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sports
Article
Exercise-Induced Muscle Damage and Recovery in
Young and Middle-Aged Males with Different
Resistance Training Experience
John F. T. Fernandes 1,2,*, Kevin L. Lamb 2and Craig Twist 2
1Sport, Health and Well-being Arena, Hartpury University, Hartpury GL19 3BE, UK
2Department of Sport and Exercise Science, University of Chester, Chester CH1 4BJ, UK;
k.lamb@chester.ac.uk (K.L.L.); c.twist@chester.ac.uk (C.T.)
*Correspondence: jfmtfernandes@hotmail.co.uk
Received: 3 April 2019; Accepted: 23 May 2019; Published: 29 May 2019
Abstract:
This study compared the time course of recovery after a squatting exercise in trained young
(YG; n=9; age 22.3
±
1.7 years) and trained (MT; n=9; 39.9
±
6.2 years) and untrained (MU; n=
9; age 44.4
±
6.3 years) middle-aged males. Before and at 24 and 72 h after 10
×
10 squats at 60%
one-repetition maximum (1RM), participants provided measurements of perceived muscle soreness
(VAS), creatine kinase (CK), maximal voluntary contraction (MVC), voluntary activation (VA), and
resting doublet force of the knee extensors and squatting peak power at 20% and 80% 1RM. When
compared to the YG males, the MT experienced likely and very likely moderate decrements in MVC,
resting doublet force, and peak power at 20% and 80% 1RM accompanied by unclear differences
in VAS, CK, and VA after the squatting exercise. MU males, compared to MT, experienced greater
alterations in peak power at 20% and 80% 1RM and VAS. Alterations in CK, MVC, VA, and resting
doublet force were unclear at all time-points between the middle-aged groups. Middle-aged males
experienced greater symptoms of muscle damage and an impaired recovery profile than young
resistance trained males. Moreover, regardless of resistance training experience, middle-aged males
are subject to similar symptoms after muscle-damaging lower-body exercise.
Keywords: squatting; ageing; muscle damage
1. Introduction
The number of middle-aged (i.e., 30 to 59 years old) people in the U.K. is increasing [
1
]. Alongside
this is a growing number of middle-aged athletes, many of whom want to maintain or improve their
athletic performances despite the natural age-related declines [
2
]. Specifically, these impairments are
because of losses in muscle mass [
3
] and strength and power [
3
,
4
], of which, the lower-body undergoes
the greatest losses [
3
–
5
]. Importantly, resistance training can provide a potent method of ameliorating
these age-associated losses in muscle mass, strength, and power [6].
When used acutely, resistance exercise can cause exercise-induced muscle damage (EIMD; [
6
]),
for which the mechanisms have been discussed extensively before (see [
7
]). EIMD symptoms include
increases in muscle soreness, intramuscular enzymes in the blood serum, and plasma, and, of most
importance to the athlete, an impaired muscle function [
8
]. Importantly, changes in muscle function
provide the best indication of EIMD [
7
,
8
]. Although highly individualised [
9
], these symptoms typically
peak between 24 and 48 h after the initial bout and are recovered by seven days [
7
]. A muscle’s
susceptibility to damage might also be affected (reduced) in subsequent bouts where prior eccentric
exercise has occurred [
10
,
11
]. Two studies have noted that this protection from eccentric exercise is less
pronounced (~29% in MVC) in untrained older, compared to younger, men [
12
,
13
], which suggests
Sports 2019,7, 132; doi:10.3390/sports7060132 www.mdpi.com/journal/sports
Sports 2019,7, 132 2 of 13
that older resistance-trained men might exhibit symptoms of EIMD that are not dissimilar to their
untrained counterparts.
Studies examining the recovery of older and younger untrained adults after muscle-damaging
exercise are equivocal. Some studies have reported greater symptoms of EIMD in younger, compared
to older, males [
14
,
15
], while others have observed greater EIMD in older (~59 to 66 years), compared
to younger, males (~23 years) (17). Moreover, a number of studies have reported no difference in
symptoms of EIMD after exercise for young populations (~19 years), compared to older populations
(~48 to 76 years) [
6
,
16
–
19
]. One confounding factor in the current literature might be the physical
activity and resistance training status of the participants. For example, when controlling for physical
activity, Buford et al. [
18
] noted that recovery from muscle-damaging unilateral plantar flexion was
similar among young (~23 years) and older (~76 years) adults. Despite the effectiveness of resistance
training in combating the age-associated losses, only one study has investigated the EIMD response
in older resistance trained males. Like Buford et al. [
18
], Gordon and colleagues [
16
] observed
no differences in indirect markers of EIMD between recreationally trained young (~22 years) and
middle-aged (~47 years) males after damaging knee extensor exercise. Despite these novel findings,
no study has yet reported on the recovery characteristics from multi-jointed lower-body exercise in
middle-aged (35 to 55 years), resistance trained males. Indeed, Gordon et al. [
16
] advised that future
studies might adopt a more ecologically valid exercise protocol. Data from such a study would be
highly applicable to those athletes seeking to prolong their athletic careers. Consequently, the primary
aim of the study was to determine the time course to recovery from EIMD in young and middle-aged
resistance trained males. A secondary purpose was to determine if the recovery profile of middle-aged
males is altered by resistance training experience. Given the variability in the current data regarding
EIMD and ageing and a lack of studies in trained populations, we propose the null hypothesis, i.e.,
that the EIMD response would not be different between groups.
2. Materials and Methods
2.1. Design
The study used a two-way repeated measures design (age group x time), whereby participants
attended the laboratory on four separate occasions, the initial visit for estimations of body composition
and the back squat 1RM (Figure 1). On the same visit they were habituated with the measurements
of squatting peak power and MVC, VA, and resting doublet force during isometric knee extension.
Participants were considered ‘habituated’ when they could complete three consecutive repetitions that
produced power or force values each within 10% [
20
]. Participants returned to the laboratory 2–4 days
later for measurements comprising squats at 20% and 80% 1RM, MVC, VA, resting doublet force,
muscle soreness, and creatine kinase (CK) activity, and an exercise bout comprising 10
×
10 squats at
60% 1RM [21]. Repeated measures were then conducted 24 and 72 h after the initial exercise bout.
Sports 2019, 7 FOR PEER REVIEW 2
which suggests that older resistance-trained men might exhibit symptoms of EIMD that are not
dissimilar to their untrained counterparts.
Studies examining the recovery of older and younger untrained adults after muscle-damaging
exercise are equivocal. Some studies have reported greater symptoms of EIMD in younger, compared
to older, males [14,15], while others have observed greater EIMD in older (~59 to 66 years), compared
to younger, males (~23 years) (17). Moreover, a number of studies have reported no difference in
symptoms of EIMD after exercise for young populations (~19 years), compared to older populations
(~48 to 76 years) [6,16–19]. One confounding factor in the current literature might be the physical
activity and resistance training status of the participants. For example, when controlling for physical
activity, Buford et al. [18] noted that recovery from muscle-damaging unilateral plantar flexion was
similar among young (~23 years) and older (~76 years) adults. Despite the effectiveness of resistance
training in combating the age-associated losses, only one study has investigated the EIMD response
in older resistance trained males. Like Buford et al. [18], Gordon and colleagues [16] observed no
differences in indirect markers of EIMD between recreationally trained young (~22 years) and
middle-aged (~47 years) males after damaging knee extensor exercise. Despite these novel findings,
no study has yet reported on the recovery characteristics from multi-jointed lower-body exercise in
middle-aged (35 to 55 years), resistance trained males. Indeed, Gordon et al. [16] advised that future
studies might adopt a more ecologically valid exercise protocol. Data from such a study would be
highly applicable to those athletes seeking to prolong their athletic careers. Consequently, the
primary aim of the study was to determine the time course to recovery from EIMD in young and
middle-aged resistance trained males. A secondary purpose was to determine if the recovery profile
of middle-aged males is altered by resistance training experience. Given the variability in the current
data regarding EIMD and ageing and a lack of studies in trained populations, we propose the null
hypothesis, i.e., that the EIMD response would not be different between groups.
2. Materials and Methods
2.1. Design
The study used a two-way repeated measures design (age group x time), whereby participants
attended the laboratory on four separate occasions, the initial visit for estimations of body
composition and the back squat 1RM (Figure 1). On the same visit they were habituated with the
measurements of squatting peak power and MVC, VA, and resting doublet force during isometric
knee extension. Participants were considered ‘habituated’ when they could complete three
consecutive repetitions that produced power or force values each within 10% [20]. Participants
returned to the laboratory 2–4 days later for measurements comprising squats at 20% and 80% 1RM,
MVC, VA, resting doublet force, muscle soreness, and creatine kinase (CK) activity, and an exercise
bout comprising 10 × 10 squats at 60% 1RM [21]. Repeated measures were then conducted 24 and 72
h after the initial exercise bout.
Figure 1. Schematic of study design.
Habituationn and
anthropometry
Markers of EIMD
Muscle damaging exercise
Markers of EIMD
Markers of EIMD
48-96 hrs
72 hrs
24 hrs
Figure 1. Schematic of study design.
Sports 2019,7, 132 3 of 13
2.2. Participants
Nine young resistance trained (YG; range: 21 to 25 years), nine middle-aged (MT; range: 35 to
54 years) resistance trained, and nine untrained middle-age males (MU; range: 35 to 53 years) were
recruited for this study using convenience sampling. Thirty-five years was selected as the lower
boundary for the middle-aged group because it is the entry age for ‘Masters’ athletes (see British
Masters Athletic Federation and World Masters Athletics). As age-related studies typically use older
groups (60 years and over), 55 was selected as the upper-limit for the middle-aged group. An overall
sample size of approximately 27 (nine per group) was estimated using Batterham and Atkinson’s [
22
]
nomogram. This was calculated using a coefficient of variation and typical change of 6.1% [
23
] and 5%,
respectively. The YG and MT had a minimum of two years’ resistance training experience and regularly
used squats as part of their resistance training programmes. The MU group had no resistance training
experience, but was screened by the lead researcher to ensure they could perform the correct squat
technique. All participants had been active in sport for a minimum of two years and were competitive.
Participants completed a pre-test health questionnaire and provided written consent for the study,
which was approved by the Ethics Committee of the Faculty of Life Sciences at the host institution.
Participants were instructed not to consume any ergogenic supplements (for example, caffeine) on the
day of testing and to refrain from exercise, other than that performed as part of the study, throughout
their involvement.
2.3. Procedures
2.3.1. Anthropometric Measurements
Body density was estimated via skinfold thickness measurements (Harpenden, British Indicators,
Burgess Hill, UK) taken at the triceps, axilla, abdominal, suprailiac, chest, subscapular, and
mid-thigh [
24
]. Body fat percentage (%BF) was estimated [
25
] from which quantities (kg) of fat-mass
(FM) and fat-free mass (FFM) were derived.
2.3.2. Resistance Training History and Sports Participation
The YG and MT participants completed a questionnaire to record how many years they had
participated in regular resistance training, their weekly training frequency and session duration,
and the main reason for their training. A second questionnaire detailed how many years they had
participated in organised sport, their weekly frequency and session duration, and the type of sport
they in which participated (i.e., team, endurance, racket, or other).
2.3.3. Maximal Strength Testing
The 1RM for squat exercise was predicted using a three-repetition maximum (3RM) protocol.
Participants performed 8–10 repetitions with 50% of their estimated 1RM, followed by 3–5 repetitions
with 85% of their estimated 1RM. The load was then set at the approximate 3RM and the participants
performed three repetitions. The load was progressively increased until the participant could no longer
perform a complete repetition. The final load lifted was then used with the following equation [
26
] to
estimate the 1RM squat load:
1RM =(100 ×3RM load lifted)/[48.8 +(53.8 ×2.71828−0.075 ×repetitions). (1)
The above equation has been reported to yield accurate 1RM predictions (r=0.969, 0.02% different
from direct 1RM) [27].
2.3.4. Indirect Markers of Muscle Damage
Perceived muscle soreness of the knee extensors was measured using a 0–10 visual analogue scale
(VAS). Plasma CK activity was also determined from a capillary blood sample. A 30
µ
L sample of
Sports 2019,7, 132 4 of 13
whole blood was collected into a heparinised capillary tube and pipetted onto a test strip for analysis
(Reflotron, Type 4, Boehringer Mannheim, Mannheim, Germany).
2.3.5. Assessment of Maximal Voluntary Contraction and Voluntary Activation
Before undertaking the MVC and VA assessments, participants performed a warm-up comprising
five minutes of cycling at 100 W (Lode, Corival, Groningen, Netherlands). An isometric dynamometer
(Biodex, Multi-joint system 3, Biodex Medical, New York, NY, USA) was employed to measure the
force of the participant’s dominant knee extensor at 80
◦
knee flexion. To prevent extraneous body
movements, Velcro straps were applied tightly across the chest and thigh. Participants were provided
with strong verbal encouragement and real-time feedback via the PC monitor.
The knee extensors were electrically stimulated (5 s with two 100 Hz single square impulses
(doublet); Digitimer, D57, Hertfordshire, UK) using two 5
×
13 cm moistened surface electrodes
(Axelgaard Manufacturing Co., Ltd., Fallbrook, CA, USA); one placed distally over the quadriceps
and the other proximally over the upper quadricep. During optimisation, the amplitude of a doublet
was progressively increased, starting at 50 amps, until a point where no further increases in intensity
resulted in an increase in resting doublet force. Initially, a 230 volt electrically evoked doublet (set 20%
above the value required to evoke a resting muscle doublet of maximum amplitude) was applied to
the resting muscle (resting doublet) at 1 s. The resting doublet was used to elucidate any peripheral
alterations that might have occurred as a result of the squatting protocol [
21
]. Participants then
performed a 4 s MVC before a doublet, which was applied at the isometric plateau (superimposed
doublet). The MVC was taken as the average force over 50 ms (AcqKnowledge 3 software, Biopac
Systems, Massachusetts, MA, USA) before the superimposed doublet was applied. VA was calculated
according to the interpolated doublet ratio using the equation:
VA (%) =[1 −(size of superimposed doublet/size of resting doublet)] ×100. (2)
A similar procedure has been deemed a reliable method (CV =3.38%) for assessing VA [28].
2.3.6. Assessment of Peak Power During Squat
Peak power was assessed at loads corresponding to 20% and 80% 1RM during the back squat
exercise using a rotary encoder (FitroDyne, Fitronic, Bratislava, Slovakia), the procedures for which
have been described elsewhere [
5
,
23
]. The FitroDyne has been shown to produce reliable intra- and
inter-day measures of peak power (coefficient of variation =3.9–6.1%) at the selected loads [23].
2.3.7. Muscle-Damaging Exercise Protocol
This consisted of 10
×
10 repetitions of squat exercise at a load corresponding to 60% 1RM with 120 s
rest between sets [
21
]. Each repetition was performed in the manner outlined above. A similar protocol
has successfully induced symptoms of muscle damage in previous research [
21
,
29
]. The FitroDyne was
used to calculate the power for each repetition in the manner outlined above. The average peak power
per repetition was used to elucidate the influence of exercise intensity on recovery profiles between
groups. One participant from the MU group was unable to complete sets 8, 9, and 10 at 60% 1RM, thus
the load was reduced by 5 kg (50.1% 1RM) and power values were calculated accordingly.
2.4. Statistical Analyses
Comparisons of categorical training history and sport participation variables by group were made
using a chi-squared (
χ2
) test of association. All other data were analysed using the effect size (ES)
with 90% confidence intervals (CI) [
30
]. Magnitude-based inference statistics were used to provide
information on the size of the differences, allowing for a more practical and meaningful explanation of
the data [
31
]. Thresholds for the magnitude of the observed change for each variable were determined
as the within-participant standard deviation in that variable
×
0.2, 0.6, 1.2, and 2 for a small, moderate,
Sports 2019,7, 132 5 of 13
large, and very large effect [
32
]. Threshold probabilities for a meaningful effect, based on the 90%
confidence limits (CL) were as follows: Less than 0.5% most unlikely, 0.5–5% very unlikely, 5–25% unlikely,
25–75% possibly, 75–95% likely, 95–99.5% very likely, and >99.5% most likely. Effects with confidence
limits across a likely small positive or negative change were classified as unclear [
30
]. All calculations
were completed using predesigned spreadsheets (www.sportsci.org). Data are presented as ES, lower
CI, and upper CI.
3. Results
3.1. Biometric Measures and Training History
Age and sum of skinfolds were most likely and likely higher, respectively, in the MT groups
compared to the YG group (Table 1). Differences in FM and body fat percentage between the YG and
MT groups were very likely, while mass and squat 1RM were unclear. Age and FFM differences between
the MT and MU groups were likely moderate, whilst all other biometric characteristics demonstrated
unclear differences.
The MT group had most likely regularly resistance trained for longer than the YG (ES 2.29, CI 1.46,
3.13; Table 2), though their training was associated with a lower weekly frequency (
χ2
=32.5, p<0.05)
and shorter session duration (
χ2
=36.4, p<0.05). Moreover, the MT group typically chose resistance
training for strength and fat loss, whereas the YG trained for strength (χ2=31.8, p<0.05).
Table 1.
Biometric characteristics (mean
±
SD) and comparisons of young (YG) and middle-aged
trained (MT) and untrained (MU) groups.
Measure Group Comparison
YG (n=9) MT (n=9) MU (n=9) YG v MT MT v MU
Age (years) 22.3 ±1.7 39.9 ±6.2 44.4 ±6.3 Most likely ↑
3.70 (2.87, 4.53)
Likely ↑
0.71 (−0.10, 1.52)
Mass (kg) 82.0 ±9.0 79.1 ±10.3 83.4 ±9.56 Unclear
0.29 (−1.10, 0.52)
Unclear
0.42 (−0.39, 1.23)
Fat-free mass
(kg) 71.4 ±7.9 63.9 ±6.5 68.6 ±7.1 Very likely ↓
−
1.02 (
−
1.83,
−
0.22)
Likely ↑
0.68 (−0.13, 1.49)
Fat-mass (kg) 10.5 ±4.5 15.2 ±5.7 14.8 ±7.0 Likely ↑
0.89 (0.09, 1.70)
Unclear
−0.07 (−0.88, 0.74)
Body fat (%) 12.8 ±4.7 18.8 ±5.8 17.4 ±6.7 Very likely ↑
1.13 (0.32, 1.94)
Unclear
−0.23 (−1.04, 0.58)
Sum of
skinfolds (mm) 82.3 ±24.6 102.4 ±31.9 91.7 ±32.7 Likely ↑
0.69 (−0.12, 1.50)
Unclear
−0.32 (−1.13, 0.48)
Squat 1RM (kg)
130.8 ±26.8 109.3 ±22.5 98.4 ±14.25 Unclear
−
0.85 (
−
1.65,
−
0.04)
Unclear
−0.56 (−1.37, 0.25)
The comparison panel details the qualitative descriptor, effect size, and upper and lower confidence limits.
Table 2. Resistance training characteristics of the young (YG) and middle-aged trained groups (MT).
Resistance Training Characteristics YG (n=9) MT (n=9)
Years of resistance training (mean ±SD) 4.6 ±1.3 18.0 ±5.6
Weekly frequency *
1 to 2 2 (22.2) 6 (66.7)
3 to 4 4 (44.4) 2 (22.2)
5+3 (33.3) 1 (11.1)
Session duration *
0 to 30 min 0 (0.0) 1 (11.1)
31 to 60 min
3 (33.3) 7 (77.8)
61 to 90 min
5 (55.6) 1 (11.1)
90+min 1 (11.1) 0 (0.0)
Reason for resistance training *
Strength 6 (66.7) 4 (44.4)
Hypertrophy
1 (11.1) 0 (0.0)
Fat loss 1 (11.1) 4 (44.4)
Health 1 (11.1) 1 (11.1)
* Categorical variables are significantly associated (p<0.05). Bracketsdenote percentage of responses in each category.
Sports 2019,7, 132 6 of 13
There were very likely large and moderate differences in sports participation for the MT compared
to the YG and MU, respectively, with MT having more years compared to the YG (ES 1.47, CI 0.66, 2.28)
and less than the MU group (ES 1.17, CI 0.36, 1.98; Table 3). No relationship (p>0.05) was observed
between groups for weekly frequency, session duration, or type of sport played.
Table 3. Sports participation characteristics of the young and middle-aged trained groups.
Sports Participation Characteristics YG (n=9) MT (n=9) MU (n=9)
Years of sports participation (mean ±SD) 11.2 ±4.8 22.0 ±7.8 30.3 ±7.8
Weekly frequency
1 to 2 4 (44.4) 2 (22.2) 0 (0.0)
3 to 4 4 (44.4) 4 (44.4) 6 (66.7)
5+1 (11.1) 3 (33.3) 3 (33.3)
Session duration
0 to 30 min 0 (0.0) 0 (0.0) 0 (0.0)
31 to 60 min 3 (33.3) 4 (44.4) 7 (77.8)
61 to 90 min 3 (33.3) 3 (33.3) 1 (11.1)
90+min 3 (33.3) 2 (22.2) 1 (11.1)
Type of sport
Team 5 (55.6) 3 (33.3) 3 (33.3)
Endurance 3 (33.3) 5 (55.6) 4 (44.4)
Racket 0 (0.0) 1 (11.1) 2 (22.2)
Other 1 (11.1) 0 (0.0) 0 (0.0)
3.2. External Load Response during the Muscle-Damaging Protocol
There was a likely moderate lower average peak power (ES
−
0.71 CI
−
1.53, 0.10) in the MT (603.2
±
162.6 W) compared to the YG (770.4
±
278.0 W). Differences between the MT and MU (547.0
±
75.0 W)
groups were unclear (ES −0.43, CI −1.25, 0.39).
3.3. Indirect Markers of Muscle Damage
At Pre, differences in muscle soreness between the YG and MT and MT and MU were unclear
(ES 0.00, CI
−
0.81, 0.81 and ES 0.42, CI
−
0.39, 1.22, respectively; Figure 2). When the three groups were
combined, perceived muscle soreness demonstrated most likely very large (ES 4.20, CI 3.74, 4.65) increases
at 24 h and, likewise (ES 1.82, CI 1.36, 2.27), at 72 h after muscle-damaging exercise. Between-group
differences for the YG and MT comparison were unclear at 24 and 72 h after muscle-damaging exercise.
Increases in muscle soreness were likely moderately higher in the MU group compared to the MT group
at 24 and 72 h.
Sports 2019, 7 FOR PEER REVIEW 6
There were very likely large and moderate differences in sports participation for the MT compared
to the YG and MU, respectively, with MT having more years compared to the YG (ES 1.47, CI 0.66,
2.28) and less than the MU group (ES 1.17, CI 0.36, 1.98; Table 3). No relationship (p > 0.05) was
observed between groups for weekly frequency, session duration, or type of sport played.
Table 3. Sports participation characteristics of the young and middle-aged trained groups.
Sports Participation Cha
r
acteristics YG (n = 9) MT (n = 9) MU (n = 9)
Years of sports participation (mean ± SD) 11.2 ± 4.8 22.0 ± 7.8 30.3 ± 7.8
Weekly frequency
1 to 2 4 (44.4) 2 (22.2) 0 (0.0)
3 to 4 4 (44.4) 4 (44.4) 6 (66.7)
5+ 1 (11.1) 3 (33.3) 3 (33.3)
Session duration
0 to 30 min 0 (0.0) 0 (0.0) 0 (0.0)
31 to 60 min 3 (33.3) 4 (44.4) 7 (77.8)
61 to 90 min 3 (33.3) 3 (33.3) 1 (11.1)
90+ min 3 (33.3) 2 (22.2) 1 (11.1)
Type of sport
Team 5 (55.6) 3 (33.3) 3 (33.3)
Endurance 3 (33.3) 5 (55.6) 4 (44.4)
Racket 0 (0.0) 1 (11.1) 2 (22.2)
Other 1 (11.1) 0 (0.0) 0 (0.0)
3.2. External Load Response during the Muscle-Damaging Protocol
There was a likely moderate lower average peak power (ES −0.71 CI −1.53, 0.10) in the MT (603.2
± 162.6 W) compared to the YG (770.4 ± 278.0 W). Differences between the MT and MU (547.0 ± 75.0
W) groups were unclear (ES −0.43, CI −1.25, 0.39).
3.3. Indirect Markers of Muscle Damage
At Pre, differences in muscle soreness between the YG and MT and MT and MU were unclear
(ES 0.00, CI −0.81, 0.81 and ES 0.42, CI −0.39, 1.22, respectively; Figure 2). When the three groups were
combined, perceived muscle soreness demonstrated most likely very large (ES 4.20, CI 3.74, 4.65)
increases at 24 h and, likewise (ES 1.82, CI 1.36, 2.27), at 72 h after muscle-damaging exercise.
Between-group differences for the YG and MT comparison were unclear at 24 and 72 h after muscle-
damaging exercise. Increases in muscle soreness were likely moderately higher in the MU group
compared to the MT group at 24 and 72 h.
Figure 2. Changes in perceived muscle soreness between YG, MT, and MU at pre, 24, and 72 h after
resistance exercise. The panel above details the qualitative descriptor, effect size, and upper and lower
confidence limits.
Figure 2.
Changes in perceived muscle soreness between YG, MT, and MU at pre, 24, and 72 h after
resistance exercise. The panel above details the qualitative descriptor, effect size, and upper and lower
confidence limits.
Sports 2019,7, 132 7 of 13
Differences in CK activity at Pre for YG and MT and MT and MU comparisons were unclear
(ES
−
0.41, CI
−
1.21, 0.40 and ES
−
0.44, CI
−
1.25, 0.38, respectively; Figure 3). The increase in plasma
CK activity for the three groups combined was very likely moderate (ES 1.19, CI 0.73, 1.64) and likely small
(ES 0.59, CI 0.13, 1.05) at 24 and 72 h, respectively, compared to Pre. Differences in plasma CK activity
over time were unclear between the YG and MT groups. Plasma CK activity was likely moderately higher
in the MU group compared to the MT group at 24 h, though differences between the groups were
unclear at 72 h.
Sports 2019, 7 FOR PEER REVIEW 7
Differences in CK activity at Pre for YG and MT and MT and MU comparisons were unclear (ES
−0.41, CI −1.21, 0.40 and ES −0.44, CI −1.25, 0.38, respectively; Figure 3). The increase in plasma CK
activity for the three groups combined was very likely moderate (ES 1.19, CI 0.73, 1.64) and likely small
(ES 0.59, CI 0.13, 1.05) at 24 and 72 h, respectively, compared to Pre. Differences in plasma CK activity
over time were unclear between the YG and MT groups. Plasma CK activity was likely moderately
higher in the MU group compared to the MT group at 24 h, though differences between the groups
were unclear at 72 h.
Figure 3. Changes in plasma creatine kinase activity between YG, MT, and MU at Pre, 24, and 72 h
after resistance exercise. The panel above details the qualitative descriptor, effect size, and upper and
lower confidence limits.
At Pre, differences in MVC force were likely moderate and unclear for the YG compared to MT (ES
−0.80, CI −1.61, 0.01) and MT compared to MU (ES 0.27, CI −0.56, 1.10), respectively (Figure 4). MVC
force had very likely moderate (ES −0.71, CI −1.16, −0.26) and likely small (ES −0.39, CI −0.84, 0.06)
decreases at 24 and 72 h after muscle-damaging exercise. Likely and very likely moderate reductions in
MVC force were observed in the MT group compared to the YG groups at 24 and 72 h, respectively.
At 24 and 72 h, differences between the MT and MU groups were unclear.
Figure 4. Changes in maximal voluntary contraction force between YG, MT, and MU at Pre, 0, 24, and
72 h after resistance exercise. The panel above details the qualitative descriptor, effect size, and upper
and lower confidence limits.
0
200
400
600
800
1000
1200
1400
1600
Pre 24h 72h
Creatine kinase (U/l)
YG MT MU
Figure 3.
Changes in plasma creatine kinase activity between YG, MT, and MU at Pre, 24, and 72 h
after resistance exercise. The panel above details the qualitative descriptor, effect size, and upper and
lower confidence limits.
At Pre, differences in MVC force were likely moderate and unclear for the YG compared to MT
(ES
−
0.80, CI
−
1.61, 0.01) and MT compared to MU (ES 0.27, CI
−
0.56, 1.10), respectively (Figure 4).
MVC force had very likely moderate (ES
−
0.71, CI
−
1.16,
−
0.26) and likely small (ES
−
0.39, CI
−
0.84, 0.06)
decreases at 24 and 72 h after muscle-damaging exercise. Likely and very likely moderate reductions in
MVC force were observed in the MT group compared to the YG groups at 24 and 72 h, respectively.
At 24 and 72 h, differences between the MT and MU groups were unclear.
Sports 2019, 7 FOR PEER REVIEW 7
Differences in CK activity at Pre for YG and MT and MT and MU comparisons were unclear (ES
−0.41, CI −1.21, 0.40 and ES −0.44, CI −1.25, 0.38, respectively; Figure 3). The increase in plasma CK
activity for the three groups combined was very likely moderate (ES 1.19, CI 0.73, 1.64) and likely small
(ES 0.59, CI 0.13, 1.05) at 24 and 72 h, respectively, compared to Pre. Differences in plasma CK activity
over time were unclear between the YG and MT groups. Plasma CK activity was likely moderately
higher in the MU group compared to the MT group at 24 h, though differences between the groups
were unclear at 72 h.
Figure 3. Changes in plasma creatine kinase activity between YG, MT, and MU at Pre, 24, and 72 h
after resistance exercise. The panel above details the qualitative descriptor, effect size, and upper and
lower confidence limits.
At Pre, differences in MVC force were likely moderate and unclear for the YG compared to MT (ES
−0.80, CI −1.61, 0.01) and MT compared to MU (ES 0.27, CI −0.56, 1.10), respectively (Figure 4). MVC
force had very likely moderate (ES −0.71, CI −1.16, −0.26) and likely small (ES −0.39, CI −0.84, 0.06)
decreases at 24 and 72 h after muscle-damaging exercise. Likely and very likely moderate reductions in
MVC force were observed in the MT group compared to the YG groups at 24 and 72 h, respectively.
At 24 and 72 h, differences between the MT and MU groups were unclear.
Figure 4. Changes in maximal voluntary contraction force between YG, MT, and MU at Pre, 0, 24, and
72 h after resistance exercise. The panel above details the qualitative descriptor, effect size, and upper
and lower confidence limits.
0
200
400
600
800
1000
1200
1400
1600
Pre 24h 72h
Creatine kinase (U/l)
YG MT MU
Figure 4.
Changes in maximal voluntary contraction force between YG, MT, and MU at Pre, 0, 24, and
72 h after resistance exercise. The panel above details the qualitative descriptor, effect size, and upper
and lower confidence limits.
Sports 2019,7, 132 8 of 13
Differences in VA at Pre were unclear for YG compared to MT (ES 0.03, CI
−
0.77, 0.84) and MT
compared to MU (ES 0.07, CI
−
0.76, 0.90; Figure 5). When all groups were combined VA decreased over
time, with values at 24 and 72 h demonstrating very likely moderate decreases (ES
−
0.87, CI
−
1.33,
−
0.41
and ES
−
0.88, CI
−
1.34,
−
0.41, respectively). Differences between groups were unclear at all time-points.
Sports 2019, 7 FOR PEER REVIEW 8
Differences in VA at Pre were unclear for YG compared to MT (ES 0.03, CI −0.77, 0.84) and MT
compared to MU (ES 0.07, CI −0.76, 0.90; Figure 5). When all groups were combined VA decreased
over time, with values at 24 and 72 h demonstrating very likely moderate decreases (ES −0.87, CI −1.33,
−0.41 and ES −0.88, CI −1.34, −0.41, respectively). Differences between groups were unclear at all time-
points.
Figure 5. Changes in voluntary activation between YG, MT and MU at Pre, 24, and 72 h after resistance
exercise. The panel above details the qualitative descriptor, effect size, and upper and lower
confidence limits.
Higher mean resting doublet values for the YG were likely moderate compared to the MT (ES
−0.96 CI −1.77, 0.14; Figure 6). Similarly, higher values for MU (ES 0.95, CI 0.12, 1.78) were likely
moderate compared to the MT group. Mean doublet values were likely small and unclear at 24 and 72
h, respectively, (ES −0.52, CI −0.98, −0.06 and ES −0.04, CI −0.50, 0.42, respectively) after squatting
exercise. Differences in resting doublet were very likely moderate and likely moderate between YG and
MT groups at 24 and 72 h, respectively. MT and MU comparisons were unclear at 24 and 72 h.
Figure 6. Changes in resting doublet force between YG, MT and MU at Pre, 24, and 72 h after
resistance exercise. The panel above details the qualitative descriptor, effect size, and upper and lower
confidence limits.
75
80
85
90
95
100
Pre 24h 72h
Voluntary activation (%)
YG MT MU
Figure 5.
Changes in voluntary activation between YG, MT and MU at Pre, 24, and 72 h after
resistance exercise. The panel above details the qualitative descriptor, effect size, and upper and lower
confidence limits.
Higher mean resting doublet values for the YG were likely moderate compared to the MT (ES
−
0.96
CI
−
1.77, 0.14; Figure 6). Similarly, higher values for MU (ES 0.95, CI 0.12, 1.78) were likely moderate
compared to the MT group. Mean doublet values were likely small and unclear at 24 and 72 h, respectively,
(ES
−
0.52, CI
−
0.98,
−
0.06 and ES
−
0.04, CI
−
0.50, 0.42, respectively) after squatting exercise. Differences
in resting doublet were very likely moderate and likely moderate between YG and MT groups at 24 and
72 h, respectively. MT and MU comparisons were unclear at 24 and 72 h.
Sports 2019, 7 FOR PEER REVIEW 8
Differences in VA at Pre were unclear for YG compared to MT (ES 0.03, CI −0.77, 0.84) and MT
compared to MU (ES 0.07, CI −0.76, 0.90; Figure 5). When all groups were combined VA decreased
over time, with values at 24 and 72 h demonstrating very likely moderate decreases (ES −0.87, CI −1.33,
−0.41 and ES −0.88, CI −1.34, −0.41, respectively). Differences between groups were unclear at all time-
points.
Figure 5. Changes in voluntary activation between YG, MT and MU at Pre, 24, and 72 h after resistance
exercise. The panel above details the qualitative descriptor, effect size, and upper and lower
confidence limits.
Higher mean resting doublet values for the YG were likely moderate compared to the MT (ES
−0.96 CI −1.77, 0.14; Figure 6). Similarly, higher values for MU (ES 0.95, CI 0.12, 1.78) were likely
moderate compared to the MT group. Mean doublet values were likely small and unclear at 24 and 72
h, respectively, (ES −0.52, CI −0.98, −0.06 and ES −0.04, CI −0.50, 0.42, respectively) after squatting
exercise. Differences in resting doublet were very likely moderate and likely moderate between YG and
MT groups at 24 and 72 h, respectively. MT and MU comparisons were unclear at 24 and 72 h.
Figure 6. Changes in resting doublet force between YG, MT and MU at Pre, 24, and 72 h after
resistance exercise. The panel above details the qualitative descriptor, effect size, and upper and lower
confidence limits.
75
80
85
90
95
100
Pre 24h 72h
Voluntary activation (%)
YG MT MU
Figure 6.
Changes in resting doublet force between YG, MT and MU at Pre, 24, and 72 h after
resistance exercise. The panel above details the qualitative descriptor, effect size, and upper and lower
confidence limits.
Sports 2019,7, 132 9 of 13
3.4. Peak Power during Squat Exercise
At Pre, avery likely moderate lower peak power was at 20% and 80% 1RM (ES
−
1.03, CI
−
1.84,
−
0.22 and ES
−
1.03, CI
−
1.84,
−
0.21, respectively) was observed in the MT compared to YG (Table 4).
Differences at Pre for MT and MU were most likely very large and unclear for 20% and 80% 1RM (ES
−
3.34,
CI
−
4.18,
−
2.50 and ES
−
0.47, CI
−
1.28, 0.33, respectively). When all groups were combined, peak power
for 20% and 80% 1RM demonstrated possibly small (ES
−
0.25, CI
−
0.71, 0.20 and ES
−
0.36, CI
−
0.81, 0.09,
respectively) and unclear (ES
−
0.23, CI
−
0.69, 0.22 and ES
−
0.19, CI
−
0.64, 0.26, respectively) decrements
at 24 and 72 h, respectively. For 20% and 80% 1RM, between group differences at 24 and 72 h were very
likely moderate between the YG and MT groups. Similarly, reductions in 20% 1RM peak power at 24
and 72 h for the MT vs. MU comparison were very likely moderate. Peak power at 80% 1RM illustrated
likely moderate and very likely large differences at 24 and 72 h, respectively.
Table 4. Peak power at Pre, 24 and 72 h.
Intensity Group Pre 24 h 72 h Comparison (90% CI)
Pre v 24 h Pre v 72 h
20% 1RM
(W)
YG 507.9 ±134.6 473.8 ±119.9 476.6 ±119.7 YG v MT
Very likely ↓Very likely ↓
MT 387.4 ±87.9 360.3 ±76.1 366.3 ±76.4
−1.07(−1.85, −0.28) −1.04 (−1.82, −0.25)
MT v MU
MU 320.7 ±47.9 291.7 ±40.1 289.7 ±40.2 Very likely ↓Very likely ↓
−1.06 (−1.84, −0.27) −1.17 (−1.96, −0.39)
80% 1RM
(W)
YG 1295.3 ±369.1 1207.5 ±328.2 1275.9 ±338.3 YG v MT
Very likely ↓Very likely ↓
MT 977.1 ±211.1 869.8 ±195.0 964.9 ±212.1
−1.07 (−1.96, −0.39) −1.04 (−1.83, −0.25)
MT v MU
MU 886.0 ±163.2 746.7 ±153.3 735.1 ±134.8 Likely ↓Very likely ↓
−0.67 (−1.45, 0.12) −1.22 (−2.01, −0.43)
The comparison panel details the qualitative descriptor, effect size, and upper and lower confidence limits.
4. Discussion
Contrary to our hypothesis, the current findings highlight the magnitude of exercise-induced
muscle damage and time-course of recovery after lower body resistance exercise is greater in trained
middle-aged males than their young counterparts. Moreover, regardless of resistance training
experience, middle-aged males experienced like symptoms of muscle damage and a similar recovery
profile in the days after.
4.1. Confirmation of EIMD
The small to moderate loss of force at 24 and 72 h observed in the current study confirms that
the prescribed lower-body resistance exercise caused EIMD. Although not indicative of myofibrillar
disruption [
7
,
8
], the small to very large increases in muscle soreness and CK activity indicate that
tissue damage occurred after squatting exercise. The losses in MVC support previous observations of
isometric strength loss after lower-body eccentric exercise in younger resistance trained males [
21
].
The reductions in MVC at 24 h possibly owe to both peripheral and central impairments, given the
contemporaneous decrements in resting doublet and VA. However, that resting doublet scores were
recovered by 72 h, but VA remained suppressed, suggests that the reductions in MVC at the later
time point were caused by central alterations. Potential central mechanisms include a reduction
in drive to the muscle caused by neural impairments and reduction in excitability to the alpha
motor-neuron [28,33].
Sports 2019,7, 132 10 of 13
4.2. Changes in Indirect Markers of EIMD in Trained Young and Middle-Aged Males
That differences between trained groups on plasma CK activity after resistance exercise were
unclear reaffirms the findings of previous studies [
15
,
18
,
34
], suggesting that membrane permeability is
similar between trained young and middle age groups. Likewise, the comparable changes in muscle
soreness observed in the two resistance trained groups is consistent with the work of Buford et al. [
18
],
albeit in a non-resistance trained sample, in the plantar flexors, though contradictory to reports of
greater soreness experienced by younger males after muscle-damaging elbow flexor exercise [
14
,
19
].
Increases in muscle soreness might reflect damage to connective tissue and decreases in range of
motion, rather than damage to the contractile machinery per se [
7
,
8
]. Consequently, these data indicate
that CK and muscle soreness responses to lower-limb muscle damaging exercise are similar in young
and middle-aged resistance trained males.
4.3. Changes in Muscle Function in Trained Young and Middle-Aged Males
Reductions in MVC, VA, and resting doublet occurred in both resistance trained groups after EIMD.
The finding that Pre VA values were not different between groups contrasts previous suggestions that
older healthy adults are unable to activate the muscle to the same extent as their young counterparts [
35
],
possibly owing to the trained nature of the MT group [
36
]. That the time course of VA recovery after
high volume squatting exercise was not different between the MT and YG groups is also a novel
finding. The moderately greater reductions in MVC in the MT group, compared to the YG group after
EIMD, appear to be mediated by peripheral alterations (i.e., disruptions of sarcomeres and impaired
excitation-contraction coupling), as reflected by the lower resting doublet values in the older trained
participants. Given that differences in VA were unclear between the resistance trained groups after
EIMD suggests that central alterations are not responsible for the greater reductions in MVC in the
MT group.
The lower Pre peak power values at 20% and 80% 1RM in the MT group, compared to the YG
group, are similar to those previously reported in resistance trained middle-aged males [
5
]. For the
first time, this study has highlighted that the decrements in peak power after EIMD are of a greater
magnitude in middle-aged males, compared to young resistance trained males. Work in young athletes
indicates that lower-body power output has strong relationships with a variety of sporting tasks [
37
,
38
].
Thus, it is plausible that the impaired power output due to EIMD may inhibit these movements in
trained young and middle-aged males. Applied practitioners should therefore be cognisant of this
and consider adopting different recovery practices for young and middle-aged male athletes after
muscle-damaging lower-limb exercise.
4.4. Differences in Recovery Between Trained and Untrained Middle-Aged Males
The two middle-aged groups produced similar peak power during the muscle-damaging protocol,
which was followed by similar changes in MVC, VA, resting doublet, and CK. The repeated bout effect
(RBE) [
7
,
10
] suggests that resistance trained males should experience less muscle damage after eccentric
exercise compared to untrained males. However, the attenuated protection offered to the muscle with
ageing [
12
,
13
] might explain the similar recovery profiles in these age groups. Moreover, the similar
sporting characteristics of the two middle-aged groups might also explain why both demonstrated
a comparable recovery profile. That is, the training experienced by both groups during their sports
participation might have provided a similar protection to the muscle-damaging squatting exercise.
A further explanation might be owed to the similar peak power produced during the muscle-damaging
protocol. It has been noted previously that the magnitude of EIMD and recovery were positively
related to the workload during the muscle damaging protocol in young and older adults [
39
]. Given
that both middle-aged groups produced a similar peak power during the exercise protocol, it is perhaps
not unexpected that the recovery profile was similar. After high volume squatting, differences between
middle-aged groups in perceived muscle soreness and peak power were moderate to large. After
Sports 2019,7, 132 11 of 13
muscle damaging exercise, the MU group demonstrated greater losses in peak power compared to the
MT group. It is plausible that the resistance training experience of the MT group served to preserve or
enhance the type 2 fiber cross-sectional area [
40
], thus accounting for their smaller losses in peak power.
Consequently, resistance training in middle-aged males might help to maintain lower-body peak power
after muscle-damaging exercise, but does not appear to alter other indirect markers of EIMD.
4.5. Limitations
Readers should be aware of the cross-sectional nature of this study. That is, cause and effect
cannot directly be established, but rather, only associations between age groups and different training
status. However, given the large differences between age groups (>18 years), designing a study that
spanned over ~18 years would be unfeasible. Whilst the high variability in plasma CK in our sample
is concerning, it should be noted that CK alterations show a poor temporal pattern with muscle
function [
41
]. As such, the CK alterations should be used to confirm tissue damage, rather than indicate
the magnitude of muscle damage.
5. Conclusions
This study reports that the magnitude of EIMD, as indicated by a reduction in muscle function,
and time-course of recovery after high volume resistance exercise is greater in trained middle-aged
males compared to their young counterparts. Practically, trained middle-aged males should be
cognisant of requiring greater recovery time and adopt appropriate strategies. Moreover, resistance
training in middle-aged males could attenuate the losses in peak power after high volume squatting
exercise, but does not alter the recovery profile of other indirect markers of muscle damage. Applied
practitioners should be mindful of these alterations in trained and untrained middle-aged males and
should programme training accordingly.
Author Contributions:
Conceptualization, J.F.T.F., K.L.L. and C.T.; Methodology, J.F.T.F., K.L.L. and C.T.; Formal
Analysis, J.F.T.F.; Investigation, J.F.T.F.; Resources, J.F.T.F.; Data Curation, J.F.T.F.; Writing—Original Draft
Preparation, J.F.T.F.; Writing—Review & Editing, J.F.T.F., K.L.L. and C.T.; Supervision, K.L.L. and C.T.
Funding: This research received no external funding.
Conflicts of Interest: There are no conflict of interest.
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