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Journal of
Clinical Medicine
Article
The Effect of High-Intensity Interval Training Periods on
Morning Serum Testosterone and Cortisol Levels and Physical
Fitness in Men Aged 35–40 Years
Tadeusz Ambro˙
zy 1, Łukasz Rydzik 1,* , Zbigniew Obmi ´nski 2, Wiesław Błach 3, Natalia Serafin 4,
Blanka Błach 3, Jarosław Jaszczur-Nowicki 5and Mariusz Ozimek 1
Citation: Ambro˙
zy, T.; Rydzik, Ł.;
Obmi´nski, Z.; Błach, W.; Serafin, N.;
Błach, B.; Jaszczur-Nowicki, J.;
Ozimek, M. The Effect of
High-Intensity Interval Training
Periods on Morning Serum
Testosterone and Cortisol Levels and
Physical Fitness in Men Aged 35–40
Years. J. Clin. Med. 2021,10, 2143.
https://doi.org/10.3390/jcm10102143
Academic Editors: Michal Wilk,
Michał Krzysztofik,
Aleksandra Filip-Stachnik and Joel
T. Cramer
Received: 8 April 2021
Accepted: 14 May 2021
Published: 15 May 2021
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Copyright: © 2021 by the authors.
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This article is an open access article
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conditions of the Creative Commons
Attribution (CC BY) license (https://
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4.0/).
1Institute of Sports Sciences, University of Physical Education, 31-571 Krakow, Poland;
tadek@ambrozy.pl (T.A.); mariusz.ozimek@awf.krakow.pl (M.O.)
2Department of Endocrinology, Institute of Sport-National Research Institute, 01-982 Warsaw, Poland;
zbigniew.obminski@insp.pl
3Faculty of Physical Education & Sport, University School of Physical Education, 51-612 Wroclaw, Poland;
wieslaw.judo@wp.pl (W.B.); blanka.blach@awf.wroc.pl (B.B.)
4Faculty of Physical Education and Sport, Institute of Social Sciences, University of Physical Education in
Krakow, 31-571 Kraków, Poland; natalia.ambrozy@gmail.com
5Department of Tourism, Recreation and Ecology, University of Warmia and Mazury in Olsztyn,
10-719 Olsztyn, Poland; j.jaszczur-nowicki@uwm.edu.pl
*Correspondence: lukasz.gne@op.pl; Tel.: +48-730-696-377
Abstract:
Background: Intensive physical activity largely modulates resting concentrations of blood
cortisol (C) and testosterone (T) and their molar ratio, which is defined as the anabolic–catabolic
index and expressed as T/C
×
10
2
. The aim of the study is to evaluate the effect of the author’s
high-intensity training program on T, C, T/C
×
10
2
, and selected physical fitness indices in men
between 35 and 40 years of age. Methods: The experiment was conducted on a group of 30 healthy
men, divided into control and experimental groups. The experimental group followed a high-
intensity 8-week training program, which included three sessions per week, each of them lasting
1 h and consisting of intensive-interval exercises followed by strength circuit exercises. The controls
did not change their previous recreational physical activity. T, C, and T/C
×
10
2
were measured
before and after the experiment for all participants. Physical performance was examined using a
standardized laboratory exercise test to determine maximal oxygen uptake (VO
2max
). Results: There
were statistically significant increases in T (by 36.7%) and T/C
×
10
2
(by 59%), while C somewhat
dropped (by 12%) in the experimental group. No changes in the hormonal indices were found in
the control group. After completing the experimental training, there were no statistically significant
changes in aerobic capacity, but it improved muscle strength in the men studied. Conclusions:
High-intensity interval training, continued over an 8-week period, modulates (significantly and
positively) the balance between testosterone and cortisol levels and improves physical capacity in
men aged 35–40 years.
Keywords: men; training period; blood; hormones; physical performance
1. Introduction
As people age, adverse physical and physiological changes occur in the human body.
Maximum skeletal muscle strength, mass, and endurance decrease as a consequence of
morphological and physiological changes in the cells of the organs responsible for the
production of important metabolic regulators and target tissues. There are several markers
of these changes that are identifiable in blood and help distinguish biological age from
chronological age [
1
]. Among these markers are steroid hormones belonging to the group
of androgens, mainly testosterone. In men, the process of a gradual decrease in testosterone
(T) synthesis begins at around 35–40 years of age [
2
,
3
]. In this age range, testosterone
J. Clin. Med. 2021,10, 2143. https://doi.org/10.3390/jcm10102143 https://www.mdpi.com/journal/jcm
J. Clin. Med. 2021,10, 2143 2 of 11
levels are reported to decrease by an average of 1.6% each year [
4
]. Testosterone plays an
important role in regulating the metabolism, reproductive system, and mental status of
men. A significant decrease in blood testosterone (<7.0 nM) is considered hypogonadism,
which results in poor somatic and mental health. It should be emphasized that lifestyles
and healthy behavior have a substantial effect on the androgenic status in men aged
30–40 years [5], and, as a consequence, it affects the general state of health [6,7].
Studies relating to the effect of training on testosterone levels have focused on the
changes as a result of physical activity [
8
,
9
]. It should be noted, however, that if the training
activity is too high, i.e., not adjusted to the biological capabilities of the human body,
chronic fatigue syndrome (overtraining) may develop, leading to a decrease in testosterone
production [
10
,
11
]. As mentioned earlier, some authors have suggested that circuit training
alternated with high-intensity interval training may have a beneficial effect on general
health, including muscular and skeletal systems [
12
]. Multiple observations have been
made to determine the age- and exercise-dependent effects of a multi-week training period
on changes in resting testosterone levels [
13
–
16
]. The results showed little or no change
in testosterone levels, which is likely to have depended on the training modality. Apart
from testosterone, the second most important hormone that regulates metabolism and
exercise adaptations is cortisol (C). This hormone is produced by the adrenal cortex and is
commonly called the stress hormone because its blood levels increase in response to strong
stressors, including psychophysical stimuli (such as sports competitions) [
12
,
17
–
19
] and
environmental stimuli (such as exposure to accelerations of >5G) [
20
] or sudden hypobaric
hypoxia induced in laboratory settings (5000 m) [
21
]). As cortisol and testosterone have
opposing effects on the rate of endogenous protein metabolism, the idea has emerged to
assess the anabolic–catabolic balance as an index of the quotient of molar concentrations
(T/C) [
22
,
23
]. A significant decrease in the T/C index indicates that the training activity is
too high and catabolic processes prevail, which, in extreme cases, may lead to a decrease
in skeletal muscle mass. It seems that men who undertake moderate physical activity
at leisure to achieve better physical fitness and health status are not at such high risk.
Nevertheless, amateur workouts of different structures, i.e., weekly training volume,
session intensity, and the work-to-rest ratio during repeated exercises also cause different
biological responses. An optimal training program for middle-aged men with inadequate
physical activity levels should improve cardiorespiratory capacity, muscle strength, and
metabolic status. Therefore, it seems appropriate to monitor their effects using exercise
tests and hormone determinations.
The aim of the study is to evaluate the effect of the author’s high-intensity training
program on testosterone, cortisol, and selected physical fitness indices in men between 35
and 40 years of age. The application value may be the verification of a training program
that will favor the most effective hormone modulation.
2. Material and Methods
2.1. Study Group
The experiment conducted in this study involved a group of men (n= 30), aged 35 to
40 years, who were selected by purposive sampling, with the inclusion criterion being their
sports skill level, determined before the experiment based on the results of a pilot study.
Due to the high intensity of the training, only individuals who had been doing recreational
training for at least 2 years were qualified to participate in the study. The participants were
healthy and did not take any medication on a regular basis. The exclusion criteria were
endocrine disorders and fertility problems. Detailed characterization of participants is
presented in Table 1.
J. Clin. Med. 2021,10, 2143 3 of 11
Table 1. Characterization of the study participants.
Group NVariable Mean SD Min–Max 95%Cl
Experimental
Age (years) 37.7 1.9 35–40 36.81–38.78
15 Stature (cm) 180.0 5.6 174–195 177.68–183.92
BM (kg) 90.6 13.1 65.2–112.0 83.36–97.94
BMI (kg/m2)27.6 3.9 21.2–36.3 25.40–29.71
Control Age (years) 37.8 1.7 35–40 36.88–38.17
15 Stature (cm) 175.0 5.6 163–186 172.29–178.51
BM (kg) 84.8 14.2 65.4–121.2 76.96–92.63
BMI (kg/m2)27.6 3.3 22.9–34.9 25.74–29.44
SD—standard deviation, CI—confidence interval, BM—body mass, BMI—body mass index.
A pedagogical experiment was conducted in the study. The researchers conducted an
intervention that involved manipulating the form and manner of physical activity of the
participants. This was achieved by introducing an experimental training program in the
form of high-intensity endurance and strength training into their daily physical activity. To
conduct the experiment, the technique of working with two equal groups, formed from
previously qualified participants, was used. The men were randomized into two groups: a
control group and a study group (each consisting of 15 participants).
2.2. Research Program and Methodology
The first test for both subgroups was performed before the experiment. In the control
group, the participants continued their previous form of activity, and, between the first
and final examinations, they followed their previous program of recreational physical
activity. In the study (experimental) group, a special training modification was introduced
(independent variable), consisting of performing 60 min of personal endurance and strength
training. The experiment lasted eight weeks. All training sessions were performed in the
afternoon. Before and after this period, hormonal indices and physical capacity levels were
examined (Figure 1).
J. Clin. Med. 2021, 10, x FOR PEER REVIEW 3 of 13
Table 1. Characterization of the study participants.
Group N Variable Mean SD Min–Max 95%Cl
Experimental
Age (years) 37.7 1.9 35–40 36.81–38.78
15 Stature (cm) 180.0 5.6 174–195 177.68–183.92
BM (kg) 90.6 13.1 65.2–112.0 83.36–97.94
BMI (kg/m2) 27.6 3.9 21.2–36.3 25.40–29.71
Control Age (years) 37.8 1.7 35–40 36.88–38.17
15 Stature (cm) 175.0 5.6 163–186 172.29–178.51
BM (kg) 84.8 14.2 65.4–121.2 76.96–92.63
BMI (kg/m2) 27.6 3.3 22.9–34.9 25.74–29.44
SD—standard deviation, CI—confidence interval, BM—body mass, BMI—body mass index.
A pedagogical experiment was conducted in the study. The researchers conducted
an intervention that involved manipulating the form and manner of physical activity of
the participants. This was achieved by introducing an experimental training program in
the form of high-intensity endurance and strength training into their daily physical activ-
ity. To conduct the experiment, the technique of working with two equal groups, formed
from previously qualified participants, was used. The men were randomized into two
groups: a control group and a study group (each consisting of 15 participants).
2.2. Research Program and Methodology
The first test for both subgroups was performed before the experiment. In the control
group, the participants continued their previous form of activity, and, between the first
and final examinations, they followed their previous program of recreational physical ac-
tivity. In the study (experimental) group, a special training modification was introduced
(independent variable), consisting of performing 60 min of personal endurance and
strength training. The experiment lasted eight weeks. All training sessions were per-
formed in the afternoon. Before and after this period, hormonal indices and physical ca-
pacity levels were examined (Figure 1).
Figure 1. Training scheme. Source: own study
Testosterone (T) and cortisol (C) levels and the anabolic–catabolic index, i.e., the mo-
lar T-to-C ratio, were the dependent variables that were measured in fasting venous blood
collected in the morning (8:00 a.m.). The measurements were performed at the ALAB La-
boratoria health center using the Roche test and the ECLIA (enhanced chemiluminescence
immunoassay) method on the Cobas e601 system (Roche Diagnostics, Basel, Switzerland).
Figure 1. Training scheme. Source: own study
Testosterone (T) and cortisol (C) levels and the anabolic–catabolic index, i.e., the molar
T-to-C ratio, were the dependent variables that were measured in fasting venous blood
collected in the morning (8:00 a.m.). The measurements were performed at the ALAB Lab-
oratoria health center using the Roche test and the ECLIA (enhanced chemiluminescence
immunoassay) method on the Cobas e601 system (Roche Diagnostics, Basel, Switzerland).
The increase of maximal oxygen uptake (VO
2max
) during a graded exercise on a treadmill
and the postexercise increase in the strength of selected muscle groups of the upper body
J. Clin. Med. 2021,10, 2143 4 of 11
and upper and lower limbs were adopted as measures of the improvement of physical
capacity. Post-training changes in hormone indices and physical capacity, for each individ-
ual separately, were analyzed using Student’s t-test for dependent samples, whereas the
test for independent samples was used to test intergroup comparisons before and after the
experiment. The level of statistical significance was set at p< 0.05. Intraclass correlation
coefficients (ICCs) and the coefficient of variation (CV%) were calculated. The magnitude
of effect size for comparison between groups and conditions was expressed using Cohen’s
d. When d ranges from 0 to 0.2, the effect is small, i.e., negligible; it is medium from 0.2 to
0.5, large from 0.5 to 0.8, and extremely large when over 1.4. Calculations were performed
using STATISTICA software (ver. 13.3, StatSoft, Krakow, Poland). The participants in
both groups did not change their diets during the experiment. Diets, sleep duration, and
lifestyles were monitored by recording in notebooks and interviews.
Participants were informed of all research procedures prior to participation in the
study, in accordance with the ethical principles of the WMA (The World Medical Associa-
tion) Declaration of Helsinki (2000). The precondition for participation in the study was
the participant’s written informed consent and a medical certificate of no contraindications
to physical exercise. The experiment was approved by the Bioethics Committee at the
Regional Medical Chamber (No. 309/KBL/OIL/2019).
2.3. Measurement of Physical Fitness
In this study, muscle strength andVO
2max
were determined in the study groups using
the following tests [24]:
1.
Evaluation of aerobic capacity. To assessVO
2max
, a running test with graded exercise
intensity is performed on a treadmill (h/p/Cosmos, Nußdorf, Germany). The test
begins with a 2-min recording of respiratory indices at rest, during which the partici-
pants remain in a standing position. During the first 4 min of the test, the participants
run at a speed of 8 km
·
h
−1
. Next, the running speed is increased by 1 km
·
h
−1
every
2 min. The effort is continued until the extreme fatigue of the participants, which
is manifested by the inability to continue running at the set speed. During the test,
the levels of cardiorespiratory indices are recorded based on the breath-by-breath
method using an ergospirometer (Cosmed, Rome, Italy). The highest recorded value
of minute oxygen uptake is considered to be VO2max [25].
2.
Evaluation of abdominal strength (sit-ups). The tested person lies on the mattress
with feet 30 cm apart and knees bent at a right angle. Hands are intertwined, resting
on the neck. The participant is assisted by a partner who holds the participant’s feet
so that they remain in contact with the ground. At the start signal, the participant sits
up to touch their knees with their elbows and then returns to the starting position.
The exercise duration is 30 s.
3.
Evaluation of shoulder girdle strength by the number of repetitions of pull-ups on
a bar. The participant catches the bar with a pronated grip and hangs there; at the
signal, the participant bends his arms at the elbow and pulls his body up so high
that the chin is above the bar, and then, without a rest, returns to a simple hanging
position. The exercise is repeated as many times as possible without rest; the result is
the number of complete pull-ups (chin over the bar)
4.
Evaluation of the dynamic strength of lower limbs (long jump from a standing
position). The participant stands with his feet slightly apart in front of the starting
line and bends his knees and moves his arms backward at the same time; then, he
performs an arm swing and jumps as far as he can. The landing occurs on both feet
while maintaining the upright position. The test is performed twice.
2.4. Experimental Program
Implementation of the experimental program consisted of the introduction of the
strength and endurance training designed for the experiment to the daily activity of the
experimental group (n= 15). The authors made modifications to high-intensity interval
J. Clin. Med. 2021,10, 2143 5 of 11
training (HIIT) [
26
]. With its high diversity, the author’s program was supplemented
with new methods and exercise structures considered to be most effective [
27
–
30
]. The
main part of the training was based on different variants of circuit training and followed
the principles of functional training [
31
–
33
]. Each training session was based on interval
training according to the high-intensity interval training method and ended with strength
circuit training (Table 2). The control group (Co) pursued their previous recreational
physical activity, which was monitored but not programmed by the authors of this study.
Table 2.
Description of exercises of the strength and endurance training program performed by the
experimental group (Ex).
Interval Training 2:1
(HIIT)
Strength Circuit with the Example of
Resistance Training with a Kettlebell
At the beginning of the training, participants had to do
warm-up and adaptation exercises with minimal external
resistance under the supervision of a personal trainer
(duration 10–15 min).
The participant performed a form of activity that involved
high-intensity exercise alternated with rest (60 s/30 s).
It consisted of 10 exercises that were performed one after
the other to form a circuit.
1. Push-ups on dumbbells with the dumbbell pulled
alternately to the chest at the moment of straightening the
arms.
2. Squat with a dumbbell held with both hands at chest
height.
3. Standing kettlebell side bends.
4. Jumps with a change of legs (from the forward lunge
position).
5. Standing dumbbell press.
6. Half squat with a dumbbell held between legs with both
hands.
7. Dumbbell weighted sit-ups.
8. Dumbbell pull (dumbbell row).
9. Push-ups.
10. Dumbbell reverse lunges.
The participant performed a small circuit of 5
exercises with a kettlebell (25 repetitions each).
During one training session, they performed from
3 to 5 circuits, according to the principle of a
gradual increase in loads. The rest between
circuits was 1 to 2 min.
1. Kettlebell swing.
2. Standing one-handed press.
3. Squat with a kettlebell held with both hands in
front of the body.
4. Kettlebell clean.
5. Kettlebell snatch.
The duration of a training session was up to 30
min.
The first session was devoted to learning and
mastering the correct technique of the exercises.
The participant ended each training session with
stretching for about 6 min.
HIIT—high-intensity interval training, 2:1—two units of work to one unit of rest.
To avoid fatigue and training monotony, an innovative training unit that was con-
sistent with the entire program was planned for each session of the week. Each training
session was preceded by a warm-up and included well-thought-out exercises, according to
the author’s ideas. The training program was designed so that it was simple to perform
and accessible to every participant. Individual exercises were performed at a fast pace
(concentric phase: 1 s, eccentric phase: 2 s), with a particular focus on the correct technique
of the movement tasks. Each study group trained three times a week using the assumed
intensity and number of repetitions (Table 2).
3. Results
The reference range for T, determined in the laboratory, is 9.7–27.8 nmol/L. After the
8-week experiment, a statistically significant increase (36.7%) was observed in testosterone
levels in the exercising group (Ex) that performed the HIIT training, while a 6% increase
was not significant in the control group (Co). The relative intragroup variability of T
concentrations expressed by CV% = (SD/X)
×
100 (CV—coefficient of variation, SD—
standard deviation) in the Ex group decreased after training (27.5% vs. 20.8%), whereas it
remained unchanged in the Co group (30.6%). Neither baseline nor postexercise mean T
concentrations differentiated statistically between the two groups, whereas absolute mean
postexercise changes (
∆
T) showed a significant intergroup difference. It is worth noting
that the ICC (intraclass correlation coefficient) for T was statistically significant in both
groups (0.826 for Ex and 0.880 for Co). Baseline T values that were below the lower limit
of the reference range were found in one participant from the Ex group (8.1 nmol/L) and
J. Clin. Med. 2021,10, 2143 6 of 11
one from the Co group (8.0 nmol). Furthermore, two participants in the Co group had
T < 9 nmol after the experiment (Table 3).
Table 3.
Changes in morning serum testosterone levels (T) in the experimental and control groups following the 8-week
training program.
Testosterone
nmol/L
Experimental Group Control Group Between Groups
Mean Median Min Max SD Mean Median Min Max SD t1pCohen’s d
Pre 14.85 14.23 8.05 24.05 4.09 16.66 16.76 8.02 27.62 5.10 −
1.09
0.285 0.391
Post 20.30 18.60 13.85 25.85 4.23 17.66 18.46 8.05 26.82 5.45 1.48 0.150 0.541
differences 5.45 5.48 5.79 1.79 2.40 1.00 0.69 0.03 −0.80 2.32 5.05 0.001 1.89
Between
examinations t2=−9.08, p< 0.001, Cohen’s d =2.27 t2=−1.53, p= 0.149, Cohen’s d = 0.435
t
1
—Student’s t-test for independent samples; t
2
—Student’s t-test for dependent samples; p—likelihood ratio; Statistically significant values
are in bold.
Physiological resting cortisol levels were within the range of 138–633 nM. Mean
cortisol concentrations did not change significantly in both groups after the experiment.
Before the training in the Ex group, C levels were significantly higher than in the Co group,
but the differences disappeared after the experiment. This was due to a marked decrease
in C (by 12%) in the Ex group and no changes in the Co group. The ICC for cortisol was
significant (0.535) only in the Co group (Table 4).
Table 4.
Changes in morning serum cortisol levels (C) in the experimental and control groups following the 8-week training
program.
Cortisol
nmol/L
Experimental Group Control Group Between Groups
Mean Median Min Max SD Mean Median Min Max SD t1pCohen’s d
Pre 460 428 334 666 93 390 406 201 593 113 2.07 0.045 0.681
Post 405 424 274 527 85 385 378 166 702 159 0.47 0.640 0.171
differences −55 −4−60 −
139
118 −5−28 −35 109 137 −
1.21
0.235 0.391
Between
examinations t2=−2.00, p< 0.061, Cohen’s d = 0.466 t2=−0.13, p= 0.902, Cohen’s d = 0.037
t
1
—Student’s t-test for independent samples; t
2
—Student’s t-test for dependent samples; p—likelihood ratio; significant values are in bold.
The analysis of post-training changes in the anabolic–catabolic index revealed that
training according to the HIIT program resulted in a significant increase in the anabolic–
catabolic index (by 59%). Both the increase in T and the decrease in C in this group were
responsible for such a large change. In the Co group, the 16% increase in the index was
insignificant. The ICC for the index was significant in the Co group (0.690) but insignificant
(0.274) in the Ex group (Table 5).
Table 5.
Changes in the T-to-C ratio
×
100 in the experimental and control groups following the 8-week training program.
T/C ×100 Experimental Group Control Group Between
Groups
Mean Median Min Max SD Mean Median Min Max SD t1pCohen’s d
Pre 3.17 2.97 2.16 4.18 0.79 4.63 4.12 2.39 8.33 1.71 −3.37 0.002 1.09
Post 5.05 4.91 3.55 7.61 1.13 5.39 5.00 1.77 13.18 2.73 −
0.49
0.626 1.64
differences 1.88 1.94 1.39 3.43 1.19 0.76 0.88 −
0.62
4.85 1.98 2.11 0.042 0.686
Between
examinations t2=−6.90, p≤0.001, Cohen’s d = 1.58 t2=−0.198, p= 0.111, Cohen’s d = 0.384
t
1
—Student’s t-test for independent samples; t
2
—Student’s t-test for dependent samples; p—likelihood ratio; significant values are in bold.
The results in Table 6show the values of VO
2max
. The calculations showed statisti-
cally insignificant between-group differences in VO
2max
and postexercise changes in this
parameter in each subgroup, although a slight post-training rise of VO
2max
(by 11.8%) was
found in the experimental group.
J. Clin. Med. 2021,10, 2143 7 of 11
Table 6. Changes in the VO2values in the exercising and nonexercising groups following an 8-week period.
VO2max
(mL/kg/min)
Experimental Group Control Group BetweenGroups
Mean Median Min Max SD Mean Median Min Max SD t1pCohen’s d
Pre 32.2 32.0 27.4 38.1 3.6 32.8 22.7 26.3 41.2 4.3 1.08 0.288 0.151
Post 36.0 36.5 31.0 40.2 3.2 33.0 33.0 26.8 41.8 3.6 1.43 0.160 0.881
differences 3.8 3.5 0.5 10.9 −0.4 0.2 10.3 0.5 0.6 −0.7 0.17 0.869 0.315
Between
examinations t2= 0.872, p= 0.395, Cohen’s d = 9.50 t2=−0.13, p= 0.902, Cohen’s d = 0.286
t
1
—Student’s t-test for independent samples; t
2
—Student’s t-test for dependent samples; p—likelihood ratio; significant values are in bold.
A positive correlation coefficient was found between pre–post changes in the variable.
No significant relations between ∆VO2max and ∆T/C ×102were displayed in the control
group (Table 7). It has turned out that the postexercise decrease in cortisol and the increase
in the anabolic–catabolic index promotes the improvement of physical capacity.
Table 7.
Linear regression describing the relationships between pre–post changes (
∆
) in VO
2max
T/C
×102values in the experimental group.
Regression Equation Correlation (r) p-Value
∆VO2max = 0.887 + 1.425 ×∆T/C ×1020.047 0.0047
Table 8shows the changes in physical abilities in terms of the range of motion, strength,
and endurance of selected muscle parts from two testing days in both groups. In the exper-
imental group, the 8-week training resulted in significant improvements in the selected
exercise capacity of the muscles of the upper body (abdominals), upper limbs (pull up),
and lower limb power (SLJ, standing long jump). Interestingly, the control group also
showed gains in the strength of the upper body and upper limb muscle and endurance,
but these changes were much less significant.
Table 8.
Changes in the selected parameters of physical fitness in the experimental (Ex) and control (Co) groups following
an 8-week period.
Groups Testing Sit-Ups
n/30 s Pull-Ups Standing Long Jump
cm
Experimental Group
pre 20.6 ±4.9 3.7 ±4.2 208 ±28
post 26.4 ±3.1 7.1 ±4.3 222 ±25
difference t = −7.60, p=≤0.001
Cohen’s d = 1.93
t = −8.13, p=≤0.001
Cohen’s d = 2.09
t = −6.54, p=≤0.001
Cohen’s d = 1.69
Control Group
pre 20.4 ±4.8 2.7 ±3.7 201 ±16
post 21.3 ±4.7 3.1 ±3.9 205 ±18
difference t = −2.22, p= 0.043
Cohen’s d = 0.574
t = −2.44, p= 0.028
Cohen’s d = 0.634
t = −1.38, p= 0.195
Cohen’s d = 0.355
t
1
—Student’s t-test for independent samples; t
2
—Student’s t-test for dependent samples; p—likelihood ratio; significant values are in bold.
4. Discussion
The dynamics of changes in the biosynthesis of endogenous testosterone depend on
many factors. Among middle-aged men, higher testosterone levels have been observed
in men reporting the highest level of physical activity [
4
,
34
,
35
]. The survey of the Euro-
pean Prospective Investigation into Cancer and Nutrition (EPIC), involving 696 men aged
over 20 years, revealed significantly higher testosterone levels in men who claimed to be
physically active for three or more hours per week compared to those who did not show
greater physical activity [
36
]. The researchers demonstrated that there is an obvious link
between physical activity and testosterone levels by examining more physically active
men. The health benefits of testosterone result from the fact that it has a beneficial effect on
J. Clin. Med. 2021,10, 2143 8 of 11
the cardiovascular system by reducing the production of proinflammatory factors while
increasing vascular endothelial regeneration [
37
,
38
]. Research on the effect of training
type on the short- or long-term androgenic status is, therefore, important in health preven-
tion in the middle-aged male population. Strength training has been shown to increase
testosterone [39,40], whereas endurance training decreases its levels [41].
Biochemical blood tests confirmed the presented results as they showed significant
changes in the experimental group (participants following the author’s training program)
and no significant changes in the control group. It was demonstrated that regular par-
ticipation in the research experiment significantly increased the values recorded during
testosterone measurements. Since the strength and endurance training used in the present
study had a positive effect on testosterone levels in a group of men aged 35–40 years,
improvements in their health in terms of cardiovascular disease prevention can be antic-
ipated. The strength and endurance training performed during the experiment resulted
in an increase in testosterone levels, which, on the one hand, confirms the findings of the
research on the relationship between strength training and testosterone levels. On the other
hand, the novelty of our study is the increase in testosterone levels when strength training
is combined with endurance training. It can be assumed that the achieved results, i.e.,
improvement of power, explosive strength, and aerobic capacity and the shift of metabolic
balance towards anabolism, may have been influenced by the combination and sequence
of exercises in each training session.
A more detailed study on strength training (resistance training) showed that in older
men, the mean T:C ratio, 24 h after a variable intensity (VI) training session, was more
than 33% higher than the value after a constant intensity (CI) session and that lower
cortisol concentration after VI is responsible for this difference [
42
]. Similar tendencies
were observed in young men [
43
]. This trend is in line with the observation of competitive
kayakers who trained 3 times a week over a period of 3 weeks, in which 3 weeks of
endurance training resulted in a slight decrease in T/C, whereas HIIT led to an increase in
this parameter [44].
Some researchers suggest that at the beginning of the training period, hormonal re-
sponses should be examined immediately after a single training session. If the adrenal
cortex responds excessively to training, it is suggested that the intervals be extended
between repeated exercises, which allows a reduction of the stress response, i.e., gluco-
corticoid status, and, at the same time, achieves the planned physical load. In addition to
the effect of the training period on resting hormonal status, it is important to determine
the hormonal responses to a single training session consisting of repeated efforts. The
results of hormonal tests performed before and immediately after the session will provide
information about the magnitude of response to exercise stress and allow the modulation
of the intensity and intervals between subsequent efforts to minimize the cortisol response.
T/C measurements following a session with continuous aerobic exercise (CE) and after
a session with intermittent exercise (IC) showed twice the T/C after the IC session [
45
].
Observations were made on T/C behavior during the three-hour postexercise recovery
period. T/C fluctuated significantly during this period after a high-volume training session,
whereas after a HIIT session, the T/C parameter was more stable. Taking into account the
graph showing the area under the curve, it can be concluded that the time-integrated T/C
value is greater after HIIT [
46
]. The only contradiction to the data presented above was
reported by researchers when both continuous aerobic exercise and HIIT training sessions
led to significant physical fatigue; in this case, 12 h after the end of the session, a greater
decrease in T/C was noted after HIIT [
47
]. The results of our study on hormonal status, T,
C, and T/C demonstrate that the proposed training program, continued for 8 weeks in a
way that promotes protein metabolism, modulates the hormonal indices we are studying
and slightly improves the parameter of physical capacity.
Our study confirmed all findings presented in the literature about the relationship
between training and testosterone levels, which, for the experimental group, showed a
substantial (statistically significant) increase after 8 weeks of training. In the control group,
J. Clin. Med. 2021,10, 2143 9 of 11
no statistically significant differences were found despite the increase in this parameter.
It can also be assumed that this training program does not only lead to an increase in the
level of this hormone but also improves the personal and sexual satisfaction, health, and
physical fitness of the participants [26,48,49].
The results showed no post-training changes in aerobic capacity (VO
2max
), whereas
significant improvements were observed in the muscle strength of the upper body, upper
limbs, and lower limbs. This phenomenon is consistent with the current state of knowl-
edge regarding training and its physiological and biomechanical responses. Workouts
that are typically aerobic increase maximal oxygen uptake. HIIT sessions of sufficiently
high intensity and frequency improve both aerobic and anaerobic capacity and fitness
levels
[25,50–53]
. Typical resistance training improves only neural adaptation, which man-
ifests itself as an increase in strength but does not alter VO
2max
. In light of the changes
found, it can be concluded that the experimental group responded as if resistance exercises
had a dominant role in the HIIT program.
The strength of our study design is the inclusion to the experiment of a group of similar
age and anthropometric features who performed only their habitual physical activity over
the same period. The results inspire further research on the optimization of HIIT and its
monitoring based on physiological indices.
Limitation of the Study
In the present study, we focused on evaluating the effects of high-intensity training on
testosterone and cortisol levels. We did not investigate other physiological and biochemical
mechanisms that may have been altered by the applied training protocol. The results of the
study indicate that further research is needed to determine other body responses resulting
from the proposed form of physical activity. Furthermore, it would be noteworthy to see
how long, after the completion of the experiment, the favorable physical-hormonal status
would persist if the experimental group returned to its former habitual activity.
5. Conclusions
1.
The strength and endurance training, performed based on high-intensity interval
sessions (circuit training), increases testosterone levels in men aged 35–40 years and
can be used to enhance general well-being and partly inhibit harmful age-related
changes.
2.
It is worth using this type of training in adult men because it can positively affect their
quality of life and health by physiologically increasing testosterone levels, lowering
cortisol, and improving anabolic–catabolic balance and muscle strength.
3.
This type of physical activity can act as an alternative or support pharmacotherapy
for increasing testosterone levels in men.
Author Contributions:
Conceptualization, T.A. and Ł.R.; methodology, T.A., Z.O., Ł.R., and J.J.-N.;
validation, T.A., Ł.R., N.S., and B.B.; formal analysis, T.A., Ł.R., and Z.O.; investigation, T.A., Ł.R.,
and W.B.; resources, T.A., Ł.R., Z.O., and J.J.-N.; data curation, T.A., Ł.R., and M.O.; writing—original
draft preparation, T.A., Ł.R., Z.O., and J.J.-N.; writing—review and editing, T.A., Ł.R., Z.O., and
J.J.-N.; visualization, T.A., Ł.R., and M.O.; supervision, T.A., Ł.R., and J.J.-N.; project administration,
T.A. and Ł.R.; funding acquisition, W.B. and T.A. All authors have read and agreed to the published
version of the manuscript.
Funding:
Open Access was financed by the program of the Minister of Science and Higher Education,
entitled ‘Regional Initiative for Perfection’, for the years 2019–2022 (Project No. 022/RID/2018/19; a
total of PLN 11,919,908).
Institutional Review Board Statement: The experiment was approved by the Bioethics Committee
at the Regional Medical Chamber (No. 309/KBL/OIL/2019).
Informed Consent Statement: Informed consent was obtained from all subjects involved in the study.
J. Clin. Med. 2021,10, 2143 10 of 11
Data Availability Statement:
The data presented in this study are available on request from the
corresponding author.
Conflicts of Interest: The authors declare no conflict of interest.
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