Progressive Resistance Training Volume: Effects on Muscle Thickness, Mass, and Strength Adaptations in Resistance-Trained Individuals : The Journal of Strength & Conditioning Research

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Progressive Resistance Training Volume: Effects on Muscle Thickness, Mass, and Strength Adaptations in Resistance-Trained Individuals

Aube, Daniel1; Wadhi, Tanuj1; Rauch, Jacob1; Anand, Ashmeet1; Barakat, Christopher1; Pearson, Jeremy1; Bradshaw, Joshua1; Zazzo, Spencer1; Ugrinowitsch, Carlos2; De Souza, Eduardo O.1

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Journal of Strength and Conditioning Research 36(3):p 600-607, March 2022. | DOI: 10.1519/JSC.0000000000003524
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Aube, D, Wadhi, T, Rauch, J, Anand, A, Barakat, C, Pearson, J, Bradshaw, J, Zazzo, S, Ugrinowitsch, C, and De Souza, EO. Progressive resistance training volume: effects on muscle thickness, mass, and strength adaptations in resistance-trained individuals. J Strength Cond Res 36(3): 600–607, 2022—This study investigated the effects of 12-SET, 18-SET, and 24-SET lower-body weekly sets on muscle strength and mass accretion. Thirty-five resistance-trained individuals (one repetition maximum [1RM] squat: body mass ratio [1RM: BM] = 2.09) were randomly divided into 12-SET: n = 13, 18-SET: n = 12, and 24-SET: n = 10. Subjects underwent an 8-week resistance-training (RT) program consisting of 2 weekly sessions. Muscle strength (1RM), repetitions to failure (RTF) at 70% of 1RM, anterior thigh muscle thickness (MT), at the medial MT (MMT) and distal MT (DMT) points, as well as the sum of both sites (ΣMT), along with region of interest for fat-free mass (ROI-FFM) were measured at baseline and post-testing. For the 1RM, there was a main time effect (p ≤ 0.0001). However, there was a strong trend toward significance (p = 0.052) for group-by-time interaction, suggesting that 18-SET increased 1RM back squat to a greater extent compared with 24-SET (24-SET: 9.5 kg, 5.4%; 18-SET: 25.5 kg, 16.2%; 12-SET: 18.3 kg, 11.3%). For RTF, only a main time-effect (p ≤ 0.0003) was observed (24-SET: 5.7 reps, 33.1%; 18-SET: 2.4 reps, 14.5%; 12-SET: 5.0 reps, 34.8%). For the MMT, DMT, ΣMT, and ROI-FFM, there was only main time-effect (p ≤ 0.0001) (MMT: 24-SET: 0.15 cm, 2.7%; 18-SET: 0.32 cm, 5.7%; 12-SET: 0.38 cm, 6.4%—DMT: 24-SET: 0.39 cm, 13.1%; 18-SET: 0.28 cm, 8.9%; 12-SET: 0.34 cm, 9.7%—ΣMT: 24-SET: 0.54 cm, 6.1%; 18-SET: 0.60 cm, 6.7%; 12-SET: 0.72 cm, 7.7%, and ROI-FFM: 24-SET: 0.70 kg, 2.6%; 18-SET: 1.09 kg, 4.2%; 12-SET: 1.20 kg, 4.6%, respectively). Although all of the groups increased maximum strength, our results suggest that the middle dose range may optimize the gains in back squat 1RM. Our findings also support that differences in weekly set number did not impact in MT and ROI-FFM adaptations in subjects who can squat more than twice their body mass.


It is well established that resistance training (RT) is an effective modality to increase muscle mass (i.e., hypertrophy), decrease fat mass, and improve overall health in a large spectrum of populations (4,6,11). In an effort to maximize such training outcomes, researchers have investigated the effects of the manipulation of several training variables (e.g., intensity, repetition tempo, exercise selection, rest interval, etc.) on RT-induced adaptations (5,8,15,19,25). Among these variables, training volume has received considerable attention from the scientific community (2,7,10,12,18,21–24). Recent studies have referred to training volume as the number of sets performed per muscle group per week (2,10,22,23).

Although a dose-response relationship between training volume and maximum strength adaptations has been reported in untrained individuals (17), such a relationship has not been clearly established for trained ones (2,14,22). A corollary hypothesis would be that a similar dose-response relationship exists for strength endurance as well; however, there is paucity of data on this topic for resistance-trained individuals.

In regards to muscle hypertrophy, some meta-analyses have suggested a dose-response relationship between weekly sets and muscle mass accrual, mostly for untrained and physically active individuals (12,23). Regarding resistance-trained cohorts, empirical tests of this dose-response relationship have also produced equivocal results. Although a couple of studies reported that performing a high number of sets per muscle group per week was not more beneficial for increasing the muscle hypertrophic response compared with low weekly sets (range: 3–20 sets) (2,16), Schoenfeld et al. (22) showed otherwise. Performing 45 weekly sets produced greater increases in mid-thigh muscle thickness (MT) compared with 9 weekly sets, but there were no differences between 27 and 45 weekly sets. Thus, further scrutiny is required to address this issue.

One should also consider that increasing training volumes would impose relatively proportional physical and emotional stress on individuals. Evidence suggests positive linear relationship between training load and physical and emotional stress (26,27). Thus, the lack of agreement between the results of training volume studies may have been due to the fact that the high training volume groups were in the descending phase of the inverted “U” relationship because of greater accumulation of bodily and emotional stress. Therefore, monitoring the rate of perceived exertion and pleasure, while training with high volume, might help to elucidate whether the aforementioned equivocal results could be partially explained by individuals abilities to tolerate load.

Furthermore, to the best of our knowledge, previous studies have not controlled for the effect that the number of sets being performed before the commencement of the experimental period has on trained subjects (i.e., completely randomized designs), an important confounding factor. Thus, training-induced adaptation may have occurred because of sudden and random changes in training volume, instead of an actual dose-response relationship between training volume and the dependent variables. To account for this, we considered the number of weekly sets highly resistance-trained males was performing before the beginning of experimental period. Therefore, this study investigated the effects of 3 different training volumes regimens (12 [12-SET], 18 [18-SET], and 24 [24-SET] weekly sets) on lower-body muscular strength and hypertrophy, local fat-free mass adaptations, rate of perceived exertion, and training-associated pleasure. We hypothesized that increases in training volume would affect local fat-free mass accretion and muscle size in a similar fashion among volume groups. In addition, we hypothesized no differences between groups for muscular strength adaptations. Our primary dependent variable was muscle size; in case of not confirming our hypothesis, an exploratory analysis would be implemented to identify potential confounding factors affecting the association between training volume and changes in muscle size, taking into consideration the magnitude of the training-induced adaptations.


Experimental Approach to the Problem

This was a balanced (using previous RT volume) and randomized, parallel-group repeated-measures design, which investigated the effects of 3 different weekly RT volumes (12-SET, 18-SET, and 24-SET) on maximum strength (one repetition maximum [1RM]), strength endurance via repetitions to failure (RTF) at 70% of 1RM, region of interest fat-free mass (ROI-FFM), and MT in resistance-trained individuals. In addition, subjective assessments associated with training-related perceived exertion and pleasure were performed. All experimental groups trained 2 times per week for 8 weeks. Volume load (VL) (i.e., sets × repetitions × load [kg]) was calculated to assure that not only sets per week but also total work performed would be different between groups. One repetition maximum, RTF, lean-ROI, and MT at 2 different sites were assessed at baseline (pre) and at least 48 hours after the last training session. Rate of perceived exertion (RPE) and feeling scale (FS) were collected at the end of every training session, and data from weeks 1–4 and 5–8 were used for further analysis.


Forty-four resistance-trained subjects (age range: 18 to 30 years) were recruited for the study. The inclusion criteria were as follows: being a man aged between of 18 and 35 years; at least 3 years of previous RT experience with the back squat and 1RM testing; and relative squat strength of at least 1.5 times their body mass. Exclusion criteria were as follows: current/past history of joint pain (e.g., tendonitis); history of drug or alcohol abuse; daily usage of NSAIDs, any anticoagulants, or antiplatelet drugs, high blood pressure; heart arrhythmias; or reported sensitivity to caffeine. Subjects were classified into terciles based on the total number of sets performed per week for quadriceps before the study commencement. Next, subjects from each tercile were randomly assigned to one of the 3 experimental groups. After baseline testing, 6 subjects (12-SET: 1; 18-SET: 1; 24-SET: 4) withdrew because of personal reasons not related to the current study, and 3 withdrew because of joint pain (lower back and/or knee) during the training intervention (12-SET: 2; 18-SET: 1; 24-SET: 0). Therefore, data from 35 subjects were included in the statistical analysis (Table 1). All subjects were informed of the inherent risks and benefits before signing a written informed consent form. The University of Tampa Institutional Review Board approved the current study. A CONSORT flow diagram of the study is presented in Figure 1.

Table 1 - Subjects' characteristics (mean ± SD).*
Variable 12-SET 18-SET 24-SET
N 13 12 10
Age (yrs) 21.5 ± 2.3 20.7 ± 2.4 23.5 ± 5.7
Body mass (kg) 79.3 ± 14.8 77.0 ± 9.4 79.8 ± 10.0
BF% 13.4 ± 4.6 12.4 ± 3.8 13.7 ± 4.5
ROI-FFM (kg) 26.2 ± 4.7 25.7 ± 2.2 26.0 ± 3.3
ΣMT (cm) 9.3 ± 1.9 8.8 ± 1.8 8.9 ± 1.4
PSN (sets·wk−1) 13.5 ± 9.0 13.8 ± 7.0 12.1 ± 8.5
SETDIFF (sets·wk−1) −1.5 ± 8.7 4.1 ± 6.7 11.9 ± 8.0
1RM squat: BM (a.u) 2.03 ± 0.22 2.04 ± 0.23 2.21 ± 0.29
*BF% = body fat percentage; ROI-FFM = region of interest fat-free mass; ΣMT = sum of mid-thigh and distal muscle thickness; PSN = previous set number performed per week for lower body; SETDIFF = PSN—number sets assigned to experimental group; 1RM:BM = squat one repetition maximum:body mass ratio.

Figure 1.:
CONSORT diagram.


Muscular Strength Assessments

All subjects completed one familiarization session followed by the 1RM and strength endurance testing session interspersed by a minimum of 48 hours before the commencement of the study. During the familiarization session, subjects performed a general warm-up consisting of 3 minutes at ∼5.0 km·h−1 on a treadmill (Tuff Tread; White Phoenix, LLC., Willis, TX). After warming up, subjects were given a thorough explanation of the testing procedures including the 1RM squat and repetitions-to-failure at 70% 1RM (RTF). Subjects performed maximum strength testing on the back squat at a depth of 100° of knee flexion. Knee flexion was measured with a goniometer measuring the angle between the femur and the fibula. Briefly, the stationary arm of the goniometer was placed parallel to the long axis of the femur along a line extending from the greater trochanter to the lateral condyle, and the moveable arm was placed parallel to the long axis of the fibula in line with the head of the fibula and the lateral malleolus. Squat depth was constrained up to 100° knee flexion using an adjustable seat. Subjects were asked to “tap and go” to validate each repetition. After a general warm-up, the subjects performed 2 warm-up sets on the back squat using loads of 50% for 8 repetitions and 70% for 3 repetitions of the estimated 1RM load. Subjects then had up to 5 attempts to achieve their 1RM, of which the first attempt was standardized to 90% of their estimated 1RM. A linear position transducer (GymAware; Kinetic Performance Technology, Canberra, Australia) was used to monitor the intensity of each attempt based on the bar velocity (9). One repetition maximum load was determined when the subject reached volitional failure or if the barbell speed dropped below 0.28 m·s−1. If the subject hits volitional failure, the load from the previous set was considered their 1RM load. Testing took place on 2 separate days during pre-testing to ensure the subjects were familiarized with the squat depth and the highest value was used for the statistical analysis. Subjects were considered acquainted to 1RM testing if the coefficient variation (CV) between the familiarization session and 1RM testing was lower than 5%.

Strength Endurance

Upon completion of maximum strength testing, subjects were given 5 minutes of rest, after which they were asked to perform repetitions-to-failure at 70% of their 1RM load, where they had to “trap and go” the adjustable seat for the repetition to be valid. Failure was reached when subjects could not perform the concentric phase of the lifting, if the speed fell drastically below 0.28 m·s−1 or if the subjects took more than 2 seconds between repetitions.

Muscle Mass and Thickness Assessments: Muscle Mass

A Lunar Prodigy dual-energy X-ray absorptiometry (DEXA) apparatus (Hologic, Bedford, MA) was used to measure body composition. Furthermore, the lower body was subdivided to measure the fat-free mass of a region of interest (ROI-FFM) marked from the iliac crest to the lateral condyle for each scan at pre-testing and post-testing (Figure 2C). To measure lower-body composition more accurately, subjects were asked to cross their arms across their chest to avoid interference with the lower body. On the day of the scan, subjects were weighed on a mechanical scale and placed on the DEXA machine laying in a supine position with knees extended and were instructed not to move for the entire duration of the scan. To increase test-retest consistency, subjects' positions on the machine bed were recorded. In addition, subjects were strictly instructed to come to the laboratory after refraining from food and water for at least 10 hours before the scan. Region of interest for fat-free mass measures were acquired at pre-testing and post-testing at the same time of the day. The CV was determined before the study using 5 different subjects with similar characteristics to the current subjects (CV = ≤1.5%).

Muscle Thickness

Anterior thigh MT was measured using a mode B ultrasound (LOGIQ e; GE Healthcare, Chicago, IL) using a linear probe (12L-RS, GE Healthcare) with frequencies between 8.0 and 12.0 Mhz. The femur length was measured in the sagittal plane from the greater trochanter to the lateral condyle. Marks were made at 50 and 75% of the femur length, both in the lateral and frontal aspects of the thigh. The overall thickness of the vastus intermedius and rectus femoris muscles was measured at the medial MT (MMT—50% of the femur length) and distal MT (DMT—75% of the femur length) points (Figures 2A, B). The sum of both sites (ΣMT) was also used for further statistical analysis. A water-soluble transmission gel (Aquasonic 100; Parker Laboratories, Inc, Fairfield, NJ) was applied to the probe, which was then placed on the skin with minimal pressure to avoid indentation of the skin. Zoom and frequency were adjusted until a clear picture of the femur and muscle fascia could be seen. The thickness was measured from the highest point on the femur to the bottom edge of the muscle fascia of the rectus femoris. Three measurements were taken at each point, and the median value was used for further analyses. All images were obtained on the right thigh. The MT assessments were performed at the same time of the day at pre-testing and post-testing. To increase test-retest reliability, each site was marked with henna ink and weekly remarked. In an attempt to minimize training-induced muscle swelling, images were obtained 48–72 hours after their last lower-body training session before the commencement of the study and 48–72 hours after the last training session at the end of the study. The same 2 researchers (i.e., probe handling and screen measurement) performed all of the pre-training and post-training measurements and were blinded to the experimental groups. The CV of the MT assessments was <2.0%.

Figure 2.:
Overview of medial muscle thickness (A), distal muscle thickness (B), and region of interest fat-free (C) assessments.

Perceptual Assessments

The RPE and pleasure-displeasure rate were assessed immediately after each training session. Rate of perceived exertion was measured based on the CR-10 RPE scale. Pleasure and displeasure were acquired utilizing the FS that uses a bipolar sorting from 11 points, varying by +5 to −5, with an anchor of zero (neutral) and all the odd whole numbers corresponding to description of “very good” (+5) to “very bad” (−5). Instructions and procedures for using the RPE and FS scales were given to all individuals during the familiarization sessions. In addition, all assessments were performed in isolation from other subjects to ensure accuracy. The perceptual assessments of the 2 different training blocks (i.e., first and second half) were averaged for further analysis.

Training Intervention

Subjects underwent an 8-week hypertrophy-oriented lower-limb RT program twice a week, totaling 16 sessions per subject. A minimum adherence of 90% was required. Experimental groups performed the same exercises at the same intensities. The only difference between groups was the total number of sets performed for the quadriceps muscle per week (12-SET, 18-SET, and 24-SET groups). Sets were equally divided between the back squat and leg-press exercises. In addition, all subjects performed the same set and repetition schemes for glute-ham–raise exercise (Table 2). The intensity of the lifts was set at 2 reps in reserve until the last set for each exercise. The last set was always performed up to concentric failure. If the subjects were unable to complete the required repetitions, the load was decreased for the next set. On the contrary, if the subjects were able to complete the prescribed repetitions with ease, the set was stopped at the higher end of the range and the load was increased for the next set. A 2-minute and 3-minute rest interval was allowed between sets and exercises, respectively. Subjects were asked to report their RPE for the average of all sets for each exercise, as soon as they completed the last set on each exercise. At the end of the training session, the subjects were asked to report a cumulative RPE for the entire session, along with FS, which measures the enjoyment (i.e., pleasure and displeasure) of the workout. All subjects were strictly instructed to refrain from performing any additional lower-body exercises outside of the prescribed training intervention. To help ensuring proper nutrition throughout the experimental period, subjects received a supplement on training days containing 24 g of protein and 1 g of carbohydrate (Iso-100 Hydrolyzed Whey Protein Isolate; Dymatize Nutrition, Dallas, TX) under the supervision of the research staff.

Table 2 - Training regimens throughout 8 weeks.
Exercise 12-SET 18-SET 24-SET
Weeks 1–4: first mesocycle
 Day 1
  Back squat 3 × 6-8 4 × 6-8 6 × 6-8
  Leg press 3 × 6-8 5 × 6-8 6 × 6-8
  Glute-ham raise 2 × 4 2 × 4 2 × 4
 Day 2
  Back squat 3 × 12-15 5 × 12-15 6 × 12-15
  Leg press 3 × 12-15 4 × 12-15 6 × 12-15
  Glute-ham raise 2 × 4 2 × 4 2 × 4
Weeks 5–8: second mesocycle
 Day 1
  Back squat 3 × 6-8 5 × 6-8 6 × 6-8
  Leg press 3 × 6-8 4 × 6-8 6 × 6-8
  Glute-ham raise 2 × 6 2 × 6 2 × 6
 Day 2
  Back squat 3 × 12-15 4 × 12-15 6 × 12-15
  Leg press 3 × 12-15 5 × 12-15 6 × 12-15
  Glute-ham raise 2 × 6 2 × 6 2 × 6

Statistical Analyses

After normality (i.e., Shapiro-Wilk) and variance assurance (i.e., Levene), one-way analysis of variances were implemented to test for between-group differences for all the dependent variables at baseline. Because there were no significant differences between groups for all variables (p > 0.05), a mixed-model analysis was performed for each dependent variable (1RM, RTF, MT, and perceptual assessments) assuming group (12-SET, 18-SET, and 24-SET) and time (pre and post) as fixed factors and subjects as a random factor. For the perceptual assessments, levels for time were training blocks one and 2 (i.e., average of first and second 4-week period). In addition, VL was analyzed assuming only groups (i.e., 12-SET, 18-SET, and 24-SET) as the fixed factor and subjects the as random factor (SAS 9.4; SAS Institute, Inc., Cary, NC). Whenever a significant F-value was obtained, a post hoc test with a Tukey adjustment was performed for multiple comparison purposes. In addition, 95% confidence intervals of the within-group absolute difference (95% CI) are presented. Within-group effect sizes (ES) were calculated as follows: mean post-minus mean pre-divided by the pooled SD of pretest values. Afterward, an exploratory analysis of the individual responses was performed. The subjects were divided into terciles based on their absolute changes in ΣMT. Subsequently, the 3 clusters were labeled as high responders (third-Tercile), moderate responders (second-Tercile), and low responders (first-Tercile). Multiple analyses of covariance (ANCOVAs) were performed on the absolute pre-to-post changes with ΣMT (i.e., Δ-ΣMT) as the dependent variable and terciles (i.e., first, second and third) as a fixed factor. Different covariates (i.e., SETDIFF, previous weekly sets number [PSN], VL, and delta volume between weeks 1 and 8 [Δ-VL]) were tested to identify likely variables that may have affected Δ-ΣMT responses across different terciles. In case of significant F values, a Tukey adjustment was implemented for multiple comparison purposes. The significance level was previously set at p ≤ 0.05. Results are expressed as mean ± SD.


Volume Load and Perceptual Assessments

The 24-SET produced the greatest VL (95% CI: 333,438–430,888 kg) compared with 18-SET (95% CI: 238,718–329,255 kg) and 12-SET (157,853–203,939 kg). For RPE and FS, there were no significant differences between groups and time (p ≥ 0.05). Mean data are presented in Table 3.

Table 3 - Rate of perceived exertion and feeling scale assessments (mean ± SD).*
Variable Group Pre Post ES 95% CI
RPE (a.u) 12-SET 7.16 ± 1.56 7.37 ± 1.07 0.19 −0.22 to 0.64
18-SET 7.55 ± 0.50 7.50 ± 0.78 −0.04 −0.51 to 0.39
24-SET 7.64 ± 0.63 7.90 ± 0.62 0.24 −0.23 to 0.39
FS (a.u) 12-SET 3.17 ± 0.47 2.57 ± 1.31 −0.53 −1.14 to −0.05
18-SET 2.89 ± 1.35 2.83 ± 0.87 −0.05 −0.62 to 0.50
24-SET 3.83 ± 1.38 3.66 ± 1.55 −0.15 −0.79 to 0.45
*RPE = rate of perceived exertion; FS = feeling scale; ES = effect sizes; 95%-CI = 95% confidence intervals.

Muscular Strength Assessments

There was a main time effect (p < 0.0001) for the back squat 1RM load, indicating that all groups increased maximum strength similarly across time. However, there was a strong trend toward a significant (p = 0.052) group by time interaction, suggesting that 12-SET and 18-SET increased 1RM back squat to greater extent compared with 24-SET (estimated differences: 24-SET: 9.5 kg, 5.4%; 18-SET: 25.5 kg, 16.2%; 12-SET: 18.3 kg, 11.3%). For RTF, there was a main time effect (p = 0.0003) indicating that all groups increased strength endurance similarly across time (estimated differences: 24-SET: 5.7 reps, 33.1%; 18-SET: 2.4 reps, 14.5%; 12-SET: 5.0 reps, 34.8%). Although we had just a time effect for RTF, 95% CI of the 18-SET suggests no changes from pre- to post-training (i.e., 95% CI crosses zero: −1.27 to 6.11). Mean data are presented in Table 4, and individual responses for pre-to-post changes in 1RM back squat are presented in Figure 3A.

Figure 3.:
Individual responses for pre–to-post changes in 1RM squat (A), sum of muscle thickness (B), and region of interest for fat-free mass (C). 1RM = one repetition maximum.
Table 4 - Maximum strength and strength endurance assessments (mean ± SD).*
Variable Group Pre Post ES 95%-CI
1RM squat (kg) 12-SET 161.9 ± 37.4 180.2 ± 35.5 0.58 10.03 to 26.59
18-SET 157.4 ± 28.2 182.9 ± 23.1 0.81 16.89 to 34.13
24-SET 175.7 ± 25.7 185.2 ± 24.2 0.30 0.07 to 18.96
RTF 12-SET 14.6 ± 5.7 19.6 ± 4.0 0.80 1.45 to 8.54
18-SET 16.4 ± 7.9 18.8 ± 3.4 0.39 −1.27 to 6.11
24-SET 17.2 ± 4.2 22.9 ± 5.9 0.91 1.65 to 9.7
*ES = effect sizes; 95%-CI = 95% confidence intervals, 1RM = one repetition maximum; RTF = repetitions to failure at 70% of 1RM.

Muscle Mass and Thickness

For the ROI-FFM, there was a main time effect (p < 0.0001) indicating that all groups increased local FFM similarly across time (estimated differences: 24-SET: 0.70 kg, 2.6%; 18-SET: 1.09 kg, 4.2%; 12-SET: 1.20 kg, 4.6%) (Table 5). Similarly, in regard to MT, there was a significant main time effect (p < 0.0001) for MMT, DMT, and ΣMT indicating that all groups increases MT similarly across time on both sites (estimated differences: MMT: 24-SET: 0.15 cm, 2.7%; 18-SET: 0.32 cm, 5.7%; 12-SET: 0.38 cm, 6.4%—DMT: 24-SET: 0.39 cm, 13.1%; 18-SET: 0.28 cm, 8.9%; 12-SET: 0.34 cm, 9.7%—ΣMT: 24-SET: 0.54 cm, 6.1%; 18-SET: 0.60 cm, 6.7%; 12-SET: 0.72 cm, 7.7%, respectively) (Table 5). Individual responses from pre-to-post changes in ROI-FFM and MT are presented on Figure 3B, C, respectively.

Table 5 - Region of Interest for fat-free mass and muscle thickness assessments (mean ± SD).*
Variable Group Pre Post ES 95%-CI
ROI-FFM (kg) 12-SET 26.2 ± 4.7 27.4 ± 4.7 0.33 0.63 to 1.79
18-SET 25.7 ± 2.2 26.8 ± 2.2 0.31 0.48 to 1.69
24-SET 26.0 ± 3.3 26.7 ± 3.1 0.19 0.03 to 1.36
MMT (cm) 12-SET 5.8 ± 1.0 6.2 ± 1.0 0.41 0.16 to 0.60
18-SET 5.7 ± 0.9 6.0 ± 0.7 0.34 0.09 to 0.55
24-SET 5.9 ± 0.7 6.0 ± 0.7 0.16 −0.09 to 0.40
DMT (cm) 12-SET 3.4 ± 0.9 3.8 ± 1.0 0.38 0.12 to 0.55
18-SET 3.1 ± 0.9 3.4 ± 0.9 0.31 0.05 to 0.50
24-SET 3.0 ± 0.7 3.4 ± 0.4 0.44 0.14 to 0.64
ΣMT (cm) 12-SET 9.3 ± 1.9 10.0 ± 2.0 0.43 0.33 to 1.0
18-SET 8.8 ± 1.8 9.4 ± 1.6 0.34 0.21 to 0.99
24-SET 8.9 ± 1.4 9.5 ± 1.1 0.31 0.12 to 0.97
*ES = effect sizes; 95%-CI = 95% confidence intervals, 1RM = one repetition maximum; ROI-FFM = region of interest fat-free mass; MMT = middle muscle thickness, DMT = distal muscle thickness; ΣMT = sum of muscle thickness.

Exploratory Analysis

The summary of the exploratory analysis is presented on Table 6. Analysis of covariance revealed significant differences (p < 0.0001) between the terciles for the Δ-ΣMT in which the third tercile had the greatest improvement in MT compared with the second and first ones. In addition, the third tercile had greater increases than the first tercile in ROI-FFM (p = 0.01); however, there were no significant differences (p = 0.36) between the third and second terciles. In addition, ANCOVA analyses did not reveal any significant covariate effect (p > 0.05) for PSN, SETDIFF, VL, and/or Δ-VL (p > 0.05).

Table 6 - Exploratory analysis of the subjects' responses having Δ-ΣMT as the criterion variable.*
Variable First-tercile Second-tercile Third-tercile
Group/n 12-SET/5; 18-SET/4; 24-SET/3 12-SET/4; 18-SET/4; 24-SET/3 12-SET/4; 18-SET/4; 24-SET/4
Δ-ΣMT (cm) 0.0 ± 0.2 0.5 ± 0.1 1.3 ± 0.5
ROI-FFM (kg) 0.3 ± 0.9 1.1 ± 0.6 1.6 ± 1.0
PSN (sets·wk−1) 15.1 ± 7.0 13.0 ± 8.1 11.4 ± 9.0
SETDIFF (sets·wk−1) 1.8 ± 6.7 4.3 ± 9.3 6.6 ± 12.4
Volume load (kg) 294,784 ± 102,774 260,824 ± 86,454 265,502 ± 87,611
Δ-Volume load (8 wk−1 wk) (kg) 11,426.7 ± 9,407.3 12,626.9 ± 6,780.4 12,521.9 ± 5,892.1
*ΣMT = sum of mid-thigh and distal muscle thickness; ROI-FFM = region of interest fat-free mass; PSN = previous set number performed per week for lower body; SETDIFF = PSN—number sets assigned to experimental group.
Indicates p ≤ 0.05 when compared with third tercile.


The purpose of this study was to examine the effects of 3 different RT volumes (12-SET, 18-SET, and 24-SET) on lower-body muscle accretion and strength in highly resistance-trained males. We have not confirmed our initial hypothesis as our main findings demonstrated similar increases in muscle mass and thickness across the experimental groups. There was also a strong trend toward the 18-SET group being more effective to increase maximum strength than the 24-SET group.

Recently, the effects of training volume on RT-induced adaptations have been a topic of a heated debate. In regard to maximum strength adaptations, the average strength gain across groups in our study was ∼10.9% (∼17.8 kg). Interestingly, despite of important differences on training statuses across studies, we have observed almost identical findings compared with Schoenfeld et al. (22) and Marshall et al. (14). However, contrary to our hypothesis, 95%-CI and ES suggest that 18-SET had higher strength gains compared with 12-SET and 24-SET (i.e., 24-SET: 9.5 kg, ES: 0.30; 18-SET: 25.5 kg, ES: 0.81, and 12-SET: 18.3 kg, ES: 0.58). Nonetheless, our findings suggest that training-volume and RT-induced adaption relationship seems to follow an inverted “U” pattern, as 12-SET and 24-SET likely produced suboptimal adaptations. Finally, an important limitation on the understanding of training volume–induced maximum strength adaptations is the fact that most of studies had muscle growth as the primary dependent variable, and therefore, the current study and former ones have used training repetitions ranges above 5RM (1,14,16,20,22), as opposed to heavier intensities traditionally used when resistance-trained individuals are training to improve maximum strength performance.

In regard to muscle endurance, although the time effect might suggest that all experimental groups increased repetitions-to-failure at 70% of 1RM in a similar fashion, our ES and 95%-CI suggested otherwise. Interestingly, the 18-SET group demonstrated the greatest gains in 1RM adaptations, but it exhibited the lowest development in muscle endurance, or even a lack of increase in RTF as the 95%-CI of the pre-to-post difference crossed zero (24-SET: ES: 0.91, 95%-CI: 1.65–9.7 reps; 18-SET: ES: 0.39, 95%-CI: −1.27 to 6.11 reps, 12-SET: ES: 0.80, 95%-CI: 1.45–8.54). This specific finding is very intriguing, and we do not have a plausible explanation for these strength endurance adaptations as they seem to follow a regular “U” pattern instead of an inverted “U,” as observed for maximum strength. To date, only Schoenfeld et al. (22) have investigated the dose-response effects of training volume on muscular endurance in resistance-trained individuals. They found similar improvements in strength endurance on bench-press exercise across groups that performed 6, 18, and 30 weekly sets. In addition, they reported a similar magnitude of improvement compared with the current study (i.e., ∼4.0 repetitions vs. ∼4.2 repetitions, respectively). Despite similarities observed in the training outcomes, Schoenfeld et al. (22) had different exercise/muscle group assessments, training intensity, and training background of the subjects. Yet, Schoenfeld et al. (22) and the current study suggest that the repetitions zone (i.e., specificity) might be more important than the utilized training volume to optimize the gains in strength endurance. Nevertheless, this warrants further investigation in resistance-trained individuals.

Regarding muscle hypertrophy, we did not find a dose-response relationship between weekly training volume, VL, and muscle mass accretion as assessed by 2 distinct methods (i.e., MT and region of interest fat-free mass). Despite the important differences in weekly sets and VL, the average increases across our experimental groups were similar for MMT, DMT, and ROI-FFM (i.e., ∼4.9, ∼10.5 and ∼3.8%, respectively). Recently, other studies have also investigated the effects of distinct RT volumes on muscle mass accretion in resistance-trained individuals, and results are equivocal. For example, Barbalho et al. (2) found no differences between groups on mid-thigh MT in individuals performing 5, 10, 15, or 20 weekly sets for 24 weeks, which corroborates with previous shorter-duration studies that investigated the effects of different training volumes on muscle hypertrophic adaptations (10,16). However, it is important to note that Barbalho et al. (2) had their subjects performing all the sets in 1 weekly session compared with 2 or 3 weekly sessions in the current study and other former studies (10,14,16,22). On the other hand, Schoenfeld et al. (22) reported that 45 weekly sets increased mid-thigh thickness to a greater extent compared with 9 weekly sets, with no differences between 45 and 27 weekly sets after an 8-week training regimen. The average gains in MT between Schoenfeld et al. (22) and our study for the mid-thigh assessment were 0.42 and 0.26 cm, respectively. In addition, the high-volume group in the Schoenfeld et al. study demonstrated an increase in MT of approximately 0.66 cm, which is ∼60% more than we observed in our highest response group (i.e., 12-SET: 0.41 cm). Therefore, the findings from Schoenfeld et al. (22) suggest that resistance-trained individuals need to perform high training volumes (e.g., ≥27 weekly sets) to enhance muscle hypertrophic adaptations. This discrepancy may be due to the differences in training status between Schoenfeld et al. and this current study (i.e., 1RM:BM about 1.31 vs. 2.09). However, the precise impact that training status has on RT-induced adaptations in response to volume progressions remains to be investigated.

It is difficult to reconcile the results on the effects of training volume on muscle hypertrophy in resistance-trained individuals for a few reasons. Although studies on the topic have important differences, majority of them did not provide information pertaining to the VL (1–3,10,16,22). Consequently, it is difficult to determine whether differences in weekly sets performed actually produced differences in work performed (i.e., tonnage). For example, in our study, a ∼194,000-kg difference in VL was not sufficient to enhance muscle hypertrophy adaptations when comparing the 24-SET and 12-SET groups, after 8 weeks of training. Another important confounding factor that has been disregarded by researchers is the weekly sets performed by subjects before commencement of the actual study (1–3,10,16,22). A completely randomized design is unlikely to balance training groups regarding training volume history, which may introduce an important bias to the results. Taking this information in consideration, one may suggest that observed outcomes might be associated with sudden changes in training volume rather than with a dose-response relationship between volume and the muscle mass accretion. In the current study, previous training history (i.e., weekly sets per muscle group) was balanced between experimental groups; on average, subjects were performing ∼13.2 sets per muscle group per week before the intervention period. As a consequence, our experimental groups started the protocol with very similar baseline volume (Table 1). Although our randomization procedure assured similar baseline volumes between groups, another important confounding variable was overlooked; some subjects reduced their volume when they were assigned to the lower volume training groups. For example, 8 of 13 subjects (i.e., 61.5%) in the 12-SET group reduced their weekly volume compared with what they were previously performing, 3 of 12 subjects reduced their weekly sets in the 18-SET group, and only 1 of 10 reduced his weekly sets in the 24-SET group. In fact, the 24-SET group almost doubled their weekly sets (+11.9 sets) (Table 1). Interestingly, doubling the weekly volume for the 24-SET group did not translate in augmented muscle growth.

As results across our groups were very similar in regard to muscle hypertrophy, we performed an exploratory analysis to identify possible factors that could explain differences between subjects (Table 6). For the criterion variable Δ-ΣMT, the higher responders demonstrated significant differences compared with moderate and lower responders (third tercile: 1.3 cm; second tercile: 0.5 cm; first tercile: 0.0 cm). Individual response analyses revealed that an important proportion of the subjects (34.2%, 12 subjects) did not responded to variations in training volume. Interestingly, this was similarly distributed across the groups (24-SET = 4 subjects; 18-SET = 4 subjects and 12-SET: 4 subjects). Despite impressive and significant differences between terciles, none of the tested covariates (i.e., SETDIFF, PSN, VL [VL, change in VL] influenced MT responses. It is noteworthy to mention that there were no differences between third and second quartiles for ROI-FFM. Intriguingly, the moderate- and high-responder clusters added more sets to their previous weekly set number than the lower responders (third tercile: 6.6 sets; second tercile: 4.4 sets; first tercile: 1.8 sets), although the ANCOVAs failed to demonstrate a statistically significant covariate effect.

Regarding perceived effort and pleasure assessments, our findings do not corroborate with previous acute research that demonstrated a relationship between training volume and internal load parameters (13). However, our data are in agreement with the previous literature that demonstrated trained individuals responded similarly in measures of internal load despite of important differences in work performed after 9 weeks of training (19). Therefore, our results from the rate of perceived exertion and pleasure analyses did not suggest a psychological/emotional inability to deal with the distinct training volumes and consequently to an overreaching condition throughout the experimental period. It seems logical to conclude that difference in RT-adaptations might be more associated with limitations in muscle activation capacity (maximum strength) and protein synthesis machinery (muscle hypertrophy) than overreaching due to high training volumes. Even when using high training volumes, RT-induced adaptations seem to be individualistic. However, our perceived effort and pleasure findings should be interpreted with a degree of caution because of our short-term study design.

Certainly, our study has limitations that need to be addressed; first, we tried to track the macronutrients and calories intake. However, poor adherence by our subjects did not provide a good picture of their nutritional status during the experimental period. Although the subjects were strictly instructed to maintain their normal diet and they received postworkout protein, the lack of diet control might be a confounding factor modulating muscle mass accretion. Second, it would be interesting to have longer training periods in such trained cohort to verify the effects of different RT volumes on muscular adaptations. Finally, and most important, although this study was the first to assure groups started with a very similar weekly sets background, approximately one third of our sample (i.e., 34.2%) reduced their weekly sets during the experimental groups. Therefore, we need to certify that experimental units increase work performed compared with their PSN to gain a better insight on the effect of RT volume on muscle hypertrophy.

In conclusion, although all of the groups increased maximum strength, our results suggest that approximately 18 weekly sets targeting the quadriceps muscle may optimize the gains in back squat 1RM in resistance-trained individuals. Our result also support that differences in work performed, measured by weekly number of sets per muscle group and VL, did not impact the gains in MT and region of interest fat-free mass in subjects who can squat more than twice their body mass. Although, our findings suggest the presence of a ceiling effect for such population that performing 12–24 weekly sets per muscle and VL ranging from 191,648 to 386,374 kg will produce similar muscle mass accretion outcomes in lower-body muscles. The effects of increasing weekly sets proportionally to what the subjects were previously performing would provide a better insight on the effects of varying training volumes on muscle hypertrophy.

Practical Applications

Although maximum strength significantly increased in all experimental groups, strength and conditioning professionals, as well as practitioners aiming at improving 1RM squat, should consider performing the middle dose range (e.g., 18 weekly sets) as we observed the largest effects. Concerning hypertrophic adaptations, when training lower-body muscles twice a week over an 8-week training block (i.e., 16 training sessions), they should be aware that more sets per muscle group per week might not translate in enhanced muscle growth for highly trained individuals (i.e., 1RM:BM > 2.0). Therefore, the prescription of RT volume needs to be individualized as the responses to a given volume may vary considerably across resistance-trained individuals. However, when more work is necessary to achieve a specific goal, individuals who squat twice their body mass can handle more weekly sets as well as more VL without increasing RPE and FS or performance and morphological adaptations during a short-term training regimen (i.e., 8 weeks).


The researchers thank Dymatize Nutrition for supplying both the preworkout and protein powder. In addition, there are no conflicts of interests involved in this manuscript. Finally, the results of this study do not constitute endorsement by the authors or the NSCA.


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volume load; muscle hypertrophy; maximum strength; strength endurance; perceptual assessments

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