Acute Responses of A Drop-Set Session on Muscular Activation and its Applicability in Resistance-Trained Individuals

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INTRODUCTION
The potential for further functional and morphological improvements appears to diminish as maximal strength and training status increase (Stone, Plisk et al. 1998).Consequently, trained individuals present lower magnitudes of gains than untrained individuals (American College of Sports 2009), thus requiring the application of advanced training stimuli.
The drop-set is an advanced training method for enhancing muscle hypertrophy in trained individuals (Schoenfeld 2011).The method is proposed to be performed by taking a set to muscular failure (FAIL) at a given load and then immediately reducing the load and performing as many additional reps as possible (Schoenfeld 2011).Nevertheless, a possible limitation of the technique could relate to load reductions.Henneman's size principle ensures that the central nervous system recruits motor units (MUs), starting with the smallest and most excitable, then progressing to the higher and most difficult to excite to maintain or increase force (Henneman, Somjen et al. 1965).Considering that MU recruitment International Journal of Strength and Conditioning.2023 is necessary for subsequent adaptations, training stimuli aimed at maximizing muscular development would be performed with higher loads (Fry 2004).On the other hand, the concept that high loads are mandatory to induce muscle hypertrophy has been challenged (Mitchell, 2017).Possible explanations may be that during fatiguing contractions, the excitability threshold for recruitment of type II motor units is reduced (Adam and De Luca 2003) or motor units may 'cycle' to maintain force (Westad, Westgaard et al. 2003).Additionally, the increased neural drive to the agonist and synergist muscles and the higher metabolic stress generated by the glycolytic contribution during FAIL may increase motor unit activation (Gandevia 2001, Gorassini, Yang et al. 2002).Despite the drop-set being proposed to be performed by taking a set to FAIL (Schoenfeld 2011), this procedure is not adopted in studies that represent a mischaracterization of the technique regarding its primary objectives (Kassiano, de Vasconcelos Costa et al. 2021).
The present study aimed to investigate neuromuscular activation during a drop-set session with repetitions to FAIL in trained individuals.We hypothesized that the possible decreased neuromuscular activation due to load reduction would be counterbalanced by the fatigue induced by performing repetitions to FAIL.Since longitudinal studies remain equivocal on the utility of drop-set sessions, conceivably understanding the acute responses can help to explore its potential practical applications.) participated in the study.The inclusion criteria for participants were as follows: 1) having no intake of exogenous anabolic-androgenic steroids or dietary supplements with potential effects on physical performance; 2) being engaged in strength training programs for at least one year before the experiments; and 3) having no current history of injuries on the lower limbs that could impair the execution of the session.After being advised about the purposes and risks of the study, all subjects signed informed consent forms.The Local Research Ethics Board approved this study.

Study design
We investigated quadriceps femoris electromyography activity during a drop-set session.The protocol was performed on 45° leg press equipment.Since this setup already characterized the proposed drop-set protocol, we used the first set as a control situation.To investigate the effect of the whole session, we compared the electromyographic responses of the second and third sets with those presented during the first.We assessed the fatigue and metabolic stress induced by observing the peak force changes-extracted from the maximum voluntary contraction test-and blood lactate concentrations measured immediately before and after training, as well as 30 min after training.
Participants made four visits to the laboratory 72 hours apart.The first three visits were destined for familiarization with the leg press equipment, range of motion, and frequency of the repetitions.Additionally, for the training load prescription, they performed the 1-RM test.On the fourth day, the volunteers performed the drop-set protocol.During the execution of the protocol, we monitored muscle activation through the amplitude of the surface electromyography signals of the vastus lateralis, vastus medialis, and rectus femoris muscles.We asked the participants to remain absent from strenuous exercise, alcohol intake, tobacco, and ergogenic supplements (such as caffeine) for 48 hours before visits.

Training protocol
The training session was performed on the same 45° leg press equipment used for the 1-RM assessment.Before the beginning of the session, subjects performed a dynamic warm-up consisting of 8 repetitions with 50% 1-RM and 3 repetitions with 70% 1-RM load.The drop-set protocol consisted of three sets with two drops (80 to 60 and to 40% 1-RM load), and repetitions performed to FAIL were defined as the inability to complete a repetition at a given load over a full range of motion without external We controlled the repetition frequency by a digital metronome set at 60 beats per minute.We instructed the volunteers to synchronize the transition of concentric to eccentric and eccentric to concentric phases with the equipment's audible signals resulting in execution times of ~1s for each phase of the repetitions.The range of motion for the knee and hip joints was ~90° of flexion to full extension at ~0°.Three researchers involved in the study fully monitored the session.We quantified the volume load by multiplying the repetitions by the load (in kg) employed.

One repetition maximum test (1-RM)
The 1-RM test was employed to evaluate muscular strength and to define the relative loads employed.
The test was performed in the same 45° leg press machine (Tonus Fitness Equipment®, model RT 009) used for the drop-set session and was conducted according to the methods described by Brown and Weir (Brown and Weir 2001).

Surface electromyography (EMG)
Surface EMG data were recorded from the vastus lateralis (VL), vastus medialis (VM), and rectus femoris (RF) muscles of the right limb using an eightchannel EMG system (MyosystemBr1, Data Hominis Tecnologia Ltda).After preparation of the skin (shaving, lightly abrading, and cleansing with 70% ethanol), bipolar active surface electrodes were placed over each muscle.This protocol followed the guidelines of Surface Electromyography for the Non-Invasive Assessment of Muscles (SENIAM http://www.seniam.org)for electrode placement and orientation.The reference electrode (Bio-Logic Systems Corp® -made of stainless steel, with circular format and 30 mm diameter) was placed on the clavicula of the individuals.
The same investigator placed the electrodes in the same relative position on all participants during the experiment.The surface EMG signal was acquired at a rate of 1000 Hz, pre amplified (20x) and synchronized with the force signal using the same analog-to-digital converter.The data were full-wave rectified and bandpass-filtered between 20 and 500 Hz using a fourth-order Butterworth digital filter.The amplitude of the signal was calculated as the root mean square (EMG RMS ) of the concentric phase of each repetition.A synchronized linear potentiometer (Magneto Pot, Spectra Symbol, Salt Lake City, UT, USA) was attached to the equipment and allowed the separation of the concentric and eccentric phases of the signal.The quadriceps femoris EMG RMS signals were considered the averaged EMG RMS values of the three muscles [(VL+VM+RF)/3].To minimize inter subject changes, the EMG RMS values were normalized to the dynamic warm-up performed with 50%1-RM (Burden 2010).

Muscular activation analysis
The EMG RMS values of each repetition were distributed equally on a 0-100 scale, which represented the % of repetitions to FAIL.The first repetition was always equal to zero, and the last was equal to 100.This procedure allowed the load and number of repetitions to be relativized according to individual values (% of 1RM and % of repetitions to fail).In this way, we evaluated the individuals under comparable, experimental conditions, which allowed the grouping of individual results and the comparison of their neuromuscular responses.

Maximal isometric voluntary contraction (MVC)
The a digital fourth-order Butterworth filter using a cutoff frequency of 15 Hz.Before the MVC tests, the subjects performed a set with a fixed load of 40 kg followed by one specific warm-up consisting of 10 isometric voluntary contractions of ~1s separated by ~1s of rest.Throughout the ten isometric contractions, the subjects were instructed to gradually increase their force.The testing protocol consisted of three MVCs of ~3s with 2 min of rest.The relative knee angle was set at 70° (0° for full extension), and during all MVCs, the subjects were instructed to exert force as fast as possible and then relax.We provided real-time biofeedback of the MVC force on a computer monitor and vigorously encouraged the participants to exceed the achieved peak force value.The maximum value of the three attempts was considered for further analysis.

Blood lactate
Approximately 25μL of blood was collected from the fingertip by using a digital puncture and disposable lancets (Roche®) before training, as well as 8-and 30-min post training.Lactate concentrations were measured using the portable lactate analyzer Accutrend® (Roche® Diagnostics, Basel, Switzerland, also marketed as Accusport® by Boehringer Mannheim, Mannheim, Germany).

Statistical analysis
We created specific routines in MATLAB 2019a (The MathWorks, Inc, Massachusetts, USA) for all the analyses.Data normality was verified using the  load's reduction effects on neuromuscular activation decreasing significantly from 136.8 ± 4.8 at 80%1-RM to 119.3 ± 5.5 and 110.6 ± 5.5 at 60 and 40%1-RM, respectively.The slopes significantly differed from zero, indicating an increased rate of muscle activation during repetitions, observed at 80 and 60%1-RM loads only (see Table 1).This first set configuration allowed an average of ~30 repetitions.

Muscle activation in the whole session
Table 1 presents the intercepts and slopes of the EMG RMS linear regression analysis.
Curiously, the addition of the second and third sets led to higher EMG RMS intercept values at 80 and 60%1-RM loads than those found in the first one.The EMG RMS intercept at 40%1-RM remained significantly lower than those at 80 and 60% 1-RM in all sets.EMG RMS slopes were not significantly different from zero in the second and third sets at all loads except for the 40%1-RM load in the third set.
Table 2 shows the number of repetitions and the volume load of the session.
Only one volunteer was unable to perform a complete repetition with 80%1-RM in the second and third sets; another was unable to perform a complete repetition with the same load in the third set.The total volume load of the second and third sets decreased significantly by ~45 and 53%, respectively.However, the inclusion of 2 more sets doubled the volume load performed in the first.

DISCUSSION
The present study investigated neuromuscular activation of trained individuals during a dropset session performed to FAIL.To assess the behaviour of muscular activation during the experimental protocol, we developed two innovative methodological procedures: a) applying a linear regression model to the EMG RMS values extracted from repetitions and b) using the first set as the control.The linear regression models to EMG RMS values allowed us to observe the initial muscle activity (intercept) and its behaviour throughout the repetitions (slope).The relativization of the repetitions  As expected, the load reductions in the first set decreased the initial muscle activation by 12.8% and 7.3% at 60 and 40%1-RM loads, respectively (Table 1 and Fig. 2).The slopes significantly different from zero may indicate an increased action potential firing rate and neural drive for the active muscles at each load to sustain the repetitions to FAIL (Gandevia 2001).The high number of movements and volume loads observed at all loads reinforces this interpretation.
Worthy of attention was the change in the activation pattern observed in the second and third sets.The EMG RMS responses remained high during all repetitions performed at 80 and 60% 1-RM loads without changes in the activation rate (slope ~0; see Table 1).In contrast, the EMG responses during the 40%1-RM load were significantly lower in all sets, suggesting a possible load threshold for a significant increase in the initial muscle activity during repetitions to FAIL.Sustained, low-load dynamic activities may be important to induce substitution and rotation between motor units (Bawa and Murnaghan 2009).Although this condition has only been confirmed by spectroscopy and phosphorus-31 magnetic resonance analysis (Dankel, Mattocks et al. 2017), we may speculate that the 40%1-RM load was essential for maintaining higher activation at 80 and 60%1-RM loads of the subsequent sets.In addition, lower loads may also increase the hypertrophic response in type I fibers (Fry 2004).
In summary, we show herein that our drop-set session induced a higher time under tension and increased muscle activation; these conditions considered essential for optimizing muscle strength and hypertrophic responses in trained individuals (Schoenfeld 2013, Dankel, Mattocks et al. 2017).Despite a possible study limitation in using a multiarticular exercise and not monitoring the activation of all muscle groups participating in the movements, we have shown herein that the neuromuscular activation responses were dependent on the magnitude of the loads and the addition of more sets.This knowledge may support coaches and athletes interested in optimizing strength training responses incorporating multiple drop-set sessions into their periodization.
Twelve trained men (age: 23 ± 3 years; body mass: 78.4 ± 10.4 kg; height: 1.74 ± 0.06 m; % of body fat: 11.4 ± 5.0) with 3-9 years of resistancetraining experience (1-RM on 45° leg press: 489 ± 111 kg; relative strength [1-RM/body mass] = 6.3 ± 1.1 MVC test was performed before and 30 min after training in the same 45° leg press machine used for the training session and the 1-RM test.Peak force was measured by a load cell (Reaccion®, CZCB-500) connected to the equipment calibrated before each subject's session.We acquired the force signal by the auxiliary channel of the electromyography system.During the late offline analysis, the load cell signal was converted to Newtons and smoothed by International Journal of Strength and Conditioning.2023 Acute Responses of A Drop-Set Session on Muscular Activation and its Applicability in Resistance-Trained Individuals

Figure 2 Figure 2 .
Figure 2 presents the adjusted lines of the EMG RMS behavior in the first set and repetitions performed in each load.The intercept behavior (% of warm-up) indicates the

1 -
RM = one repetition maximum test; *Significant difference from the first set.# Significant difference (p<0.05) to 80%1-RM of the same set; †Significant difference (p<0.05) from the total of the first set; Data expressed as the mean ± SD.International Journal of Strength and Conditioning.2023 Acute Responses of A Drop-Set Session on Muscular Activation and its Applicability in Resistance-Trained Individuals 6 Copyright: © 2023 by the authors.Licensee IUSCA, London, UK.This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/4.0/).allowed us to monitor the longitudinal neuromuscular activation throughout the nine phases of the session.The experimental approach using the first set as the control efficiently overcame the limitations imposed in studies that compare the drop-sets with traditional resistance training protocols equalizing their volume loads (Kassiano, de Vasconcelos Costa et al. 2021).

Acute Responses of A Drop-Set Session on Muscular Activation and its Applicability in Resistance-Trained Individuals International Journal of Strength and Conditioning. 2023 Ide
. Two min of rest intervals interspersed the sets, and ~10s interspersed the load reductions.Figure1illustrates the experimental design of the testing protocol.

Table 1 .
Comparisons of intercepts and slopes throughout the sets and intensities.

Table 2 .
Repetitions and volume load (repetitions x load) were performed throughout the sets.