Keywords
velocity-based training - strength training - neuromuscular fatigue - back-squat
Introduction
Monitoring acute responses such as neuromuscular fatigue plays an important role in
adaptations to training [1].
Neuromuscular fatigue is defined as a transient reduction in the ability to produce
force or power induced by exercise, which results in a temporary decrease in
performance [2]. The accumulated fatigue
in the RT session increases metabolic stress and the recovery time of neuromuscular
function [3]
[4]. There are training periods in which
it is desirable to attenuate the level of mechanical, metabolic, and
psychobiological stress, as well as to avoid an exacerbated reduction in
neuromuscular performance [5].
Therefore, fatigue monitoring tools are essential for adequate adjustments in the RT
program configuration.
The CMJ test is one of the most popular protocols for monitoring neuromuscular status
[6]
[7]. CMJ assessment is a valid and
reliable method for quantifying neuromuscular fatigue [8]
[9]. A meta-analysis study demonstrated that CMJ height is sensitive to
neuromuscular fatigue [10]. Several
methods, such as force platforms, contact mats, video systems, and smartphone
applications, are valid for accurately measuring CMJ performance [11]
[12]
[13]
[14]. However, some potential drawbacks
may limit its utilization for strength and conditioning coaches. Force platforms and
contact mats are considered the gold standard, but their relatively expensive cost
can restrict usability in many practical contexts. Video systems and smartphone
applications are low-cost, but the evaluation results are not available in real
time. Video processing takes time because each jump is tracked manually, which can
compromise its applicability for large groups.
Monitoring bar velocity in resistance exercise through linear encoders and
accelerometers has emerged as a methodological alternative to control neuromuscular
fatigue in RT [15]. Sánchez-Medina and
González-Badillo [4] investigated
whether the loss of bar velocity can be used as an optimal indicator of fatigue. The
results showed significant reductions in bar velocity against a moderate load
(~1 m/s-1) and countermovement jump (CMJ) height pre-post exercise.
Strong correlations were found between pre-post exercise in the velocity loss
percentage and the velocity loss percentage over sets (r=0.91–0.97), blood
lactate (r=0.97), and ammonia (r=0.86). Additionally, the velocity
loss percentage (pre-post) in the squat and CMJ height reduction were strongly
correlated (r=0.93).
Previous studies have also proposed that bar velocity monitoring can be applied in
various ways to assess and control neuromuscular fatigue in RT [4]
[16]. The magnitude of fatigue can be controlled in real-time using
intra-set velocity loss thresholds to regulate the volume [17]. Another application involves daily
monitoring of bar velocity changes against a baseline measure (i. e. load-velocity
profile) to assess residual fatigue level, readiness, and recovery status [18]
[19]
[20]. Increased or
maintained bar velocity against a given load (pre-session) compared to the baseline
measurement indicates a low level or absence of fatigue, optimal recovery, and
readiness to train [18]
[19]
[20]. Conversely, a bar velocity reduction suggests exacerbated residual
fatigue levels and compromised readiness and recovery. Despite the practical
importance of this approach, the effects of variables capable of interfering with
bar kinematics and its sensitivity to fatigue have yet to be widely examined.
The load intensity (usually considered by the percentage of the one-repetition
maximum (%1RM)) and the kinematic velocity variables (e. g. mean propulsive
velocity, peak velocity, or mean velocity) must be defined to implement bar velocity
monitoring daily. A strong inverse relationship between load and different velocity
variables has been evidenced in various resistance exercises [21]
[22]. The absolute bar velocity is load-dependent and changes according to
the load intensity magnitude. Furthermore, a recent study reported moderate
reductions in bar velocity in the back-squat (mean and peak velocity) from baseline
to 24 h and 48 h following a strength-oriented session, but only when moderate and
heavy squat loads were performed (≥60% 1 RM) [18]. No decrease in the mean or peak velocity was observed at light loads
(20% and 40% 1RM). These results suggest that load intensity and the kinematic
velocity variable can modulate their sensitivity to fatigue.
Three kinematic variables are often used for monitoring the bar velocity during the
concentric exercise phase, which include mean propulsive velocity (MPV), mean
velocity (MV), and peak velocity (PV). The kinematic variables measured depend on
the type of device used to monitor the bar velocity (e. g. encoder, accelerometer,
or smartphone application). Some technological devices only measure MV and PV or MPV
and PV, while other equipment reports MPV, MV, and PV data. However, the kinematic
variable used as a parameter for monitoring neuromuscular fatigue appears to be
arbitrarily defined [18]
[19]
[20]. Pérez-Castilla et al. [23] compared the inter-session reliability of the MPV, MV, and PV
variables measured by a linear encoder during a loaded vertical jump exercise. The
results showed that MV, MPV, and PV presented acceptable test-retest reliability
(ICC>0.70 and CV<10%), but PV demonstrated greater reliability than MV and
MPV. The findings of the study by Pérez-Castilla suggest that the kinematic velocity
variable seems to be an important factor to be considered in monitoring protocols
using encoders in the RT. However, to date it has not yet been determined whether
the encoder’s sensitivity to fatigue is influenced by the velocity variable measured
by the encoder device used in the bar. Therefore, this study compared the acute
responses of pre- to post fatigue exercise through CMJ height and bar velocity in
free parallel back-squat against three load intensities (40%, 60%, and 80% 1RM)
using two kinematic variables (MPV and PV).
Materials and Methods
Experimental design
We implemented a repeated-measures randomized cross-over design to analyze the
effects of load magnitude (40%, 60%, and 80% 1RM) and bar velocity variables
(MPV and PV) on sensitivity to neuromuscular fatigue. After familiarization
(session 1) and 1RM assessment (session 2), the actual experimental trials took
place in three visits to the laboratory, separated by 48 to 96 hours (sessions
3, 4, and 5). The participants completed a fatiguing protocol that consisted of
three sets with maximum repetitions of back squat exercise at 75% 1RM in each
experimental session. The eccentric phase of back-squat was performed at a
continuous and controlled cadence (2–3 s) with a momentary pause (~1.5 s) before
starting the concentric phase [22].
The participants performed the concentric phase with maximum intentional
velocity [4]. They also performed
assessments of CMJ height (3 trials with 30 s of rest) and bar velocity (MPV and
PV) during a back-squat exercise (1 set x 3 repetitions at 40%, 60%, or 80% 1RM)
before and after the intervention (-10 min,+5 min, and+20 min, respectively).
The exercise load was randomized in the experimental sessions. The same
evaluators conducted the tests at the same time of day (±1 hour) under similar
environmental conditions.
Participants
The estimated minimum sample size was 16 participants, considering a power of
0.80, alpha=0.05, and effect size of 0.35. A total of 20 male university
students aged 18 to 35 years old were recruited to participate in the study.
After three subjects failed to complete the experimental sessions, the final
sample consisted of 17 participants (age=25.7±4.9 years; height: 177.0±7.2 cm;
body mass=77.7±12.3 kg; strength training experience=4.2±2.5 years; back-squat
1RM=145.0±33.9 kg; relative strength ratio - 1RM/body mass=1.86). All the
participants were recreationally trained with at least 12 months of experience
in RT. The participants were physically active and had experience in RT ranging
from 1.5 to 6 years (4.2±2.5 years), with a weekly training frequency of three
to five RT sessions per week in the last 12 months. They were accustomed to
performing the back-squat exercise with correct technique in training routines.
No recent musculoskeletal injuries were reported. Consent for participation in
the study was obtained individually after the participants received information
about the experimental procedures. The Institutional Research Ethics Committee
approved the experimental protocol, and it was then carried out in conformity
with the Declaration of Helsinki.
Procedures
Familiarization – session 1
The first session was used to familiarize participants with the Total Quality
Recovery scale (TQR), OMNI-RES perceived exertion scale, CMJ, and back-squat
exercise at maximal intended velocity in the concentric phase. Maximal
intended velocity aimed to familiarize participants with lifting loads with
different intensities (% 1RM) as fast as possible. We used the average and
confidence interval of the bar velocity reported in a previous study as a
reference to ensure the appropriate application of the maximal intended
velocity [22]. We adopted
anchoring procedures based on previous studies to ensure accuracy and
consistency of responses when employing perceptive scales [24]
[25]. Anchoring procedures
consisted of teaching participants to report responses based on their
sensations, perceptions and memory with maximum accuracy. Standardized
instructions were used to differentiate the scale scores and their
respective descriptors [26]
[27]. Participants also received
cues to facilitate the association of the score with the appropriate scale
descriptor and possible sensations experienced.
Each participant completed a general warm-up, including dynamic stretching,
joint mobility, light aerobic cycling, and 2 sets of 3 unloaded vertical
jumps. Then, they completed 2 sets of 5 CMJ (10 s between attempts and
3 minutes rest between sets). The CMJ consisted of a maximal concentric
movement preceded by a rapid eccentric movement to a knee flexion of
approximately 90°. After 5 minutes of rest, the participants performed a
specific warm-up in the back-squat exercise (2 sets x 5 reps x 20 kg).
Familiarization procedures for applying the maximal intended velocity were
performed as previously described [22]. The participants completed three different back-squat
protocols during familiarization as follows: a) 3 sets x 12 reps x 40% 1RM
estimated; b) 3 sets x 8 reps x 60% 1RM estimated; and c) 3 sets x 5 reps x
80% 1RM estimated. The interval between the sets was 2 minutes. The
researchers provided verbal feedback on performance throughout the
familiarization session.
1RM assessment – session 2
A progressive load test was performed in session 2 to determine 1RM in the
back-squat exercise from the individual load-velocity profile [20]. The participants completed a
general and specific warm-up as previously described. The initial load was
30 kg for all participants, followed by gradual increments of 20 kg until
reaching a mean propulsive velocity (MPV)<0.9 m.s-1, and then
15 to 5 kg until MPV was<0.50 m.s-1. Three, two and one
repetitions for a given load were performed for light, moderate, and heavy
loads, respectively. We considered only the fastest MPV repetition at each
load for the load analyses [21].
Inter-set resting ranged from 2 to 3 min for light and moderate loads, and
5 min for heavy loads. Finally, the participants squatted down until their
thighs were parallel with the floor (90-degree angle), pushing their hips
backward and flexing their knees, and then they returned to the initial
position. The execution technique was carefully supervised in all
repetitions. The eccentric phase of the movement was performed in a
controlled manner imposing a momentary pause (i. e. the bar was held on the
ground for 1.5 s between the eccentric and concentric phases) [22]. Verbal encouragement and
real-time velocity feedback were provided in every repetition.
Experimental trials – sessions 3–5
The experimental sessions involved assessments of neuromuscular fatigue (pre-
and post-exercise) using the CMJ and back-squat bar velocity against three
load intensities (40%, 60%, or 80% 1RM). The experimental phase consisted of
three sessions spaced 48 to 96 hours apart. The interval between sessions
was evaluated to ensure that the participants were in the best recovery
state between experimental tests. The experimental session only occurred
when participants reported a minimum score of 15 points, equivalent to the
“good recovery” descriptor on the TQR scale. During the experimental
sessions, three subjects reported scores below the established criteria. In
these particular circumstances, experimental sessions were rescheduled in
order to guarantee sufficient recovery and equalization between the
experimental conditions. All participants answered the TQR scale and
performed the standardized warm-up before each experimental session. Then,
assessments of neuromuscular fatigue using vertical jump (CMJ) and maximal
concentric velocity (MCV) in the back-squat exercise were performed
immediately after the warm-up (detailed in the next section). After
10 minutes, the participants were submitted to a fatiguing exercise protocol
consisting of 3 sets with the maximum repetitions at 75% 1RM and 3 min of
rest in the back-squat exercise. The participants completed as many
repetitions as possible until the MPV fell below the velocity corresponding
to 1RM (0.30 m.s-1) [22]. The maximum repetitions protocol with a load of 75% 1RM was
programmed to achieve a velocity loss percentage of approximately 50% in
each set. The percentage of velocity loss was established by the percentage
difference between the maximum intended velocity for the load of 75% 1RM
(0.60 m.s-1) and the velocity corresponding to 1RM
(0.30 m.s-1) for the back-squat exercise [22]. Velocity loss thresholds
above 40% per set result in high magnitudes of mechanical and metabolic
stress [17]. The repetitions
were performed with the maximum intended velocity and verbal encouragement
throughout the protocol. Neuromuscular fatigue was assessed again 5 and
20 minutes after fatiguing exercise. Finally, the participants reported
their perceived exertion at the end of the experimental session. They were
asked to maintain their sleep behavior, avoid alcohol consumption, and usual
diet habits before each of the following experimental sessions. They were
also requested to avoid caffeine consumption for 3 h before the experimental
condition and consume a light meal 2 h before the experiment. Participants
were instructed to interrupt their resistance training routines during the
experimental period to minimize the potential effects of confounding
variables. These data were self-reported before each experimental session.
Moreover, we use the TQR responses to control for optimal readiness status
before the experimental session, as previously described.
Neuromuscular fatigue assessment
The CMJ height and MCV in the back-squat exercise with sub-maximal loads were
used as mechanical measures to assess neuromuscular fatigue in the
experimental sessions. Mechanical measures to assess neuromuscular fatigue
were obtained before (−10 min) and after (+5 min and+20 min) the fatiguing
resistance exercise protocol. The participants completed a general warm-up
prior to the start of testing, as previously described. After 2 minutes of
rest, the participants performed three maximal CMJs interspersed by
10-second intervals. The mean of three jumps was used for analysis.
The evaluation of MCV in the back-squat against different load intensities
started after 3 minutes of rest from the jump test. The load order (40%,
60%, and 80% 1RM) was distributed randomly among the experimental sessions
for each participant. The participants performed three repetitions of the
back-squat with maximum intentional velocity in the concentric phase. The
mean of three repetitions based on bar velocity variable (MPV and PV) was
used in the analysis. Vertical jumps and MCV protocols followed the
procedures described in previous studies [28]
[29]. The researcher provided
verbal encouragement during both tests.
Measurement equipment and data recording
CMJ height was assessed on a contact platform (Elite Jump; S2 Sports, São
Paulo, Brazil). The height (h) was estimated from the flight time (t) in the
system (i. e. h=gt2/8). The validity and usability of this
contact platform system have been previously reported [12]. A linear encoder (Vitruve,
Madrid, Spain) was attached to the bar to measure the lifting kinematics for
the MCV assessment. This system measures the cable displacement in response
to changes in the bar position during the concentric phase at a sampling
rate of 100 Hz [30]. Two
velocity variables were used for analyses: 1) mean propulsive velocity (MPV)
and 2) peak velocity (PV). The MPV was defined as a mean velocity value of
the portion of the concentric phase during which barbell acceleration was
greater than the acceleration due to gravity [31]. The PV was established as
the maximum instantaneous velocity value reached during the concentric phase
[23]. The Vitruve encoder
(previously named Speed4Lifts) has been demonstrated to be valid and
reliable to record movement velocity with an absolute error below the
acceptable maximum error criterion (<5%) [30].
Recovery status, perceived exertion and internal load
The level of perceived recovery was assessed through the Total Quality
Recovery scale (TQR) before each experimental session [26]. TQR scores range from 6 to
20, in which a higher level of perceived recovery is related to higher
scores. Participants reported their RPE 30 min after the end of each
experimental session using the OMNI-RES scale [27]. All participants were
familiarized with both scales in sessions 1 and 2 for the anchoring
procedures. The same investigator obtained the perception variables
individually during all sessions. The internal training load of each
experimental condition was quantified using the session-RPE method [32]. The internal training load
was the product of the RPE scale by the total number of repetitions
completed in the fatiguing exercise.
Statistical analysis
The data are presented through mean and standard deviation. Percentual delta
(pre-post) of the mechanical measures of neuromuscular performance was
calculated for each test. We used the variation in CMJ performance as a
reference measure for assessing bar velocity sensitivity during the MCV
protocol. Normal distribution was confirmed by Shapiro-Wilk test (p>0.05).
Repeated-measures analysis of variance (ANOVA) compared the perceived recovery,
perceived exertion, total of repetitions, volume load, and internal load between
the experimental conditions. Changes in CMJ and MCV neuromuscular performance
(40%, 60%, and 80% 1RM) were compared at different time points (pre, post+5 min,
and post+20 min) using two-way repeated measures ANOVA. The Bonferroni post hoc
test was used to identify significant differences. Effect sizes (ES) were
calculated using Cohen’s d (mean difference divided by the SD of the change
score) [33]. The ES magnitude was
interpreted as trivial (<0.2), small (0.21–0.59), moderate (0.60–1.19), large
(1.20–1.99), and very large (≥2.0) [34]. Intraclass correlation coefficient (ICC), coefficient of
variation (CV), and Bland-Altman plots with a 95% confidence interval verified
the agreement between CMJ and MCV protocols. The following criteria were used to
interpret the ICC: poor (<0.5), moderate (0.5–0.75), good (0.75–0.9), and
excellent (>0.9) [35]. CV values
lower than 10% were considered acceptable [36]. The statistical significance level was set at p≤0.05. All
analyses were conducted in the Statistical Package for the Social Sciences
program (SPPS version 21.0 for Windows).
Results
Neuromuscular performance changes in fatigue tests (CMJ and MCV) from pre-exercise to
post-exercise (+5 min and+20 min) are shown in [Table 1] and [Fig. 1]. There was a significant
decrease in CMJ height from pre- to post-exercise (∆%=−7.5 to −10.4; p<0.01;
ES=0.37 to 0.60). MPV reduced also significantly at 40% 1RM (∆%=−7.5 to −8.9;
p<0.01; ES=0.43 to 0.60), 60% 1RM (∆%=−5.3 to −6.2; p<0.01; ES=0.35 to 0.39),
and 80% 1RM (∆%=−10.7 to −12.5; p<0.01; ES=0.53 to 0.66) from pre- to
post-exercise. Similarly, there was a decrease in PV at 40% 1RM (∆%=-4.0 to −4.5;
p<0.01; ES=0.32 to 0.36), 60% 1RM (∆%=−4.6 to −4.9; p<0.01; ES=0.30 to 0.34),
and 80% 1RM (∆%=−9.2 to −9.2 p<0.01; ES=0.43 to 0.47).
Fig. 1 Comparison of changes in performance dext-linkng post-exercise
(+5 min and+20 min) versus pre-exercise (baseline) between the assessment of
fatigue in the CMJ protocol and maximum concentric velocity in the
back-squat using different loads (40%, 60%, and 80% 1RM) and kinematic
variables (MPV and PV). The data are presented in mean and 95%CI. Note: CMJ:
countermovement jump; MPV: mean propulsive velocity; PV: peak velocity.
*Significant difference to pre-exercise (p≤0.05). †Significant
difference to CMJ (p≤0.05).
Table 1 Comparisons of neuromuscular performance within (pre-
to post-exercise) and between experimental protocols. Data are presented
in mean and standard deviation.
|
Protocols
|
Variables
|
Pre
|
Post 5 min
|
Post 20 min
|
∆% Post 5 min
|
ES Post 5 min
|
∆% Post 20 min
|
ES Post 20 min
|
|
40% 1RM
|
CMJ (cm)
|
40.4±6.34
|
37.3±5.64
|
36.5±6.07
|
−7.5±3.7*
|
0.37 (S)
|
−9.6±6.0*
|
0.46 (S)
|
|
MPV (m.s−1)
|
0.95±0.12
|
0.88±0.11
|
0.86±0.09
|
−7.5±5.3*
|
0.43 (S)
|
−8.9±7.0*
|
0.60 (M)
|
|
PV (m.s−1)
|
1.68±0.15
|
1.61±0.16
|
1.60±0.16
|
−4.0±3.3*†
|
0.32 (S)
|
−4.5±4.4*†
|
0.36 (S)
|
|
60% 1RM
|
CMJ (cm)
|
40.6±6.40
|
36.9±4.24
|
36.2±5.15
|
−9.1±7.3*
|
0.60 (M)
|
−10.4±5.9*
|
0.54 (S)
|
|
MPV (m.s−1)
|
0.74±0.08
|
0.70±0.08
|
0.69±0.10
|
−5.3±6.2*†
|
0.35 (S)
|
−6.2±9.2*†
|
0.39 (S)
|
|
PV (m.s−1)
|
1.42±0.14
|
1.36±0.14
|
1.35±0.15
|
−4.6±4.5*†
|
0.30 (S)
|
−4.9±6.7*†
|
0.34 (S)
|
|
80% 1RM
|
CMJ (cm)
|
40.5±7.07
|
36.5±5.43
|
36.3±6.50
|
−9.7±5.4*
|
0.45 (S)
|
−10.2±5.9*
|
0.52 (S)
|
|
MPV (m.s−1)
|
0.52±0.08
|
0.45±0.07
|
0.46±0.08
|
−12.5±8.3*
|
0.66 (M)
|
−10.7±9.0*
|
0.53 (S)
|
|
PV (m.s−1)
|
1.19±0.17
|
1.08±0.16
|
1.08±0.19
|
−9.2±8.6*
|
0.47 (S)
|
−9.2±8.1*
|
0.43 (S)
|
Note: CMJ: there was a decrease in countermovement jump; MPV: mean propulsive
velocity; PV: peak velocity; ES: effect size; S: small effect size; M:
moderate effect size; *Significant difference to pre-exercise (p≤0.05).
†Significant difference to CMJ (p≤0.05).
The decrease magnitude in neuromuscular performance was similar between the CMJ, MPV
(40% 1RM; and 80% 1RM; p=1.00), and PV (80% 1RM; p=1.00). However, the decrease in
CMJ height percentage was higher than in MPV at 60% 1RM (p=0.05) and PV (40% 1RM;
p=0.05 and 60% 1RM; p<0.01).
Bland-Altman plots revealed low systematic bias in the different MCV protocols ([Fig. 2]). However, the lowest systematic
biases values were observed in the MPV with 40% and 80% 1RM. In addition, acceptable
levels of agreement between CMJ and bar velocity post 5 min were only found in the
MPV protocols at 40% 1RM (ICC=0.71, 95%CI=0.19–0.89; CV=5.1, 95%CI=3.5–6.7) and 80%
1RM (ICC=0.58, 95%CI=0.65–0.84; CV=8.5%, 95%CI=6.2–10.7), but both were moderate
([Table 2]). Moderate agreement was
also observed at the post-20 min moment between CMJ and bar velocity at 80% 1RM
(ICC=0.58, 95%CI=0.65–0.84), while the CV was not acceptable (CV=12.1,
95%CI=8.7–15.4).
Fig. 2 Bland-Altman plots for the changes in performance (∆%) between
CMJ performance (reference measure) and bar velocity assessment (MPV and
PV). Note: CMJ=countermovement jump; MPV=mean propulsive velocity; PV=peak
velocity; SD=standard deviation; LoA=limits of agreement.
Table 2 Intraclass correlation coefficients and coefficients
of variation with 95% confidence intervals of the agreement of fatigue
assessment between CMJ and bar velocity protocols with different loads
(40%, 60%, and 80% 1RM) and kinematic variables (MPV and
PV).
|
Protocols vs CMJ
|
ICC
|
CV (%)
|
ICC
|
CV (%)
|
|
Post 5 min
|
Post 5 min
|
Post 20 min
|
Post 20 min
|
|
MPV (40% 1RM)
|
0.71 (0.19–0.89)
|
5.1 (3.5–6.7)
|
0.09 (−1.76–0.68)
|
5.4 (3.8–7.1)
|
|
MPV (60% 1RM)
|
0.10 (−1.36–0.62)
|
5.4 (3.7–7.1)
|
0.23 (−0.81–0.70)
|
5.5 (4.0–6.9)
|
|
MPV (80% 1RM)
|
0.58 (−0.65–0.84)
|
8.5 (6.2–10.7)
|
0.51 (−0.40–0.82)
|
12.1 (8.7–15.4)
|
|
PV (40% 1RM)
|
0.37 (−0.29–0.73)
|
2.8 (2.2–3.3)
|
−0.08 (−1.34–0.56)
|
3.1 (2.0–4.2)
|
|
PV (60% 1RM)
|
−0.27 (−1.77–0.48)
|
4.8 (2.4–4.8)
|
0.21 (−0.53–0.66)
|
4.5 (1.9–7.0)
|
|
PV (80% 1RM)
|
0.43 (−0.65–0.80)
|
8.2 (5.5–11.0)
|
0.38 (−0.77–0.78)
|
10.3 (6.3–14.3)
|
Note: CMJ: countermovement jump; MPV: mean propulsive velocity; PV: peak
velocity; ICC: intraclass correlation coefficients; CV: coefficients of
variation.
The total quality recovery, total of number repetitions, load volume, perceived
exertion, and internal training load from the fatigue protocol are shown in [Table 3]. There were no statistical
differences between experimental condition sessions (p>0.05).
Table 3 Perceived recovery, characterization of the fatigue
protocol, and perceived exertion in the three experimental
conditions.
|
Variables
|
40% 1RM
|
60% 1RM
|
80% 1RM
|
|
TQR
|
17.0±2.1
|
17.0±1.7
|
18.0±1.4
|
|
Total of RMs
|
27.0±6.0
|
28.0±8.6
|
28.0±9.1
|
|
Volume load (kg)
|
2960±1029
|
3104±1278
|
3101±1220
|
|
RPE
|
7.0±1.0
|
7.0±1.4
|
8.0±1.7
|
|
Internal load (AU)
|
189±50
|
196±89
|
224±95
|
Note: TQR: total quality recovery; RMs: sum of the maximum number of
repetitions in the 3 sets of the fatigue protocol (75% 1RM); RPE: rating of
perceived exertion; AU: arbitrary units.
Discussion
Previous studies have proposed the use of bar velocity during resistance exercise as
a method of monitoring neuromuscular fatigue [4]
[18]
[19]
[20]. However, no study to date has investigated whether load magnitude
and the mechanical velocity variable influence the sensitivity to fatigue. To our
knowledge, this is the first study to compare the effects of the load magnitude of
resistance exercise and bar velocity kinematics variables on sensitivity to fatigue.
Our findings revealed an acute reduction in neuromuscular performance during
post-session in both CMJ and back-squat exercise at MCV, independent of load
intensity (40%, 60%, and 80% 1RM) and variable kinematic velocity (MPV or PV).
Furthermore, our results indicated greater agreement and similarity in the acute
decrease pattern of neuromuscular performance between CMJ and back-squat exercise
using MPV at intensities of 40% and 80% 1RM during fatigue monitoring by bar
velocity.
Similar to previous studies [18]
[19]
[20], we found meaningful reductions in MPV and PV during the back-squat
at 40%, 60% and, 80% 1RM after fatiguing exercise. These findings provide additional
evidence that supports the use of monitoring changes in bar velocity as a simple,
practical, and valid alternative to assess exercise-induced neuromuscular fatigue
[4]
[18]
[19]
[20], which can be used as
an alternative to other protocols such as isometric maximal force measurement,
electromyography, voluntary muscle activation, motor evoked potential, and vertical
jumps [10]
[37]. Neuromuscular fatigue results in a
temporary reduction in the capacity of the neuromuscular system to produce force and
power [2]. The gradual development of
fatigue affects central and peripheral mechanisms, which results in a reduction in
shortening velocity, an increase in relaxation time, and a decrease in the
efficiency of the contractile capacity of muscle fibers [2]. Regardless of the load magnitude, the
acute post-exercise decreases in both kinematic measurements of bar velocity (MPV
and PV) in the back-squat exercise can be thought of as a behavioral manifestation
of impaired central and peripheral mechanisms involved in muscle contraction
resulting from fatigue.
Based on delta percentage magnitude, our results revealed that MPV was more sensitive
than PV during fatigue monitoring using bar velocity in back-squat exercise. These
findings can be interpreted as a result of the greater sensitivity of the MPV to
discriminate the fatigue level during the concentric phase of the movement. The
concentric phase of the movement can be subdivided into two portions: 1) propulsive
(F>0) and braking (F<0) [31]. The
hip and knee extensor muscles work collectively to produce high levels of force to
break inertia and accelerate the bar throughout the propulsive phase of the
back-squat. During the concentric phase of the back-squat from a static position,
the initial velocity of the movement is zero, achieves peak velocity at the final
part of the propulsive portion of the lift, and returns to zero velocity at the end
of braking [31]. The presence of
neuromuscular fatigue impairs muscle contractile function and, consequently, the
capacity of the neuromuscular system to produce force and velocity in the propulsive
portion of the concentric phase [2].
Thus, a measure with high sensitivity to fatigue is one that is able to detect the
impairment degree of neuromuscular function accurately during the propulsive portion
of movement. MPV is a kinematic velocity variable representative of the entire
propulsive portion of the movement. Conceptually, MPV is a mean velocity value
arising from the portion of the concentric phase in which the bar acceleration is
higher than gravity. In contrast, PV is a measurement derived from a single value
equivalent to the maximum instantaneous velocity reached in the bar’s acceleration
phase [31]. Additionally, the ability of
muscle fibers to generate force depends on the length-tension relationship [38]
[39]. Force generation is enhanced when the length of the sarcomere
provides an optimal overlap between the actin and myosin filaments for the formation
of cross-bridges [38]
[39]. Given that MPV is a measure that
depicts the force application across the propulsive phase, its increased amplitude
of capture of variations in force production of the muscle groups in various
length-tension relationships during the back-squat may explain why it is more
sensitive to fatigue. Therefore, it is possible that MPV showed greater sensitivity
to fatigue because it encompasses more information about the application of force to
lift the bar during the propulsive portion of the concentric phase.
The results of the current study suggest that the load magnitude of resistance
exercise affect the agreement of bar velocity-based protocols with CMJ to detect
neuromuscular fatigue. The post-exercise performance reduction profile (+5 min
and+20 min) during the CMJ showed higher concordance (based on ICC) with the
back-squat at 40% and 80% 1RM than 60% 1RM. These effects can be attributed to the
different load intensities of the back-squat since the experimental sessions were
similar in relation to the recovery status, volume, intensity, effort, and internal
load measures ([Table 3]). The
biomechanical similarity between the back-squat at 40% 1RM and CMJ may have been
decisive for its high sensitivity to fatigue. The force application to perform the
CMJ occurs under low external load conditions, similarly to the back-squat execution
at a light load. Both exercises are characterized by the predominance of velocity
from a mechanical point of view of the force-velocity profile [40]. In contrast, we also observed high
sensitivity to fatigue of the protocol employing MPV during the back-squat at 80%
1RM. The participants completed three sets with the maximum repetitions at 75% 1RM
in the RT program to induce fatigue. Resistance exercise of maximum repetitions with
moderate-heavy load (i. e. 75% 1RM) can predominantly cause fatigue of
high-threshold motor units. Contractions against heavy loads preferentially recruit
high-threshold motor units [41], which
may explain the high sensitivity to fatigue of the back-squat exercise in the 80%
1RM protocol. It is relevant to note that the measures used in the current study are
limited to providing strong explanations of why the 60% 1RM load did not produce
similar results to the 40% and 80% 1RM. It is possible that this result was a chance
event because we did not use a direct measure of fatigue in this study. New studies
with direct measurements of fatigue may clarify this issue.
Our findings have important practical implications for fatigue management and load
control in resistance training. Strength and conditioning coaches can implement bar
velocity monitoring protocols (MPV or PV) in the back-squat at 40% or 80% 1RM before
the start of the training session. A reduction in bar velocity indicates an
impairment of neuromuscular function due to fatigue, which can be used to establish
an optimal volume for the session, adjust the training dose, and minimize the risk
of injury. Despite the practical implications of our findings, the present study has
some limitations. The fatigue assessments through bar velocity monitoring only
involved a single lower limb exercise (back-squat exercise) after a session with the
same exercise. Fatigue assessment protocols involved two kinematic measurements of
bar velocity (MPV and PV). The sensitivity to fatigue tests with different bar
velocity monitoring protocols occurred acutely (+5 min and+20 min post-exercise).
Finally, the study sample only included resistance-trained men. Therefore,
generalizing our results applies to contexts with similar characteristics to those
described above. Future studies should investigate the fatigue sensitivity of
protocols using bar velocity in other resistance exercises, with delayed acute
(+24 h) and accumulated fatigue induced by different training programs and other
populations (e. g. elite athletes, women, young athletes).
The CMJ test was employed as a reference in this study because it has biomechanical
properties akin to the back-squat [41]
[42]
[43] and is assumed to be a valid
indicator of neuromuscular fatigue [8]
[9]
[10]. Mechanical and neuromuscular factors
linked to the capacity to generate and transmit vertical force are comparable
between the two exercises. For example, both exercises show similarities in the
recruitment pattern of lower limb muscles, such as quadriceps, hamstrings, and
glutes [41]
[42]
[43]. There is also a similarity in the kinematic pattern of movement that
includes flexion and extension of the hip, knee, and ankle joints [41]
[42]
[43]. Although vertical
jump assessment is an applicable method for monitoring neuromuscular fatigue, the
cost of some devices (e. g. force platforms and contact mats) and the impossibility
of accessing real-time results when using video systems can restrict or make its use
difficult for most strength and conditioning coaches. Nevertheless, there is a
growing popularity of portable devices (i. e. linear encoders) among strength and
conditioning coaches due to their applicability, accuracy, and relatively low cost
[44]
[45]. Linear encoders allow monitoring bar
velocity in real time, which makes it possible to manipulate the volume and
intensity of resistance training in an objective, precise and practical way [15]. Additionally, previous studies have
proposed using movement velocity as an alternative method to monitor neuromuscular
fatigue [18]
[19]
[20]. However, the best choice of velocity variables and the optimal load
to detect changes in neuromuscular status have not yet been defined. Based on the
results of this study, strength and conditioning coaches can quantify neuromuscular
fatigue resulting from RT by monitoring MPV or PV during the back-squat exercise,
regardless of the load magnitude. However, using the MPV variable and loads
equivalent to 40% 1RM and 80% 1RM in the back-squat allows greater consistency in
the CMJ results to detect the neuromuscular fatigue.
Conclusion
In conclusion, our findings showed that MCV back-squat exercise, regardless of the
load intensity (40%, 60% and 80% 1RM) and the kinematic variable velocity (MPV or
PV), can be used to monitor acute fatigue caused by RT. However, the back-squat
exercise at 40% or 80% of 1RM using MPV provides higher sensitivity for monitoring
fatigue through changes in bar velocity. Therefore, strength and conditioning
coaches can use the MPV and light (40% 1RM)or heavy (80% 1RM) loads to accurately
manage neuromuscular fatigue during RT.
