Key words combined endurance and strength training - training intensity - training effect -
exercise physiology - strength and aerobic gains - exercise cessation
Introduction
Concurrent training (CT), which involves a combination of resistance and aerobic regimens,
has attracted strong attention from the scientific community in recent years due to
its potential to simultaneously induce aerobic and strength gains [1 ]
[2 ]. While some researchers have shown that CT affects the development of muscle strength
and power (i. e., interference effect) [3 ]
[4 ]
[5 ]
[6 ]
[7 ], others have indicated that CT has no inhibitory effect on strength and aerobic
development compared to strength training alone [8 ]
[9 ]
[10 ]
[11 ]
[12 ]
[13 ]
[14 ]
[15 ]. The interference between strength and aerobic training can be explained by the
training program configuration, such as the volume, intensity, and training frequency
[16 ]
[17 ]
[18 ] or even physical fitness level and age [19 ]
[20 ].
The management of both resistance and aerobic exercise variables can maximize performance
but also expose athletes to overreaching or overtraining if they are incorrectly manipulated
[21 ]. Varying modalities, intensities, frequencies and volumes of training have been
shown to affect the magnitude of molecular signaling and protein synthesis [22 ]
[23 ], which will therefore influence the degree of interference between exercise modes.
Thus, the degree of the interference effect can vary depending on depending on how
the training variables are configured [21 ]
[23 ]. Several studies have indicated that an interference effect exists between aerobic
training and resistance training when the weekly training volume is high [21 ]
[24 ]
[25 ]
[26 ]
[27 ]
[28 ]. It seems quite clear that high volumes of aerobic training, either due to an increase
in the frequency and/or duration of aerobic exercises, results in the inhibition of
strength gains, in contrast to low volumes of aerobic training [27 ]
[28 ]. According to these studies [27 ]
[28 ], it seems that an increase in the volume of aerobic training induces a higher degree
of fatigue, which compromises the quality of strength training and, consequently,
the possible chronic adaptations. Nevertheless, it is still not clear what happens
on both cardiorespiratory and neuromuscular performance when the intensities of the
aerobic or/and strength training performances are manipulated.
Researchers focused on CT have recently tried to understand its effects by studying
the detraining period after a CT program [1 ]
[29 ]
[30 ]. In fact, interruptions in the training process due to illness, post-season vacation,
or other factors are ordinary in most of sports [26 ]
[31 ]. The magnitude of this reduction may depend on the duration of the detraining period
and on the training level of the subject. It seems that the longer training period,
the longer the detraining period needed for severe performance decrements [1 ]
[32 ]. Knowing the effects of training on subsequent detraining period will allow to better
understand how to design a training program, either to optimize and reduce performance
losses, or to better understand how to combine the periodization models regarding
the training load and recovery phases to maximize gains and competitive performance
[33 ].
In recent years, several reviews have been published analyzing CT [21 ]
[27 ]
[28 ]
[34 ], but, to the best of authors’ knowledge, no systematic review has comprehensively
examined the literature regarding the effects different resistance and/or aerobic
intensities in CT and the effects of the subsequent detraining period on performance.
Analyzing studies that have evaluated CT intensities would provide coaches and sports
scientists with valuable knowledge and strategies to effectively combine aerobic exercise
with bouts of resistance training when seeking improved performance across training
and competition. Therefore, the current review aims to analyze and compare the effect
of training intensity during resistance and aerobic training in CT programs on changes
in strength and endurance variables. A secondary purpose was to analyze the effect
of training intensity during CT on the detraining period.
Materials and Methods
Search strategy
A systematic review was conducted according to PRISMA (Preferred Reporting Items for
Systematic reviews and Meta-analyses) guidelines [35 ]. A disciplined literature search was independently conducted by two researchers
using the Web of Science, PubMed, ScienceDirect, Scholar Google, and Scopus databases.
An extensive literature search was conducted from January 1, 1980, to April 30, 2019,
to identify studies related to concurrent training with different aerobic or/and resistance
training intensities, and the effects of detraining period in young adults. The search
was performed using the Boolean search method, which limited the search results (including
AND/OR) to only those documents containing key terms relevant to the scope of this
review. The search terms used were “concurrent training”, “resistance training”, “aerobic
training”, “detraining”, “intensities”, and “young adults”. The review was conducted
in accordance to the International Journal of Sports Medicine ethical standards in
sport and exercise science research [36 ].
Eligibility criteria
The included studies focused on experimental interventions related to CT and detraining
in young adults (between 18 and 35 years old) with performance-related outcomes (i. e.
time, velocity, strength, aerobic capacity and power). Studies written in English
that were published in a peer-reviewed journal, assessing different intensities of
resistance and/or aerobic training during CT programs, and studies on the effect of
training intensity during CT on the detraining period, in healthy young adults were
included. Review articles (qualitative review, systematic review, and meta-analysis),
thesis, dissertations, conference abstracts and proceedings were not considered. Regarding
the research question, studies were categorized into the following two main groups:
i) effects of concurrent training and ii) effects of detraining. The information extracted
from the selected studies was based on research design, aim, subjects, procedures
and findings.
Study selection
The initial search identified 2 580 initial studies. After removing duplicates and
studies with different types of intervention (e. g., different than longitudinal studies),
subjects with other chronological ages (children, elderly), and subjects who did not
include a session of CT in the protocol, 2 498 studies were excluded. From the remaining
studies, the full texts of 13 original research articles were assessed for eligibility,
and those that did not meet the inclusion criteria were excluded (e. g., inconclusive
information on study procedures). For the qualitative analysis, a total of 9 studies
were considered relevant for a detailed analysis. The earliest of these studies was
published in 2007 [37 ], and the most recently published study was from 2019 [30 ]. The articles were grouped according to the CT intervention (n=8) or to the presence
of detraining (n=3). A detailed flow chart describing the process of selecting the
relevant studies is shown in [Fig. 1 ].
Fig. 1 PRISMA study flow diagram.
Data analysis
Assessment of risk of bias
Quality analysis of the identified studies was conducted independently by two researchers
using methods recommended by the Cochrane Collaboration [38 ]. Any conflict was resolved by including an independent researcher. All relevant
biases, such as sequence generation, allocation concealment, blinding of participants,
personnel and outcomes, incomplete outcome data, selective outcome reporting, and
other sources of bias, were checked, and the studies were graded. The following classifications
were used: low risk, high risk, or unclear risk (either lack of information or uncertainty
regarding the potential for bias). Review Manager software (RevMan, Copenhagen, the
Nordic Cochrane Centre) version 5.3.5 was used to create risk-of-bias graphs.
Statistical analysis
The results of the included studies were used to quantify the percentage of change
for each variable during training programs ([post – pre/pre] × 100). Moreover, the
results of the included studies were recalculated to determine effect sizes (ES) as
a measure of the difference between averages in terms of standard deviation units,
which provides information about the magnitude of the observed differences. This analysis
was calculated using Cohen’s d [39 ], where the post-training values were subtracted from the pre-training values and
divided by the combined standard deviation. This method allowed us to determine the
magnitude of differences for the studies that provided means and standard deviations.
The magnitude of the ESs was considered trivial (<0.2), small (0.2–0.59), moderate
(0.60–1.19), large (1.2–1.99) or very large (>2.00) [40 ].
Results
[Table 1 ] presents a summary of the studies that monitored the intensity variations of CT
in young adults (athletes and nonathletes). Of the nine studies included in the current
review, most included assessments of strength performance and aerobic capacity [29 ]
[30 ]
[41 ]
[42 ]
[43 ]
[44 ]. The tests most commonly used to evaluate strength and power-output were the one-repetition
maximum (1RM; 67% of the studies) test [7 ]
[29 ]
[30 ]
[41 ]
[42 ]
[43 ] and the countermovement jump (CMJ; 67% of the studies) [29 ]
[30 ]
[41 ]
[42 ]
[43 ]
[44 ], respectively. Aerobic speed and/or oxygen uptake were variables used to evaluate
cardiorespiratory fitness in 89% of the studies [1 ]
[29 ]
[30 ]
[37 ]
[41 ]
[42 ]
[43 ]
[44 ]. Most of the subjects were males between 20 and 30 years of age. Another important
issue was related to the training program duration, which ranged from 5 weeks [1 ] to 20 weeks [37 ] of implementation.
Table 1 Characteristics of the studies included in the review.
Author
Subjects
Age
Duration
Outcomes
Esteve-Lanao et al. [37 ]
12 runners (male)
27.0
20 weeks
HR; HRpeak; VO2 max; Running performance (10.4 km)
Fyfe et al. [41 ]
23 physically active (male)
29.6
8 weeks
1RM leg press and bench press; CMJ; VO2 peak; LT; Body composition
Joo, C.H. [1 ]
20 semi-professional soccer players (male)
22.1
5 weeks
Sprint (30 m); Repeated sprints (34.2 m); Yo-Yo test; Arrowhead agility test
Petré et al. [42 ]
16 high-level athletes (male)
27.3
6 weeks
VO2 max; VO2 max Time limit; [LA-]; MLSS; 1RM; CMJ.
Silva et al. [7 ]
44 physically active (female)
23.5
11 weeks
Knee extension; Leg press; Bench press; 1RM; Isometric and isokinetic peak torque
Sousa et al. [29 ]
32 physically active (male)
20.6
12 weeks
Sprint (20 m); CMJ; 1RM; VO2 max
Sousa et al. [30 ]
36 physically active (male)
21.0
12 weeks
Sprint (20 m); CMJ; 1RM; VO2 max
Varela-Sanz et al. [43 ]
35 sport science students (male and female)
18–27.0
8 weeks
Sprint (10 m; 30 m); CMJ; 1RM; VO2 max
Wong et al. [44 ]
39 professional soccer players (male)
24.6
8 weeks
Jump height; Ball-shooting; Sprint (10 m; 30 m); Yo-yo test; MAS; HRmax
[LA-]: Blood lactate concentration; 1RM: 1 maximal repetition; CMJ: Countermovement
jump; HR: heart rate; HRmax: maximal heart rate; HRpeak: heart rate peak; LT: lactate
threshold; MAS: maximal aerobic speed; MLSS: maximal lactate steady-state; VO2 max: maximal oxygen uptake; VO2 max time limit: time at maximal oxygen uptake;
The analyzed studies were mainly focused on the exercise intensities during the aerobic
component of CT training ([Table 2 ]). From the 8 selected studies, all experimental interventions induced improvements
in the variables assessed, regardless of the intensity used in the resistance training
or in the aerobic training. When higher aerobic intensities were combined, the magnitude
of changes of maximal oxygen uptake (VO2 max) and aerobic power were moderate [41 ]
[42 ]. Moreover, moderate to large neuromuscular adaptations were found when higher resistance
training loads were combined with low to moderate aerobic training intensities [29 ]
[30 ]
[42 ].
Table 2 Effects of intensity during concurrent training in performance.
References
Main aim
Intervention
Main findings*
Esteve-Lanao et al. [37 ]
Compare different loads distribution
G1 (n=6): Z1=80%; Z2=10%; Z3=10%
[10.4 km time]**G1: −7%, ES=2.1; G2: −5%, ES=1.4
Fyfe et al. [41 ]
Effects of different intensities and types of CT
G1 (n=7)=moderate continuous training 80–100% LT+~ 65–90%1RM
[1RM LP]** G1: 27%, ES=0.8; G2: 29%, ES=1.2; G3: 39%, ES=1.3
Petré et al. [42 ]
Effects of different combinations of AT
G1 (n=8)=≥ 80% 1RM+CT low volume and HIIT at ~150% VO2 max (4–12 min)2 max (40–80 min)
[VO2 max] G1: 5%, ES=0.6
Silva et al. [7 ]
Effects of different intensities and types of aerobic exercise
G1 (n=10)=RT+20 min continuous running 95% VT2 ;2 max, 1 min of active recovery at 50% vVO2 max;2
[1RM LP] G1: 41%, ES=2.3; G2: 47%, ES=2.2; G3: 39%, ES=1.8; G4: 53%, ES=2.0
Sousa et al. [29 ]
Compare different external loads of RT during CT
G1 (n=9)=40–55% 1RM+20 min (75% MAS)
[CMJ] G1: 12%, ES=0.7; G2: 14%, ES=1.0; G3: 12%, ES=1.22 max] G1: 15%, ES=0.7; G2: 12%, ES=0.8; G3: 12%, ES=1.1
Sousa et al. [30 ]
Compare different aerobic intensities during CT
G1 (n=10)=70–85% 1RM+16–20 min at 80% MAS
[CMJ] G1: 9%, ES=0.7; G2: 10%, ES=0.6; G3: 7%, ES=0.62 max] G1: 10%, ES=0.5; G2:11%, ES=0.6; G3: 10%, ES=0.5
Varela-Sanz et al. [43 ]
Influence of intensity distribution
G1 (n=12)=Traditional-based training: 24–37 min of running at 65–75% MAS+3–5 ×10–12RM;
[CMJ] G1: −7%, ES=0.4; CG: −8%, ES=0.7
Wong et al. [44 ]
Effect of high-intensity CT
CG (n=19): Soccer training
[CMJ]** G1: 4%, ES=2.0
* only main findings and statistically significant between pre and post-training are
presented; ** p<0.05 between experimental groups; AT=aerobic training; BP=bench press;
CG=control group; CMJ: countermovement jump; CT=concurrent training; Gn=Experimental
group n; HIIT=high intensity interval training; LT=lactate threshold; LP=leg press;
MAS=maximal aerobic speed; RM=repetition maximum; RT=resistance training; SQ=squat;
VO2 max=maximal oxygen uptake; VT2=second ventilatory threshold; YYIRT=Yo-Yo intermittent
recovery test; Z1=below ventilatory threshold; Z2=between ventilatory threshold and
respiratory compensation threshold; Z3=above respiratory compensation threshold.
Focusing on the distribution of exercise intensities during a long-term CT program,
it was suggested that an undulating nonlinear periodization model intensities (polarized
model), with most of training time spent at light and very hard aerobic intensities,
with little at moderated/hard intensities, would be the most effective training intensity
distribution for reducing the interference in neuromuscular performance [37 ]
[43 ]. Running performance was approximately 2% greater with polarized training compared
to the traditional distribution [37 ]. Moreover, the upper and lower body maximum strength increased 24 and 47%, respectively,
after 8 weeks of polarized training [43 ].
Among studies on exercise intensities during CT, only 3 (33%) focused on the issue
of detraining. [Table 3 ] presents a summary of the studies that monitored the effects of detraining on physical
performance in young adults. Sousa et al. [29 ] reported that CT loads in resistance training seem not to influence the reversibility
of the training effects after a detraining period of 4 weeks. In the same study, the
gains obtained from low, moderate and high resistance training loads combined with
low-intensity aerobic training decreased between 2 and 15% after detraining. In accordance
with this finding, Joo [1 ] verified that 2 weeks of detraining after a competitive season resulted in marked
decreases in repeated sprints and agility variables of elite soccer players. Moreover,
combining different aerobic training intensities with the same resistance training
also resulted in performance decrements after 4 weeks of detraining, but smaller performance
decrements when lower aerobic training intensity was performed [30 ].
Table 3 Effects of concurrent training intensities after detraining.
References
Main aim
Intervention
Main findings
Joo [1 ]
Effects of HIT with reduced volume and training cessation
G1=DT combined with high-intensity AT (3×12 min at 80–90% of HRmax)
[Agility] G1: 2%, ES=0.7; G2: 1%, ES=0.5#
#
Sousa et al. [29 ]
Compare different external loads of RT during CT followed by 4-weeks DT
DT of 4 weeks after CT for 8 weeks
[20 m time] G1: 2%, ES=0.4; G2: 4%, ES=1.0; G3: 3%, ES=0.82 max] G2: −15%, ES=1.1; G3: −9%, ES=0.9
Sousa et al. [30 ]
Compare different aerobic intensities during CT followed by 4-weeks DT
DT of 4 weeks after CT for 8 weeks
[CMJ] G1: −3%, ES=0.3; G2: −5%, ES=0.3 2 max] G2: −5%, ES=0.3; G3: −7%, ES=0.4
AT=aerobic training; CT=concurrent training; DT=detraining period; Gn=Experimental
group n; MAS=maximal aerobic speed; RM=repetition maximum; RT=resistance training;
VO2 max=maximal oxygen uptake; # no data was available for ES calculation and for exact
percentage.
Risk of bias in the included studies
The included studies were randomized, but few described the sequence of the randomized
generation [1 ]. Most were not clear regarding the blinding outcome assessment, or this was performed
by the main researcher of the study, which reveals high risk of bias [1 ]
[29 ]
[37 ]
[41 ]
[42 ]
[43 ]
[44 ] ([Fig. 2 ], [3 ]).
Fig. 2 Judgements about each risk of bias item for each included study.
Fig. 3 Risk of bias item presented as percentages across all included studies.
Discussion
The current review aimed to analyze and compare the scientific evidence regarding
aerobic and resistance exercise intensities during CT and their effect on strength
and endurance variables. Moreover, the effect of training intensity during CT on the
detraining period was also assessed to better understand the impact on the training
cessation. The studies on this topic were relatively recent, with increased interest
in the last 2 decades. CT has been studied since the early 1980s; however, only recently
researchers have focused on the influence of training intensity during aerobic or
resistance training on neuromuscular adaptations. The few studies found have shown
improvements in strength and cardiorespiratory performance regardless of the different
intensities used in aerobic and/or resistance training during CT. Nevertheless, it
seems to exist a trend toward higher neuromuscular improvements when high resistance
training loads were combined with low to moderate aerobic training intensities. The
aerobic gains were found to be greater when higher aerobic training intensities were
used, regardless of the resistance training intensities.
The increase of interest regarding CT may be due to its potential to simultaneously
provide gains in cardiorespiratory fitness and strength [45 ]
[46 ] as well as the short time requirement and the convenience of this training program
for several sports disciplines [47 ]. In several sports, CT is a usual method of training, as it combines the specific
motions of sports, such as swimming or running, with resistance training to obtain
gains in several actions/tasks crucial for increase sport performance [48 ]
[49 ]
[50 ]
[51 ]. Our search revealed that among the studies on CT intensity, only a few reported
data on professional athletes [37 ]
[42 ]
[44 ]. Non-athletes are also an important cohort, and they were studied in several reports
[1 ]
[7 ]
[29 ]
[30 ]
[43 ]. Therefore, one of the main problems detected by the present review was that further
studies must be conducted with competitive individuals.
From the selected studies, only Sousa et al. [29 ] focused on the intensities of resistance training in CT. Sousa and colleagues [29 ] suggested resistance training programs with low, moderate and high external loads
combined with low-intensity aerobic training to produce gains in strength and aerobic
capacities. Moreover, they suggested that loads higher than 55% 1RM of resistance
training combined with low-intensity aerobic training were efficient in improving
rapid voluntary muscle contractions, such as short runs and CMJ. However, few is known
regarding the effects of concurrent resistance training with higher loads, combined
with high intensities of aerobic training on physical performance of young adults.
In this sense, it has been described that higher intensities of aerobic training can
cause greater metabolic perturbation in type II muscle fibers and potentially compromise
anabolic responses from strength training [52 ]
[53 ]. Thus, these findings highlight the importance of further knowledge on the intensities
of resistance training during CT so that coaches could minimize the interference phenomenon
and efficiently improve performance.
The study of the intensity during CT was mostly restricted to the aerobic component.
For instance, Silva et al. [7 ] reported that different intensities of aerobic training, combined with the same
resistance training, twice a week for eleven weeks, does not seem to differently affect
the strength development. Thus, it would be suggested that different intensities of
aerobic training enhance athletes’ performances in similar magnitudes. Concordantly,
Fyfe et al. [41 ] evidenced that eight weeks of high-intensity or moderate-intensity of cycling for
15–33 min combined with the same resistance training induced improvements in maximal
strength and neuromuscular performance. However, when compared with resistance training
alone, both aerobic intensities similarly attenuated improvements in maximal lower-body
strength (1RM and CMJ). Thus, we should highlight the similarity of the gains between
the different training intensities, but we should not disregard the possible interference
effect on strength gains by the moderate and high intensities used [22 ].
It was previously suggested that the high intensity aerobic exercise could impair
acute molecular interference and attenuate the anabolic response [22 ]
[54 ]. The higher aerobic intensities may increase glycogen depletion and intensify residual
fatigue, possibly compromising muscle regeneration and training adaptations [55 ]. Studies independently examining the effect of aerobic exercise intensity on concurrent
training are scarce. It seems difficult for the researchers to investigate different
aerobic intensities during CT without changing the volumes or methods of training.
This is easily explained by the high intensities used, which require some rest or
even reduction of the exercise duration, resulting in changes of training methods.
As evidenced by Fyfe et al. [41 ], two different methods of aerobic training are usually compared (continuous vs.
interval) and this could affect the conclusions obtained. Recently, Petré et al. [42 ] compared different aerobic training intensities (low volume of high intensity interval
training vs. high volume of moderate continuous training) combined with the same resistance
training (loads higher than 80% 1RM) in former competitive athletes. These authors
found that strength improved with high and low intensities of aerobic training. However,
VO2 max gains were only found when resistance training was combined with low volume of
high intensity interval training. This suggested that higher aerobic intensities should
be used during CT for greater aerobic performances [30 ]
[42 ].
Recently Sousa et al. [30 ] aimed to verify the effects of low, moderate or high intensities of running exercise
combined with the same resistance training on strength and aerobic performances. Strength
performance (1RM) was significantly higher in the low-intensity aerobic training than
moderate and high intensities and there were moderate positive effects in sprint and
CMJ performances. Moreover, these authors found that when lower aerobic intensities
were implemented during CT, smaller performance decrements were shown after detraining
period. Conversely, recent reviews [27 ]
[34 ] on CT suggested that high intensity interval training can be prescribed alongside
resistance training without negatively impacting changes in muscle mass. Despite it
seemed contradictory recommendations, all seemed to agree that an adequate rest between
aerobic and resistance training sessions should be provided for these gains to occur.
Coffey and Hawley [21 ] warned for the fact that recommending divergent exercise modes on different days
to avoid interference effect of concurrent training is simplistic and not representative
of the real training and competition context. Therefore, when training in the same
training session, it is recommended to choose lower aerobic intensities for increased
strength gains [30 ].
Research on CT issues has also focused on the distribution of training intensities
throughout the season [37 ]
[43 ]. In this regard, Esteve-Lanao et al. [37 ] sought to understand how the day-to-day aerobic training intensity should be distributed
and combined with the same resistance training program in order to obtain the greater
improvements in physical performance. The training intensity during endurance training
was typically divided into different arbitrary intensity zones (Z1=below ventilatory
threshold; Z2=between ventilatory threshold and respiratory compensation threshold;
Z3=above respiratory compensation threshold), and the authors aimed to verify the
effects of a traditional training program emphasizing moderately high-intensity aerobic
training or those of a new trend of polarized training emphasizing the low-intensity
zone. After 21 weeks of training program, the runners who combined resistance training
with polarized aerobic training emphasizing low-intensity training zones resulted
in greater 10.4 km performance enhancements than the others. Interestingly, another
recent study [43 ] suggested that 8 weeks of traditional training-based regimens (i. e., moderate volume
and intensity of CT) produced similar improvements in neuromuscular and cardiorespiratory
fitness as polarized training. Discrepancies between results of both studies [37 ]
[43 ] could be due to differences in training duration (8 vs. 21 weeks) and sample characteristic
(sports science students vs. competitive sub-elite athletes). Therefore, training
intensity distribution seems to be irrelevant for training programs lasting a few
weeks for non-athletes, but polarized training is suggested as the most effective
training intensity distribution for improving competitive performance [37 ].
Regarding the detraining period, only three studies analyzed the effects of CT intensities
during training cessation [1 ]
[29 ]
[30 ]. These studies revealed that the training-induced gains may be compromised with
short-term detraining period (2–4 weeks), leading to a return to baseline values [1 ]
[29 ]. Sousa et al. [29 ]
[30 ] demonstrated that a 4-week period of training cessation after CT with different
resistance or aerobic training loads compromised training-induced gains in young men.
In the study by Joo [1 ], only 2 weeks of detraining after a competitive season markedly decreased performance.
Therefore, despite scarce evidence, it seemed that regardless of the intensities of
the previous endurance and resistance training during CT, only 2–4 weeks of training
cessation can cause a significant and marked loss of performance. Some possible causes
for this performance impairment could be the change in skeletal muscle morphology,
a reduction in mean fast twitch fiber cross-sectional area, a reduction of oxidative
enzyme activities, in glycogen synthase activity and in mitochondrial ATP production
[32 ]. However, when different aerobic training intensities (80 vs. 90 vs. 100% of maximal
aerobic speed) were combined with the same resistance training, the lower intensities
showed smaller decrements during detraining, especially in VO2 max [30 ].
The scientific evidence and knowledge that is provided by the current review should
be helpful for coaches and professionals to improve the training program design and
consequently enhance performance. Moreover, it was clear that this issue is still
unknown, and that further research is required. It is important to understand the
effect of different resistance training intensities and/or different aerobic training
intensities and then investigate methods of combining these exercise modalities. Moreover,
more research on competitive athletes should be conducted and longitudinal studies
with longer training periods should be developed to analyze the interference of CT
at different intensities and how performance changes over time.
To the best of our knowledge, no detailed systematic review has comprehensively examined
the literature regarding the effects of the intensities used in a CT training program,
specifically in the aerobic or/and resistance training component. However, we found
some limitations in the comparison of the results presented by the different investigations,
and recommendations concerning optimal intensities to use during CT were designed
based on the present data. It is worth noting that there were differences in the subject’s
characteristics (athletes and non-athletes) and even in the training programs (frequency,
intensities, type) between the included studies that conditioned the analysis. Furthermore,
only few studies were found on this issue and some methodological quality flaws compromised
general conclusions. Moreover, longer interventions should be studied for a better
understanding of this subject.
In brief, despite the lack of longitudinal studies on different CT intensities and
performance, it seems evident that CT with different intensities positively influences
the performance of young adults. Furthermore, short-term training cessation (2–4 weeks)
compromises the training-induced gains. The few studies revealed greater strength
and neuromuscular performance gains when the CT program combined high-intensity resistance
training with low-intensity aerobic training (e. g., 70–85% 1RM and 80% of maximal
aerobic speed), and an interference effect seemed to exist for higher aerobic exercise
intensities. Higher aerobic exercise intensities (e. g., interval training at 150%
VO2 max or higher than 90% of maximal aerobic speed) should be used to improve cardiorespiratory
fitness, but improvements in strength could be compromised. Regarding the intensity
distribution during the aerobic regimen, the polarized model may be better at reducing
interference in neuromuscular performance. Nevertheless, we should be cautious and
consider these findings to be tendencies, while being aware that further research
is needed on this matter. The information shown in this review could provide useful
tools for coaches to develop efficient training programs. Athletes and coaches should
design their CT program according to their main goal of increasing aerobic or strength
capacity.
