0.81 and 4.70 +/- 0.67 W/cm2) were unchanged. The intrinsic capacity of the muscle to generate explosive power is therefore preserved in A. It is concluded that the decrease in w and w after high-altitude exposure depends only on a net loss of muscle mass.
Adaptations in skeletal muscle in response to progressive hypobaria were investigated in eight male subjects [maximal O2 uptake = 51.2 +/- 3.0 (SE)] over 40 days of progressive decompression to the stimulated altitude of the summit of Mt. Everest. Samples of the vastus lateralis muscle extracted before decompression (SL-1), at 380 and 282 Torr, and on return to sea level (SL-2) indicated that maximal activities of enzymes representative of the citric acid cycle, beta-oxidation, glycogenolysis, glycolysis, glucose phosphorylation, and high-energy phosphate transfer were unchanged (P greater than 0.05) at 380 and 282 Torr over initial SL-1 values. After exposure to 282 Torr, however, representing an additional period of approximately 7 days, reductions (P less than 0.05) were noted in succinic dehydrogenase (21%), citrate synthetase (37%), and hexokinase (53%) between SL-2 and 380 Torr. No changes were found in the other enzymes. Capillarization as measured by the number of capillaries per cross-sectional area (CC/FA) was increased (P less than 0.05) in both type I (0.94 +/- 0.8 vs. 1.16 +/- 0.05) and type II (0.84 +/- 0.07 vs. 1.05 +/- 0.08) fibers between SL-1 and SL-2. This increase was mediated by a reduction in fiber area. No changes were found in fiber-type distribution (type I vs. type II). These findings do not support the hypothesis, at least in humans, that, at the level of the muscle cell, extreme hypobaric hypoxia elicits adaptations directed toward maximizing oxidative function.
Summary The training effect on the human arm flexor was studied by subjecting 5 healthy males. The training was made by isometric maximum contraction, 3 times (10 seconds/bout) a day, every day except Sunday for 100 days. Ultrasonic photography was employed to estimate the cross-sectional area of the muscle.1. The muscle training of 100 days increased the maximum strength by 91.7% and the cross-sectional area of muscle by 23.0%. 2. The average values of strength per unit cross-sectional area of muscle increased from 6.3 to 10.0 kg/cm2 after 100th day of training at extended position of arm, from 4.7 to 7.5 kg/cm2 at flexed position of arm. 3. The increase of maximum strength was associated with the increase in cross-sectional area and the increase in strength per unit cross-sectional area.
There is a great demand for perceptual effort ratings in order to better understand man at work. Such ratings are important complements to behavioral and physiological measurements of physical performance and work capacity. This is true for both theoretical analysis and application in medicine, human factors, and sports. Perceptual estimates, obtained by psychophysical ratio-scaling methods, are valid when describing general perceptual variation, but category methods are more useful in several applied situations when differences between individuals are described. A presentation is made of ratio-scaling methods, category methods, especially the Borg Scale for ratings of perceived exertion, and a new method that combines the category method with ratio properties. Some of the advantages and disadvantages of the different methods are discussed in both theoretical-psychophysical and psychophysiological frames of reference.