Neuromuscular adaptations to concurrent
strength and endurance training
JOHN P. McCARTHY, MYRON A. POZNIAK, and JAMES C. AGRE
Departments of Orthopedics & Rehabilitation, Kinesiology, and Radiology, University of Wisconsin-Madison, Madison, WI
McCARTHY, J. P., M. A. POZNIAK, and J. C. AGRE. Neuromuscular adaptations to concurrent strength and endurance training. Med.
Sci. Sports Exerc., Vol. 34, No. 3, pp. 511–519, 2002. Purpose: The purpose of this study was to examine muscle morphological and
neural activation adaptations resulting from the interaction between concurrent strength and endurance training. Methods: Thirty
sedentary healthy male subjects were randomly assigned to one of three training groups that performed 10 wk of 3-d·wk?1
high-intensity strength training (S), cycle endurance training (E), or concurrent strength and endurance training (CC). Strength,
quadriceps-muscle biopsies, computed tomography scans at mid-thigh, and surface electromyogram (EMG) assessments were made
before and after training. Results: S and CC groups demonstrated similar increases (P ? 0.0001) in both thigh extensor (12 and 14%)
and flexor/adductor (7 and 6%) muscle areas. Type II myofiber areas similarly increased (P ? 0.002) in both S (24%) and CC (28%)
groups, whereas the increase (P ? 0.004) in Type I area with S training (19%) was also similar to the nonsignificant (P ? 0.041)
increase with CC training (13%). Significant increases (P ? 0.005) in maximal isometric knee-extension torque were accompanied by
nonsignificant (P ? 0.07) increases in root mean squared EMG amplitude of the quadriceps musculature for both S and C groups. No
changes (P ? 0.38) in the EMG/torque relation across 20 to 100% maximal voluntary contractions occurred in any group. A small 3%
increase (P ? 0.01) in thigh extensor area was the only change in any of the above variables with E training. Conclusions: Findings
indicate 3-d·wk?1concurrent performance of both strength and endurance training does not impair adaptations in strength, muscle
hypertrophy, and neural activation induced by strength training alone. Results provide a physiological basis to support several
performance studies that consistently indicate 3-d·wk?1concurrent training does not impair strength development over the short term.
Key Words: HYPERTROPHY, MUSCLE FIBERS, CT SCAN, EMG, RESISTANCE EXERCISE
no interference in strength development with concurrent
strength and endurance training over a short term
(1,14,27,31,34,38). Studies investigating the interaction of
these two diverse types of training, however, provide strong
evidence that concurrent training does not impair endurance
development as measured by maximal aerobic power
(21,22,25,27,34). It thus appears, that with concurrent
strength and endurance training, the major consideration is
that of endurance training possibly interfering with the neu-
romuscular system’s ability to generate maximal force.
Although a considerable number of studies have ad-
dressed performance adaptations with concurrent training,
only a few have attempted to address underlying physiolog-
ical mechanisms responsible for impairment in strength
development. Muscle hypertrophy and changes in motor
unit recruitment are two of the most salient factors associ-
ated with strength development (16). Kraemer et al. (25)
indicate an impairment in hypertrophy of Type I fibers when
endurance training is added to strength training. Bell et al.
(4) reported no increase in Type I fiber area with concurrent
trength and power development has been impaired
when endurance training is added to strength training
(4,11,21–23,25). Other investigators, however, report
strength and endurance training, but there were no differ-
ences in fiber type area adaptations (both Type I and II)
compared with the same type of strength training performed
alone. With concurrent training, Nelson et al. (31) reported
hypertrophy in Type I, IIa, and IIb fibers, whereas the
strength-only training employed in this study resulted in
hypertrophy of only the Type IIb fibers. None of these
studies considered muscle adaptations at the macroscopic or
whole muscle level as may be assessed with imaging tech-
nology. Sale et al. (34) reported similar muscle area in-
creases with both strength-only and concurrent training as
measured at both the microscopic (Type I and II fibers) and
macroscopic (quadriceps area as assessed using computed
tomography) levels. It is evident that the pattern of specific
fiber type hypertrophy with concurrent training is inconsis-
tent in the few investigations addressing this question and
little is known about adaptations at the whole muscle level
With strength training there are several different lines of
evidence that indicate adaptations within the nervous system
occur that are related to strength development (33). One of
the major areas of investigation with strength training fo-
cuses on adaptations in motor unit activation in prime mov-
ers as measured by electromyography (EMG) (30,33). The
integrated EMG signal attained during a maximal isometric
contraction has been shown to increase in several longitu-
dinal strength-training studies (16,17,29), although other
investigations report no change in this measure
(13,30,37). It has been suggested that an impairment in
MEDICINE & SCIENCE IN SPORTS & EXERCISE®
Copyright © 2002 by the American College of Sports Medicine
Submitted for publication January 2001.
Accepted for publication July 2001.
force development with concurrent training, as compared with
strength-only training, may be related to altered neural activa-
tion associated with maximal voluntary contractions (6,25,26).
The relatively small number of maximal or near maximal
contractions involved in strength training demand different
patterns of motor unit activation than the fairly continuous low
to date have investigated the effects of concurrent training on
neural activation of prime movers.
To investigate major mechanisms associated with changes
in strength performance, the purpose of this study was to
evaluate the influence of 3-d·wk?1concurrent strength and
endurance training on muscle morphology and neural activa-
tion in previously sedentary individuals. We tested hypotheses
that the diversity of demands of traditional endurance and
strength training on skeletal muscle would limit muscle hyper-
trophy at the macroscopic (whole muscle) and microscopic
(myofiber) levels. In addition, we tested the hypothesis that
concurrent-training adaptations are different than any changes
that may occur with strength-only training in neural activation
Subjects and experimental design. The data were
collected as part of a larger study that also examined the
effect of concurrent training on body composition, V˙O2peak,
and strength performance (27). Thirty sedentary healthy men,
start of the study, served as subjects. After approval from the
Health Sciences Human Subject Committee at University of
Wisconsin-Madison, all subjects were informed of the proce-
dures, risks, and benefits, and provided written consent before
participation. Each subject was screened via a medical history
questionnaire and a physical evaluation given by a physician.
Measures of vastus lateralis fiber area and type distribution
(from muscle biopsies), thigh cross-sectional muscle area
(from computerized tomography [CT] scans), knee-extension
isometric torque and associated quadriceps RMS-EMG ampli-
tude across 20–100% maximal voluntary contractions (MVC)
were taken before and after 10 wk of training. After all pre-
training measurements, subjects were randomly assigned to
either a strength-only (S), an endurance-only (E), or a concur-
rent (CC) strength- and endurance-training program. Charac-
teristics of subjects in the three groups are presented in Table
1. Sample size was estimated as described by Cohen (7) for
paired t-tests and analysis of variance (ANOVA) (see Statisti-
cal Analysis section) by using the variable of maximal isomet-
ric torque and data from a pilot study (27). To detect an effect
size difference of 15 N·m for paired t-tests with ? ? 0.0167,
the power (1-?) ? 81; and for ANOVA with ? ? 0.05, the
power ? 76. All subjects proceeded through all testing proce-
dures with one exception. One subject in the CC group did not
have muscle-biopsy samples taken.
Training. Conventional high-intensity strength- and
continuous-endurance-training regimens were employed in
this study and have been previously described in detail (27).
All subjects completed 10 wk of 3-d·wk?1exercise training
on alternate days. The S group performed eight weight-
training exercises for one warm-up set and three maximal-
effort sets. The goal for number of repetitions throughout
the training was six (range 5–7). After the warm-up set in
each exercise, subjects performed as many repetitions as
possible until muscular failure (could not perform another
full repetition). If a set was performed outside of the range
of 5–7 reps, the load of subsequent sets was adjusted ac-
cordingly. The goal was always to perform a 6-repetition
maximum (6RM); thus, progression was always incorpo-
rated into this program. Warm-up sets were performed with
two-thirds of the load used to perform a 6RM. During the
first week of training, starting weights were determined by
trial and error. Each exercise utilized low contraction ve-
locities with subjects performing concentric and eccentric
phases of repetitions in approximately 1–2 s each, with the
last repetition of a set generally the slowest with subjects
close to muscular failure. Rest between maximal sets was
approximately 75 s (60–90 s). All sets of each exercise were
performed in succession before moving to another exercise.
Exercises performed using barbells included parallel squats,
bench presses, and standing curls. Exercises performed us-
ing plate loaded machines (Badger Fitness Equipment,
South Milwaukee, WI) included knee extensions, leg curls,
wide-grip lat pull-downs, overhead presses, and heel raises.
The E group performed 50 min of continuous cycle er-
gometry at an intensity of 70% heart rate reserve. The first
5 min of exercise served as a warm-up and was performed
at two-thirds of the normal training workload of the follow-
ing 45 min. S and E programs were designed to elicit
substantial improvements in either strength or aerobic ca-
pacity, and both were designed to require substantial in-
volvement of the knee extensor muscle group. The CC
group completed both S and E programs in the same training
session. Order of S and E training was rotated each training
day with a 10- to 20-min rest period between training
modes. S training was performed in its entirety before pro-
ceeding to endurance training and vice versa. All training
was closely supervised and monitored by investigators.
Compliance to training was high with subjects in each
training group completing an average of 30 of 31 training
sessions. No more than two training sessions were missed
by any of the subjects.
(CSA) of the dominant knee extensor and flexor/adductor
muscles were measured using a computed tomographic
(CT) technique. Scans were obtained on a DR3 CT scanner
(Siemens, Erlangen, Germany). Scans were taken while
subjects were in the supine position with the thighs relaxed.
Radiation exposure was minimized by low-exposure factors
(125 kVp, 3 s, 180 mAs). A single-axial transverse image
TABLE 1. Subject characteristics by group.
Values are means ? SE; N, no. of subjects.
27.9 ? 1.2
26.5 ? 1.6
27.3 ? 1.7
180.4 ? 1.3
179.3 ? 2.7
179.2 ? 1.5
82.0 ? 4.4
84.5 ? 5.5
82.1 ? 4.3
Official Journal of the American College of Sports Medicinehttp://www.acsm-msse.org
was obtained through the thigh at a level 18 cm above the
superior border of the patella.
All CT scans were blinded and analyzed by the same
individual. Cross-sectional areas were digitized using an
image processing and analysis system. A COHU video
camera (no. 6415-2000) with a Javelin 18- to 108-mm
macro zoom lens was interfaced with a Sony 1343 monitor
and a microcomputer equipped with a PC Vision Plus frame
grabber board (Jandel Scientific, Corte Madera, CA). JAVA
(Jandel Video Analysis Software; Jandel Scientific) com-
puter-enhanced tracing and digitizing software was used for
the analysis. From CT images, thigh muscles were divided
into extensor and flexor/adductor compartments as follows.
On the lateral side, the dividing line was between the vasti
anteriorly, and the hamstrings and adductor magnus poste-
riorly, extending to the posterior border of the femur. From
the posterior border of the femur, the line was extended on
the medial side between the vastus medialis anteriorly, and
adductor longus and sartorius posteriorly. The femur bone
was included in the initial extensor area but was measured
separately and subtracted out to yield only the muscle area
within the extensor compartment. Muscles included in the
extensor area were limited to the four quadriceps. The
flexor/adductor area contained the hamstrings, sartorius,
gracilis, and adductor magnus and longus muscles. Coeffi-
cients of variation were calculated on 16 random samples
redigitized for extensor, flexor/adductor, and bone areas.
The coefficient of variation was less than 1% for each of
Muscle biopsy. Biopsy specimens were removed from
the vastus lateralis muscle of the dominant leg by percuta-
neous needle biopsy as previously described (10). Muscle
samples were oriented cross-sectionally using a dissecting
microscope, mounted on cork by using Gum Tragacanth,
and then quickly frozen in isopentane cooled with liquid
nitrogen. Tissue was stored at ?70°C until analyzed.
Frozen muscle tissue was serial sectioned in 10-?m sec-
tions on a Reichert Histostat cryostat microtome at ?20°C.
Myofibers were classified as slow twitch (ST, or Type I) or
fast twitch (FT, or Type II) by the myofibrillar adenosine
triphosphatase reaction at pH 9.4, after preincubation at pH
10.4 (15). Pre- and post-experimental muscle samples on the
same subject were processed simultaneously to avoid dif-
ferences attributable to the procedure.
For planimetric measurements of the cross-sectional
myofiber area, a Houston Instruments computerized digitiz-
ing tablet was attached to an IBM microcomputer. Muscle
sections were projected at 200 times original size on a Nikon
Optiphot microscope, equipped with a zoom drawing tube.
A Lovins Micro-slide Field Finder (Teledyne Gurley, Troy,
NY) was used to verify projection accuracy. Myofiber pe-
rimeters were manually traced and Bioquant software (Bi-
ometrics, Nashville, TN) was used to calculate fiber areas.
Myofibers selected for area measurement were without lon-
gitudinal tendencies, had distinct cell borders, were free of
artifacts, and were centrally located in the sample. Samples
were blinded to protect the investigator from knowledge of
treatment group and initial or final biopsy status.
Myofiber distribution (% fiber types) was determined
from samples containing an average (?SD) of 718 ? 153
fibers (range 402–1045). From the samples, 152 ? 2 fibers
(range 146–158) were traced to measure fiber area. There
were 64 ? 13 (range 51–100) ST and 87 ? 12 (range
51–100) FT fibers used to compute mean areas. In addition
to the areas for Type I and Type II fibers, mean fiber area
(MFA) was calculated to account for fiber distribution as
follows: MFA ? [(area of Type I fibers ? percent of Type
I fibers) ? (area of Type II fibers ? percent of Type II
fibers)]/100. FT/ST area ratio was calculated by dividing FT
fiber area by ST fiber area.
Isometric torque and neural activation. After sub-
maximal warm-ups, maximal voluntary isometric knee-ex-
tension torque in the dominant leg was determined at a 0.52
rad angle below horizontal using a LIDO Active isokinetic
loading dynamometer (Loredan Biomedical, Inc., Davis,
CA) as previously described (27). Three maximal 5-s iso-
metric contractions were performed with 3-min rest inter-
vals between each contraction to prevent fatigue. The con-
traction with the highest torque value was used in data
analysis. We have previously shown this method to be
highly reliable with an intraclass correlation coefficient of
0.99 between different days (27). For determining the EMG/
torque relation, submaximal isometric torque levels were
then registered in random order at 20% intervals between 20
and 80% of MVC. For posttesting, the same relative (i.e.,
same percentages of post MVC) submaximal torques were
used in assessments. The same order of performing the
submaximal contractions was maintained pre- to posttesting
for each subject. Each submaximal contraction was held at
the desired torque level for 5 s with 2-min rest intervals
between each contraction. To maintain torque at the re-
quired level, each subject matched his generated torque to
the target level imposed on a computer screen linked with
During isometric knee-extension contractions electro-
myographic (EMG) activity was recorded from the vastus
medialis muscle by using bipolar skin surface electrodes by
methods we have previously described (32). Consistency in
electrode placement was maintained pre- to post-test for
each subject (32). Skin impedance was kept below 5000 ?
for all tests by standard skin preparation. Raw EMG was
full-wave rectified and converted to root mean squared
(RMS) by a microprocessor linked to the electromyograph
(Tracor Northern, Middleton, WI) and then to a strip chart
recorder (Gould TA 550 Strip Chart Recorder, Eastlake,
OH) to record the RMS-EMG signal. We have previously
shown measures of maximal RMS-EMG amplitude to be
reliable with this method with an intraclass correlation co-
efficient of 0.82 as assessed with repeated tests over a
12-month interval (32). In recording only from the vastus
medialis, we are assuming that the behavior of RMS-
EMG amplitude in this muscle is representative of all
knee extensors. The EMG/torque relation was investi-
gated by measuring the RMS-EMG amplitude during
isometric contractions at 20, 40, 60, 80, and 100% of
CONCURRENT-TRAINING NEUROMUSCULAR ADAPTATIONS Medicine & Science in Sports & Exercise?
Statistical analysis. Group scores are reported as
means ? SE. Magnitude of changes produced by training in
the three groups was compared using a one-way analysis of
variance (ANOVA) on the difference (post minus pre)
scores. The level of statistical significance was set at 0.05.
Post hoc Fisher least significant difference tests were em-
ployed to locate specific significant differences between
groups. Effects of training within each group were assessed
using Dunn’s multiple comparison procedure incorporating
the Bonferroni correction to maintain the family-wise Type
I error rate at 0.05 (24). By using the Bonferroni correction,
the 0.05 significance level was divided by three (three
t-tests), yielding a Type I error rate of 0.0167 for each t-test.
Statistical analysis for the EMG/torque relation for the
five levels of isometric torque (20, 40, 60, 80, and 100%
MVC) was investigated via linear trend analysis across
EMG/torque data points (20). Relevant planned compari-
sons were tested among the six trends (pre- and post-training
for each of the three group).
Thigh extensor area increased (P ? 0.0001) after training
in both S (12%) and CC (14%) groups (Fig. 1). A smaller
increase (P ? 0.011) occurred with E (3%) training, which
was significantly smaller (P ? 0.0001) than changes in the
S and CC groups. Thigh flexor/adductor area increased (P ?
0.01) in both S (7%) and CC (6%) groups but did not change
in E (0%, P ? 0.924). Femur bone cross-sectional areas did
not change (P ? 0.307) pre- to post-training in any group (S:
8.8 ? 0.3 to 8.9 ? 0.3 cm2; E: 9.0 ? 0.4 to 9.0 ? 0.3 cm2;
and CC: 9.2 ? 0.4 to 9.3 ? 0.5 cm2).
Myofiber areas and distribution results are presented in
Table 2. For muscle fiber distribution, there were no
changes (P ? 0.273) pre- to post-training. The only signif-
icant finding in Type I fiber area was a pre to post increase
in the S group (18.5%, P ? 0.004). The pre to post increase
in the CC group (12.5%) approached significance (P ?
0.041). The reader is reminded that due to the application of
the Bonferroni procedure (to maintain Type I error rate at
0.05), the criterion for finding a difference within a group
was more stringent than the criterion for finding a difference
between groups. The same pattern of findings occurred in
both Type II fiber area and in mean fiber area. Both S and
CC groups had similar increases (P ? 0.002) in Type II fiber
area (24 and 28%) and in mean fiber area (21 and 23%).
These increases were significantly greater than the nonsig-
nificant increases seen in the E group (Type II fiber area ?
4.5%, P ? 0.186; and mean fiber area ? 3.9%, P ? 0.167).
Increases in FT/ST area ratio approached significance in S
(5%, P ? 0.066) and CC (15%, P ? 0.044) groups. FT/ST
area ratio showed no change in the E group (0%). There
were no significant differences in changes in FT/ST area
ratio between groups (P ? 0.081).
Knee-extension isometric torque results across 20–100%
MVC are paired with associated quadriceps RMS-EMG
amplitudes in the EMG/torque relation curves presented in
Figure 2. There were no significant differences among linear
trends pre to post training (P ? 0.384) or between groups (P
? 0.776). This indicates that for a given level of torque
output there was no change in level of neural activation.
Training-induced increases in maximal RMS-EMG ampli-
tude approached significance in the S (13.5%, P ? 0.044)
and CC (10.6%, P ? 0.073) groups (Fig. 2). The smaller
nonsignificant (P ? 0.639) increase in the E group (5.0%)
was not significantly different than changes in the other
groups. As we reported previously, maximal knee-extension
isometric torque increased similarly in both S (12%, P ?
0.004) and CC (7%, P ? 0.003) groups but did not change
with E (?2%, P ? 0.223) training (27). Increases in S and
CC groups were significantly greater (P ? 0.0006) than
changes in the E group.
Our focus on investigating neuromuscular mechanisms
related to strength development with 3-d·wk?1concurrent
strength and endurance training revealed similar findings in
all variables in both S and CC groups. Maximal isometric
torque increased in both groups (S ? 22 and CC ? 14 N·m)
FIGURE 1—Thigh extensor (top) and flexor/adductor (bottom) mus-
cle cross-sectional area. Values are means ? SE *Within group, post
significantly different from pre; P < 0.0167.
different from endurance group; P < 0.05.
Official Journal of the American College of Sports Medicinehttp://www.acsm-msse.org
without an accompanying significant increase in maximal
RMS-EMG amplitude. No changes in the EMG/torque re-
lation across 20–100% MVC occurred indicating the level
of neural activation remained the same for a given level of
torque output in all groups. At the gross muscle level,
quadriceps area (11.4 and 12.6 cm2) and flexor/adductor
area (6.2 and 5.2 cm2) increased similarly in both S and CC
groups. At the myofiber level, there was no change in fiber
type distribution or in the FT/ST area ratio in any group.
Substantial and similar levels of hypertrophy occurred in
Type II fibers (1205 and 1228 ?m2) in both S and CC
groups. Although significant Type I fiber hypertrophy oc-
curred only in the S group (820 ?m2), this change was not
different than the nonsignificant increase seen in the CC
group (537 ?m2). With the exception of a small increase
(3%) in quadriceps cross-sectional area, there were no
changes in any of the above variables in the E group. We
have previously shown that the E-training regimen, as em-
ployed in the current study, substantially increases (18%)
V˙O2peak(27). Our findings indicate that 3-d·wk?1concur-
rent training for both strength and endurance, in sedentary
subjects, does not impair the magnitude of muscle hyper-
trophy induced by S training alone. Results of the concur-
rent strength and endurance training were similar to the
strength-only training in all neuromuscular measures. These
findings provide a physiological basis to support a number
of studies addressing performance adaptations that indicate
concurrent training does not impair strength develop-
ment over theshort term
Why strength development is attenuated in only some
concurrent-training studies has generally been attributed to
differences in training-design variables (26,34). These in-
clude training volume, duration, frequency, intensity, mode
or type of S and E training, and initial training status of
subjects. However, few if any guidelines for designing con-
current-training programs for preventing or minimizing
strength development impairment have been proposed
(12,26). A striking contrast in concurrent-training regimens
that induce impairment in strength development with regi-
mens that do not induce impairment involves the frequency
of training. When both strength and endurance training is
performed on the same day and for only 3 d·wk?1on
alternate days, as in the current study, strength development
is not compromised as compared with performing the
strength training only (1,9,14,27,34,38). The similar in-
creases in isometric strength performance in our S and CC
groups agree with the similar strength increases in S and CC
groups of all other concurrent-training studies we know of
employing 3-d·wk?1CC training (1,9,14,27,34,38). Four of
these studies involved untrained or sedentary subjects
(9,27,34,38), and two of these studies involved physically fit
subjects who regularly exercised (details limited) (1,14). In
FIGURE 2—The EMG/torque relation in the quadriceps of the three
groups of subjects, pre and post training. Values are means ? SE. No
significant changes in this relation were seen pre to post training (P <
0.0167) or between groups (P < 0.05). Maximal RMS-EMG amplitude
did not change significantly pre to post training (P < 0.0167) or
between groups (P < 0.05).
TABLE 2. Myofiber areas and distribution.
Percent Type I fibers
Percent Type II fibers
Type I area (?m2)
Type II area (?m2)
Mean fiber area (?m2)
FT/ST area ratio
Values are means ? SE.
* Post significantly different from pre (P ? 0.0167).
aChange significantly different from endurance (P ? 0.05).
32.9 ? 3.5
34.5 ? 3.2
37.5 ? 5.6
31.8 ? 2.7
37.9 ? 4.6
39.5 ? 6.7
67.1 ? 3.5
65.5 ? 3.2
62.5 ? 5.6
68.2 ? 2.7
62.1 ? 4.6
60.5 ? 6.7
4429 ? 222
4661 ? 395
4279 ? 452
5249 ? 259*
4863 ? 324
4816 ? 506
4975 ? 399
5523 ? 443
4422 ? 469
6181 ? 286*a
5772 ? 418
5651 ? 528*a
4843 ? 338
5203 ? 389
4331 ? 448
5874 ? 233*a
5404 ? 346
5307 ? 485*a
1.13 ? 0.08
1.20 ? 0.05
1.04 ? 0.05
1.19 ? 0.07
1.20 ? 0.07
1.20 ? 0.05
CONCURRENT-TRAINING NEUROMUSCULAR ADAPTATIONS Medicine & Science in Sports & Exercise?
our previous study (27), we also reported almost identical
increases in one repetition maximum (1RM) barbell squats
(S ? 24 kg [23%], CC ? 23 kg [22%]) and maximum
vertical jump height (S ? 3 cm [6%], CC ? 4 cm [9%])
employing the same training regimens as in the current
study. Due to specificity of training, we expected smaller
increases in isometric strength and maximum vertical jump
height than in 1RM squat strength, because barbell squats
were used in the training (27,30). Although Craig et al. (9)
reported a significant 5.8% (7.9-kg) increase in leg strength
with S training, the CC group in this study did not signifi-
cantly increase strength. These authors concluded that leg
strength does not increase to the same degree when endur-
ance training is added to strength training. We disagree with
this conclusion in that there was no statistical analysis
performed comparing between group changes and little ac-
tual difference between the two groups in strength improve-
ment existed (the CC group showed a similar level change
of 4.6% [6.5 kg], although this was not statistically signif-
icant from pre to post training within the group). Thus, we
are assuming similar strength adaptations in both S and CC
groups for the Craig et al. (9) investigation.
The similar strength gains seen in both S and CC groups
with 3-d·wk?1training are in sharp contrast to concurrent
training that takes place 5 or 6 d·.wk?1. Concurrent-training
investigations that employ a CC group, with some type of
training (S, E, or both) being performed 5 (21) or 6 d·wk?1
(2,4,11,22,23), report impairment in some type of strength
performance measure with CC training compared with S
training. The initial training status of subject groups in these
studies varied considerably from untrained (11,22,23), to
athletes out of training for at least 4 wk (21), to experienced
rowers or experienced weight trainers (2), to physically
active subjects who were not formally training to develop
strength or endurance at the time of entry into the study (4).
In another study, Bell et al. (3) report similar strength
increases in S and CC groups with the CC group training 6
d·wk?1. But the group labeled S in this study was also
allowed to perform one E-training session per wk. Thus, as
indicated by Leveritt et al.(26), in this investigation, two
CC-training protocols were really being compared. Equiv-
ocal results have been reported in whether an impairment in
strength development occurs when endurance training is
added to strength training in studies that employ 4-d·wk?1
CC training. Whereas Kraemer et al. (25) conclude impair-
ment in strength development occurs, Nelson et al. (31)
report similar increases in strength in both S and CC groups
over 20 wk of training. It thus appears that at least over the
short term (the longest duration of training in a study em-
ploying 3-d·wk?1CC training was 22 wk (34)), designing
CC-training regimens that will not impair strength develop
may need to limit training to no more than 3 d·wk?1. In
contrast, if some type of S or E training is performed for the
same muscle group 5 or 6 d·wk?1, some type of strength
impairment is likely to occur with CC training. Whereas
frequency of training may be an important factor in influ-
encing outcomes with concurrent training, it appears no
clear pattern of results are obtained when other training-
design variables (including initial training status of subjects)
are compared across studies (1,14,26). Our current findings
provide a physiological basis to support the consistency
across studies that indicate the same level of strength im-
provements can be achieved using 3-d·wk?1concurrent
training as that which occurs with 3-d·wk?1strength-only
The current investigation indicates substantial similar
muscle hypertrophy occurred in both S and CC groups as
assessed at the macroscopic and microscopic levels. Our
whole muscle and fiber area S training results are similar in
magnitude with results reported in the literature employing
(4,18,19,28). Hakkinen et al.(19), employing a very similar
strength-training regimen for the quadriceps (10 wk of
3-d·wk?1barbell squat and machine leg-extension training)
as in the current study, reported increases in vastus lateralis
Type I, IIa, and IIb fiber areas of 23%, 26%, and 14%,
which are similar in magnitude to increases in our S group
of 19% and 24% in Type I and II fiber areas. Identical
increases in quadriceps CSA of 12.2% were found in our S
group subjects and in similar age subjects in the Hakkinen
et al. (19) investigation. With a similar 12-wk strength-
training regimen, we previously reported a 14.4% increase
in quadriceps volume (28).
Muscle morphology adaptations have only been consid-
ered in a few concurrent-training investigations. At the
whole muscle level, our findings are in agreement with Sale
et al. (34), who report similar increases in quadriceps CSA
with both S (13%) and CC (11%) 3-d·wk?1training. Similar
strength improvements from both S and CC training are also
accompanied with similar increases in both Type I and II
fiber areas for both these training conditions. Although our
results are consistent with these findings, there are substan-
tial differences in the design of the Sale et al. (34) study that
appear to limit appropriate comparisons with other concur-
rent-training investigations. The endurance-only training (3-
min bouts at 90–100% VO2max) employed in this study
produced substantial increases in strength (20%) and sub-
stantial increases in quadriceps cross-sectional area (14%).
Also, their results may be affected in employing a unilateral-
leg–training model where potential cross-over adaptations
may occur in the comparative (control) contralateral limb
In adding endurance training to strength training, a lim-
itation in muscle hypertrophy has been suggested as a mech-
anism for an interference in strength gains with concurrent
training (4,26). Bell et al. (4) and Kraemer et al. (25) both
report some type of impairment in strength development
with 12 wk of CC training compared with S training, and
both implicate an underlying factor may be a limitation in
hypertrophy of Type I fibers. Bell et al. (4) reported very
similar fiber type adaptations as in our current findings. In
their study, both Type I (27%) and II (28%) fiber areas
increased with S training, whereas in their CC group, only
a significant within group increase occurred in Type II
(14%) fiber area (Type I fiber area increased 11%, which
was not statistically significant). Although the within-group
in young adults
Official Journal of the American College of Sports Medicine http://www.acsm-msse.org
increases resulting from the S training appear to be about
twice as large as changes within the CC group, there were
no significant differences in the increases between the
groups in both Type I and II fiber areas. The critical criterion
in determining whether an impairment occurs with concur-
rent training is whether there is a difference in adaptations
between groups. Failure in the Bell et al. (4) investigation
not to find a significant difference between groups could be
due to low statistical power for this test, which may relate to
the fair amount of variability with fiber type area measures
Kraemer et al. (25), employing 4-d·wk?1training pro-
grams in physically active soldiers, found similar adapta-
tions in Type IIa fiber areas in both S (24%) and CC (20%)
trained subjects. In contrast, the S training induced a 12%
increase in Type I fiber area, which was greater than the
change seen with CC training (a nonsignificant within group
decreased of 5%). We are not considering changes in Type
IIb and IIc fibers from the Kraemer et al. investigation (25),
because after training their percentage distributions were
2% or less in both S and CC groups. Also of interest in the
Kraemer et al. (25) study was that the running type endur-
ance-only training employed in the study induced an 11%
decrease in Type I fiber area. This finding was different
from our current results and all other E-training regimens
used in concurrent-training investigations that assessed fiber
areas (4,31,34). Combining an E-training program that in-
duces a reduction in muscle fiber size with a strength-
training program would account for the attenuation in fiber
hypertrophy produced by the strength training alone. Other
concurrent-training investigations that have combined run-
ning E training with strength training have found no im-
pairment in strength development compared with the
strength training alone (9,38).
Nelson et al. (31) also reported that concurrent S and E
training produces different fiber hypertrophy patterns than S
training. However, they report significant hypertrophy oc-
curred in response to both E and CC training in all fiber type
areas (I, IIa, IIb). Their knee-extension isokinetic S training,
performed at 0.52 rad·s?1, only induced hypertrophy in
Type IIb fibers. With both S and E training producing
increases in strength, there was no impairment in strength
development when the E training was added to the S train-
ing. The apparent training effects of the individual S and E
protocols employed by Nelson et al. (31) were substantially
different that ours and other individual protocols employed
by other investigations considering fiber type adaptations.
The eclectic array of training-design variables, and some-
what limited descriptions of protocols employed in the dif-
ferent concurrent-training investigations, makes it difficult
to compare studies directly.
It has been proposed that an impairment in strength de-
velopment with concurrent training may be due to increases
in Type I fiber composition, with concomitant decreases in
Type II fiber percentage (6,12). In response to training in the
present study, fiber distribution remained the same across all
three groups. Studies reporting similar strength improve-
ments with S and CC training (31,34), and a study reporting
an impairment in strength development with concurrent
training (25), have indicated little difference in fiber type
change between S and CC training. Bell et al. (4) did not
report fiber-type distribution despite reporting fiber-area
results. In the few studies to date, none have indicated that
CC training alters the fiber distribution pattern seen with
strength training alone.
Results of the current investigation do not give any cre-
dence to the hypothesis that an interference in strength
development with concurrent training may be related to
neural activation. In response to S training in the current
study, maximal RMS-EMG amplitude was not significantly
different (P ? 0.044) from that before training, possibly due
to rather large methodological error of this measure (30).
Again, the reader is reminded that due to the application of
the Bonferroni procedure (to maintain family-wise Type I
error at 0.05), the criterion for finding a difference within a
group (level of statistical significance ? 0.0167, for each
t-test for the three groups) was more stringent than the
criterion for finding a difference between groups (level of
significance ? 0.05). Although a number of studies report
an enhancement in maximal EMG amplitude with strength
training (16,17,29), our results are in agreement with several
other investigations that report no change in maximal EMG
amplitude with strength training (5,13,30,37,39). The 13.5%
nonsignificant increase in our S group is similar in magni-
tude to significant 11.8 and 12.5% increases reported in
other strength-training regimens by Moritani and deVries
(29) and Hakkinen and Komi (17). Very similar nonsignif-
icant increases within our S (272 ?V) and CC (258 ?V)
groups indicate no interference in maximal neural activation
is likely when E training is added to S training in 3-d·wk?1
concurrent training. Further, the closeness of EMG/torque
relation changes (across 20–100% MVC), in both S and CC
groups, indicate RMS-EMG amplitude as an unlikely mech-
anism related to strength impairment with 3-d·wk?1training
(Fig. 2). We are unaware of any research investigating
neural activation changes due to endurance training. As
indicated earlier, the E training in the present study did not
affect strength, but did substantially increase V˙O2peak. No
change in maximal RMS-EMG amplitude with E training,
and the closeness of the pre- and post-EMG/torque relation
curves, indicates little effect of the continuous cycling E
training on neural activation. We cannot rule out, however,
that other possibly more intense, or other modes of E train-
ing, may influence neural activation and could potentially
interfere with strength development when added to S
In the current study, although we have investigated some
major mechanisms that may account for the possibility of
strength impairment occurring when endurance training is
added to strength training, other mechanisms may also be
involved. Concurrent-training investigations compare CC
training with other individual training regimens that are
performed less frequently or with much less volume. This
argues that overtraining and/or chronic muscle glycogen
depletion, which may occur with consecutive days of train-
ing (8,36), may be other mechanisms that negatively
CONCURRENT-TRAINING NEUROMUSCULAR ADAPTATIONS Medicine & Science in Sports & Exercise?
influence strength adaptations with concurrent training
(12,25,26). Although inconsistent in the literature, there is
evidence that a catabolic state related to increased cortisol
levels, combined with little change in concentrations of
anabolic hormones (i.e., testosterone, growth hormone),
may indicate an overtrained state with concurrent training
(2,4,25). It appears future investigations should consider
examining indices of overtraining and compare concurrent-
training protocols that are more similar in volumes to indi-
vidual S and E regimens. It should be noted, however, that
no strength development impairment has been reported with
CC training performed 6 d·wk?1(2), whereas fairly low
volume training (11) has produced significant strength de-
velopment impairments in CC versus S training. In the Bell
et al. (2) study where CC training was performed 6 d·wk?1,
there was nonrandom assignment of college students to two
groups of S and CC training. A group of experienced rowers
performed CC training 6 d·wk?1(3-d·wk?1endurance and
3-d·wk?1strength training), and a group of experienced
strength trainers (nonrowers) performed the same S training
3 d·wk?1. In this study, only female subjects showed an
impairment in strength development with CC training as
compared with S training, whereas male subjects did not
show any impairment. For the present study, we chose a
training model to help control for potential overtraining
effects by stressing the same muscle group (quadriceps)
only 3 d·wk?1with concurrent training.
In summary, our findings indicate that, in sedentary sub-
jects, 3-d·wk?1concurrent training for both strength and
endurance does not impair the magnitude of muscle hyper-
trophy induced by strength training alone. The S training
employed induced increases in maximal knee extensor
torque that was accompanied by substantial muscle hyper-
trophy of the quadriceps as measured at both the macro-
scopic and microscopic levels, whereas maximal neural
activation of the quadriceps was not significantly increased.
With S training, no changes were seen in fiber-type distri-
bution or in the EMG/torque relation across 20–100%
MVC. With the exception of a small 3% increase in quad-
riceps cross-sectional area, there were no changes in any of
the above variables with E training. Results in our study of
the concurrent strength and endurance training were similar
to the strength-only training in all neuromuscular measures.
Our findings provide a physiological basis to support a
number of studies addressing performance adaptations that
consistently indicate concurrent training does not impair
strength development over the short term with 3-d·wk?1
The authors express thanks to John Jones and Jones Barbell Ltd.
and to UW-Hospital Sports Medicine and Fitness Center, which
donated use of all free-weight training equipment; Dr. Ann Wertz
Garvin, Joel Curt, and Jim Augustine for their assistance with train-
ing and testing; Dr. Dick Calkins for statistical consultation; and
subjects for their participation. This work was partially supported
through a grant from the Naval Health Research Center, San Diego,
Address for correspondence: John P. McCarthy, Ph.D., P.T.,
University of Wisconsin-Madison, Department of Orthopedics & Re-
habilitation, Program in Physical Therapy, 4190 Medical Sciences
Center, 1300 University Avenue, Madison, WI 53706; E-mail:
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