Effect of Set-Structure on Upper-Body Muscular Hypertrophy and Performance in Recreationally-Trained Male and Female

Abstract

Davies, TB, Halaki, M, Orr, R, Mitchell, L, Helms, ER, Clarke, J, and Hackett, DA. Effect of set structure on upper-body muscular hypertrophy and performance in recreationally trained men and women. J Strength Cond Res 36(8): 2176–2185, 2022—This study explored the effect of volume-equated traditional-set and cluster-set structures on muscular hypertrophy and performance after high-load resistance training manipulating the bench press exercise. Twenty-one recreationally trained subjects (12 men and 9 women) performed a 3-week familiarization phase and were then randomized into one of two 8-week upper-body and lower-body split programs occurring over 3 and then progressing to 4 sessions per week. Subjects performed 4 sets of 5 repetitions at 85% one repetition maximum (1RM) using a traditional-set structure (TRAD, n = 10), which involved 5 minutes of interset rest only, or a cluster-set structure, which included 30-second inter-repetition rest and 3 minutes of interset rest (CLUS, n = 11). A 1RM bench press, repetitions to failure at 70% 1RM, regional muscle thickness, and dual-energy x-ray absorptiometry were used to estimate changes in muscular strength, local muscular endurance, regional muscular hypertrophy, and body composition, respectively. Velocity loss was assessed using a linear position transducer at the intervention midpoint. TRAD demonstrated a significantly greater velocity loss magnitude (g = 1.50) and muscle thickness of the proximal pectoralis major (g = −0.34) compared with CLUS. There were no significant differences between groups for the remaining outcomes, although a small effect size favoring TRAD was observed for the middle region of the pectoralis major (g = −0.25). It seems that the greater velocity losses during sets observed in traditional-set compared with cluster-set structures may promote superior muscular hypertrophy within specific regions of the pectoralis major in recreationally trained subjects.

Introduction

Resistance exercise is highly effective for improving muscular strength and size (i.e., muscular hypertrophy) (⁴²). The recommended resistance exercise prescription to develop muscular strength and hypertrophy spans a wide range of training volumes (i.e., 4–10 + sets per muscle group per week) (⁴⁷) and loading intensities (i.e., 40–100% one repetition maximum [1RM]) (⁴²). To further optimize the adaptive response, rest period length (¹⁷), exercise selection (⁵¹), and movement velocity (^7,19) should be considered when devising resistance exercise programs. The current consensus to develop muscular strength is to recommend using high-loading intensities (∼80% 1RM) (⁴²), whereas muscular hypertrophy seems to be equally developed across a range of intensities from 40 to 80% 1RM, providing a high level of effort is applied (³⁰). This has led to a recent area of inquiry when seeking to optimize muscular strength and hypertrophy development concerning the number of repetitions performed during a set relative to the RM (i.e., proximity to concentric muscular failure). This is due to motor unit recruitment being maximized when sets are performed at or close to concentric muscular failure (⁴⁵), which has been postulated to be a key driver of muscular hypertrophy and strength development (^8,44). However, there is evidence to suggest that performing repetitions at or close to concentric muscular failure may not be necessary to develop these physical qualities, rather ceasing a set multiple repetitions short of concentric muscular failure may be adequate (^10,16). However, the findings from these studies are highly variable (^27,38), as observed in recent reviews concerning muscular strength and hypertrophy (^5,48) and indicate that the current consensus is inconclusive with more research needed to build on current recommendations (⁴²).

Traditionally, resistance training is performed with repetitions that are completed continuously with no rest period within the set and a period of rest is then applied at the end of the set to facilitate recovery (⁵³). When sets are prescribed in this manner, they are known in the literature as traditional-set structures (⁵³). Traditional-set structures have been clearly demonstrated to accumulate large amounts of mechanical and metabolic fatigue during sets (^12,54). As a lifter performs repetitions and approaches concentric muscular failure and fatigue increases, there is a reduction in movement velocity (¹³), which subsequently increases the time under tension (³⁹). The degree of movement velocity loss during a set of resistance exercise is suggested to be indicative of neuromuscular fatigue (⁴⁶) and may also lead to specific muscular adaptations (e.g., muscular strength and hypertrophy). Moreover, the performance of repetitions at or close to concentric muscular failure results in large changes in the metabolic homeostasis (i.e., reduction in adenosine triphosphate concentration and increase in metabolite concentration) within working muscle fibers (¹⁵). These changes in the metabolic environment of the working muscle have been suggested to play an important role in muscular hypertrophy and strength development through indirectly increasing muscle activation and anabolic protein signaling (³⁵).

In contrast to traditional-set structures, cluster-set structures reduce the fatigue experienced during sets by including intraset rest periods in addition to traditional interset rest periods (^14,53,54). More specifically, cluster-set structures facilitate greater maintenance of velocity and power output (⁵⁴) while reducing metabolic stress (³⁹) and consequently, reduced neuromuscular fatigue for a given training volume (⁴⁶). The literature suggests that cluster-set structures are similarly effective compared with traditional-set structures in developing muscular strength and power output (⁵³). However, the evidence investigating muscular hypertrophy outcomes is inconclusive. After a 12-week intervention, Goto et al. (¹⁶) reported significantly greater changes in thigh midpoint muscle cross-sectional area (measured using magnetic resonance imaging) in the traditional-set structure (∼12% increase from baseline) compared with the cluster-set structure (4% from baseline). The authors attributed the difference to the larger metabolic and hormonal response (i.e., lactate and growth hormone) that was observed in the traditional-set structure. There have been only a couple of other studies investigating this topic (^27,38), but as to date, the findings are equivocal. However, it does seem that set structures that lead to greater fatigue and velocity loss during sets may be advantageous for the promotion of hypertrophy. This is supported by findings from Pareja-Blanco et al. (⁴⁰) where increased velocity loss (20 vs. 40% loss in velocity during sets) resulted in greater thigh muscle hypertrophy. However, Pareja-Blanco et al. (⁴⁰) did not control for training volume that may have influenced the results.

Moreover, it seems that the performance of repetitions with large velocity losses (i.e., at or close to concentric muscular failure) improves the ability to sustain force output over long periods (⁵²). This is supported by Izquierdo et al. (²⁵) who demonstrated greater changes in absolute local muscular endurance when repetitions were performed to concentric muscular failure compared with when repetitions were deliberately performed not to failure. Therefore, the experienced fatigue during sets to failure may have implications for the development of local muscular endurance; however, training studies are scarce, and more research is needed to confirm the above findings.

This study examined the effect of volume-equated traditional-set vs. cluster-set structures using the bench press on muscle thickness, body composition, muscular strength, and local muscular endurance after high-load resistance training (85% 1RM). It was hypothesized that the traditional-set group would show greater increases in muscle thickness of the prime mover muscles in the bench press (i.e., pectoralis major and triceps brachii) and greater improvements in local muscular endurance. Second, as muscular hypertrophy does not directly correlate with improvements in muscular strength (⁴⁰), we hypothesized that there will be no differences in muscular strength development between groups. Because of the disparities in gross vs. local changes in muscle size after training programs where single exercises are manipulated (^27,38), we hypothesized that there will be no differences in lean body mass between groups. Finally, confirmation of velocity loss magnitudes at the midpoint of the training intervention is hypothesized to be greater in the traditional-set group compared with the cluster-set group.

Methods

Experimental Approach to the Problem

This study used a randomized comparative design within an 8-week intervention period to examine the effects of traditional-set vs. cluster-set structures on upper-body muscle thickness and performance. To eliminate possible confounders, volume load, relative load, and exercise order were all matched between groups. Only subjects with at least 6 months of resistance training experience were eligible to participate to ensure that the high-intensity nature of the training program could be tolerated. Subjects who agreed to participate in the study completed a 3-week familiarization phase of training before baseline testing and then were randomized, using a computer-generated allocation list, into a training group. Posttesting was conducted at least 72 hours from the last completed training session within the 8-week intervention period. Moreover, posttesting was performed at a similar time of day (i.e., ± one hour) from the pretesting time. To investigate differences between groups and their effect magnitude for all dependent variables, a repeated measures analysis of variance (ANOVA) with Hedges' g was used. Analyzing the magnitude of velocity loss within each set and total session will be achieved using an independent samples t-test with effect sizes to estimate the between-group effects.

Subjects

A total of 22 healthy adults (age range: 18.5–43.2) (men, n = 13 and women, n = 9) agreed to participate in the study. Of the 22 subjects enrolled in the study, one subject withdrew immediately after pretesting because of a pre-existing shoulder injury and was therefore excluded from all analyses. Subject characteristics are presented in Table 1. Subjects were free of any cardiac, peripheral vascular, cerebrovascular, pulmonary, metabolic, renal, liver, or musculoskeletal signs and symptoms of disease or injury. Subjects reported being free of any legal or illegal agents known to increase muscle size and strength, such as creatine or anabolic-androgenic steroids within the previous 12 months before their inclusion into the study. Before entry into the study, subjects were informed of the purpose, risks, benefits, and experimental procedures involved. All subjects signed a written informed consent document before participating. The research study was approved by the University of Sydney Human Research Ethics Committee (HREC Project Number: 2016/018).

Table 1 - Characteristics of subjects at baseline.^*†

	CLUS (n = 11)	TRAD (n = 10)	p
Age (y)	26.10 ± 7.10	24.59 ± 6.90	0.628
Sex (male/female)	7/4	5/5	N/A
Height (cm)	176.57 ± 7.98	174.23 ± 7.59	0.507
Training experience (y)	3.78 ± 3.64	5.1 ± 7.72	0.617
Body mass (kg)	74.24 ± 9.99	75.57 ± 9.73	0.760
Body fat percentage (%)	22.62 ± 9.93	25.67 ± 11.33	0.511
Total fat mass (kg)	15.95 ± 7.87	18.52 ± 8.92	0.491
Total skeletal muscle mass (kg)	29.71 ± 6.47	29.88 ± 7.33	0.955
Triceps brachii MT (cm)	3.37 ± 0.81	3.29 ± 0.55	0.790
Pectoralis major PROX-MT (cm)	1.40 ± 0.39	1.21 ± 0.33	0.233
Pectoralis major MID-MT (cm)	1.82 ± 0.72	1.69 ± 0.54	0.649
Pectoralis major DIS-MT (cm)	1.85 ± 0.76	1.88 ± 0.91	0.930
Bench press 1RM at familiarization (kg)	63.41 ± 24.35	61.75 ± 29.91	0.890
Bench press 1RM at pretesting (kg)	66.59 ± 23.16	64.00 ± 29.28	0.824
Relative bench press 1RM (kg·BM−1)	0.88 ± 0.24	0.84 ± 0.33	0.708
Relative bench press 1RM (kg·LM−1)	1.18 ± 0.25	1.15 ± 0.33	0.868
Muscular endurance (repetitions)	13.91 ± 2.21	15.00 ± 2.79	0.331
Muscular endurance (volume, kg)	625.91 ± 175.68	681.95 ± 346.42	0.641

^*CLUS = cluster-set group; cm = centimeters; DIS-MT = distal muscle thickness; kg = kilograms; kg·BM⁻¹ = kilograms per kilogram of body mass; kg·LM⁻¹ = kilograms per kilogram of total lean mass; MID-MT = middle muscle thickness; MT = muscle thickness; p = p value from independent samples t-test; PROX-MT = proximal muscle thickness; TRAD = traditional-set group; y = years; 1RM = one repetition maximum.

^†Data are presented as mean ± SD.

Procedures

Familiarization and Resistance Training Intervention

Both the familiarization and resistance training intervention were presented in a previous publication from our group (⁶). The resistance training intervention involved a 3-day upper-body or lower-body split routine targeting major muscle groups progressing to 4 days at the midpoint of the intervention period. The 3-week familiarization phase was completed by all subjects and included 2 upper-body sessions and one lower-body session per week of the same resistance training program. The only exercise that was manipulated within each group was the bench press and was trained twice per week throughout the familiarization and intervention periods. Subjects were then randomized into one of 2 groups performing 4 sets of 5 repetitions for the bench press at 85% 1RM with (a) 5 minutes of interset rest only (TRAD, n = 10) or (b) 30 seconds of inter-repetition rest and 3 minutes of interset rest (CLUS, n = 11). This study used an inter-repetition rest paradigm for CLUS because it separates repetitions within a set into single efforts for specificity purposes to the muscular strength assessment (i.e., 1RM) and to produce a clear difference in experienced fatigue between conditions (³²). All subjects performed the same resistance training program with only the bench press exercise being manipulated. Subjects were required to rest a minimum of 48 hours between training sessions involving the same muscle groups to allow adequate recovery (⁴¹).

Before training, subjects performed a general warm-up of the rotator cuff muscles and upper back musculature, which consisted of band-resisted external rotations of the shoulder and reverse flyes, followed by a specific warm-up with progressive loading on the bench press (⁶). During training sessions, subjects were encouraged to perform all repetitions of the bench press in a controlled manner for the eccentric phase (∼1–2 seconds), whereas the concentric phase was performed with maximal intent with verbal encouragement from the training supervisor. Relative load was maintained throughout the intervention period using the bench press 1RM (outlined below) and was reassessed each fortnight (before the first session of that week). All training sessions were directly supervised by an accredited and experienced powerlifting coach to ensure each subject adhered to the training prescription and performed all exercises safely. Subjects were instructed at the beginning and throughout the study to refrain from performing any other resistance training and to maintain their habitual diet and physical activity regimen.

Assessment of Upper-Body Muscular Strength and Local Muscular Endurance

A 1RM test was used to determine muscular strength in the bench press and has been described in a previous publication from our group (⁶). The 1RM was performed on 2 occasions at each major time point (i.e., preintervention and postintervention) with a minimum of 48 hours between sessions for reliability purposes (Table 2). Once the 1RM score was obtained, it was expressed relative to body mass (1RM_BM) and relative to lean mass (1RM_LM). The best 1RM attempt from the 2 trials was used for analysis.

Table 2 - ICC and coefficient of variation analyses between all performance, body composition, and muscle thickness measurements.^*†

	Measure 1	Measure 2	Measure 3	ICC (95% CI)	CV% (95% CI)
Bench press 1RM (kg)	64.76 ± 25.80	65.00 ± 25.48	N/A	0.997 (0.993–0.999)	1.21 (0.28–2.14)
Relative muscular endurance (repetitions)‡	13.62 ± 2.46	14.57 ± 2.38	N/A	0.675 (0.353–0.854)	7.40 (3.54–11.26)
Relative muscular endurance volume (kg)‡	609.88 ± 252.83	656.90 ± 262.48	N/A	0.949 (0.878–0.979)	7.62 (3.58–11.20)
Triceps brachii MT (cm)	3.30 ± 0.72	3.35 ± 0.67	3.36 ± 0.70	0.965 (0.930–0.984)	4.16 (2.90–5.43)
Pectoralis major PROX-MT (cm)	1.30 ± 0.38	1.30 ± 0.36	1.32 ± 0.39	0.983 (0.960–0.993)	3.26 (2.52–4.00)
Pectoralis major MID-MT (cm)	1.75 ± 0.64	1.76 ± 0.63	1.72 ± 0.65	0.993 (0.982–0.997)	2.64 (2.01–3.28)
Pectoralis major DIS-MT (cm)	1.88 ± 0.82	1.88 ± 0.82	1.82 ± 0.82	0.994 (0.985–0.998)	3.33 (2.52–4.13)

^*CLUS = cluster-set group; CV% = coefficient of variation (expressed as a percentage); DIS-MT = distal muscle thickness; ICC = intraclass correlation coefficient; MID-MT = middle muscle thickness; MT = muscle thickness; PROX-MT = proximal muscle thickness; TRAD = traditional-set group; 1RM = one repetition maximum.

^†Data are presented as mean ± SD.

^‡Measures were taken using 70% 1RM of the predetermined 1RM bench press and may be different absolute loads depending on the performance of the 1RM test.

Muscular endurance was assessed by a maximum repetition task at 70% 1RM on the bench press 10 minutes after the completion of the 1RM assessment. The test was ceased when any of the following criteria were met: the subject could not complete a full repetition (i.e., full extension of the elbow), the subject aided the movement by lifting their feet, hips, or head from the bench, or the subject requested to stop the test. Subjects were instructed to perform the bench press with the same technique and lifting speeds as per the instructions for the 1RM. The same relative load (i.e., 70% 1RM) was used for pretraining and posttraining assessments to account for individual changes in muscular strength as per the methodology from Anderson and Kearney (³). All subjects were assessed on 2 occasions at baseline with a minimum of 48 hours between sessions for reliability purposes (Table 2). The number of repetitions completed and the volume load (repetitions × load) accrued after the best 1RM attempt were used for analysis.

Assessment of Site-Specific and Regional Muscle Thickness

Site-specific muscle thickness of the triceps brachii and regional muscle thickness of the pectoralis major were acquired using ultrasound imaging (Figure 1). All assessments were made on the right side of each subject using a Philips iU22 (Philips Ultrasound, Bothell, Washington DC) ultrasound machine and a linear 1- to 5-MHz probe. A qualified, experienced, and accredited sonographer performed all collection and analysis procedures and was blinded to group allocation of each subject. For each measurement site, the ultrasound probe was placed perpendicular to the surface of the skin, and an optimized image was obtained using an appropriate amount of water-soluble transmission gel with minimal compression of tissues. Once each scan was complete, it was assessed for preliminary image quality. Three scans for each site were taken in a single session for reliability purposes (Table 2) with the mean of the 2 closest values used for analysis (²).

Figure 1.: Mean velocity loss throughout each set and average velocity loss in the measured training session. *p < 0.05 compared with TRAD for the corresponding set. Data presented are mean ± SD. CLUS = cluster-set group, TRAD = traditional-set group. Calculated as the difference between the first and last repetition relative to the first repetition (expressed as a percentage).

For the triceps brachii, measurements were taken on the posterior surface of the upper arm at 60% distal from the acromion process to the lateral epicondyle of the humerus (¹). For the pectoralis major, panoramic images were taken in the sagittal (vertical) plane that has been demonstrated to be reliable compared with singular transverse images (²⁶). The measurement was taken from the subjects' midclavicular line (midpoint from the acromion process to the sternal notch) (³⁷) until the cessation of the muscle belly in the sagittal plane. Panoramic images were obtained by orientating the probe perpendicular to the skin on each landmark on the chest. The probe was moved manually in a slow and continuous movement along a marked vertical line on the skin, indicating the midclavicular line. The probe acquired imaging sequences from superior to inferior creating a cross-section of the pectoralis major that was used for analysis. Dimensions of muscle thickness were obtained by measuring the distance between the subcutaneous adipose tissue-to-muscle interface and the most anterior aspect of the humerus on the muscle-to-bone interface (¹). Muscle thickness (MT) was measured at 3 sites along the vertical length of the pectoralis that included the intercostal spaces of ribs 2–3 (PROX-MT), 3–4 (MID-MT) (³⁷), and 4–5 (DIS-MT).

Assessment of Body Composition

Body composition was assessed using a dual-energy x-ray absorptiometry (DEXA) scan (Lunar Prodigy, GE Medical Systems, Madison, WI). All scans were performed under standard conditions (¹⁸) by a licensed co-investigator. Subjects were instructed to arrive for the scan after a minimum 12-hour fast from solid food, a minimum 6-hour fast from liquids, and no exercise for a minimum of 24 hours. In-built analysis software (version 13.60.033; enCORE 2011, GE Healthcare) immediately calculated total lean and fat mass and regional subsites to estimate appendicular masses (i.e., upper limb and lower limb). Skeletal muscle mass was calculated using an equation described by Kim et al. (²⁸). The equation is as follows: $\text{Total}\text{Skeletal}\text{Muscle}\text{Mass}=\left(1.13\times \text{appendicular}\text{lean}\text{mass}\right)-\left(0.02\times \text{age}\right)+\left(0.61\times \text{sex }\right)+0.97$; where women are represented with a “zero” and men are represented with a “one” in the equation.

Assessment of Barbell Velocity Loss

Because of the strong correlation between the loss of barbell velocity within a set and the magnitude of neuromuscular fatigue (⁴⁶), the loss of barbell velocity during each set was used to estimate the neuromuscular fatigue experienced by the 2 training protocols. Because of equipment availability constraints, the magnitude of velocity loss between the 2 protocols was measured at the midpoint of the training intervention alone (i.e., week 4 of the intervention) to gain cross-sectional performance data. Velocity (m·s⁻¹) was calculated as the first-order derivative of the displacement-time curve in the vertical direction using the GymAware linear position transducer and associated software (Kinetic Performance Technology, Mitchell, Australia). Displacement and time of each repetition were calculated from the initiation of a vertical displacement until the cessation of movement at the lock-out position. The GymAware was placed at the junction between the shaft and sleeve on the left side of the barbell for each subject during the first session of training in the fourth week of the intervention. All subjects had reassessed their bench press 1RM in the first session of week 3; therefore, there were no differences in relative load (%1RM) between groups. Velocity loss was calculated as the difference between the first and last repetitions of each set and expressed as a percentage relative to the first repetition (¹³).

Statistical Analyses

Baseline characteristics of all subjects and variables that were not influenced by time (e.g., average relative load of training sessions and volume accrued) were analyzed using independent samples t-tests. Primary effects of resistance training on measurements of muscle thickness, body composition, muscular strength, and relative muscular endurance were analyzed using a 2 × 2 (group × time) repeated measures ANOVA with a post hoc Bonferroni adjustment. Velocity loss was analyzed using a 2 × 4 × 5 (group by set by repetition) repeated measures ANOVA with a post hoc Bonferroni adjustment. All analyses were performed using SPSS (version 24) software for Windows (SPSS, Chicago, IL) and Excel (2016) software for Windows (Microsoft, Redmond, WA).

The reliability of muscular strength, body composition, and muscle thickness measurements was calculated using intraclass correlation coefficients (ICCs) and the coefficient of variation with 95% confidence intervals (CIs) for both analyses. The results of the ICC were derived from ICC (^1,2,22) and were interpreted as; < 0.5 poor, 0.5 to 0.75 moderate, > 0.75 to 0.9 good, and >0.9 excellent reliability (²⁹). Coefficient of variation scores that were <5% were considered good reliability, 5–10% were considered moderate, and >10% were considered poor reliability (⁴). In the ultrasound measurement, with 3 trials taken, the mean of the typical error of each pairwise analysis (i.e., trial 1 against trial 2 and trial 2 against trial 3) was used as recommended by Hopkins (²¹).

Because of the small sample size presented, estimates of effect size (ES) with 95% CI were calculated using Hedges' g with the pre-post correlation set at a constant magnitude of 0.5 ([mean difference divided by pooled weighted SD] Comprehensive Meta-Analysis, Englewood, NJ). Effect sizes were interpreted in all analyses as follows; <0.2, 0.2, 0.6, 1.2, 2.0, and >4.0 for trivial, small, moderate, large, very large, and extremely large effects, respectively (²³). For between-group effects, positive effect sizes indicate that the effect favored CLUS, whereas negative effects favored TRAD. For all analyses, an alpha level of significance was set at p < 0.05.

Results

At baseline, there were no significant differences between groups in any measured variable (Table 1). Within the intervention, there was 99.38% ± 1.98% compliance in TRAD and 97.73% ± 9.74% in CLUS, respectively, with no significant difference between groups (p = 0.145). During the intervention period, CLUS completed significantly more repetitions per session compared with TRAD (TRAD: 19.04 ± 0.88 repetitions; CLUS: 19.80 ± 0.37 repetitions; p = 0.016, g = −1.15, 95% CI = −2.07 to −0.22); however, there were no significant between-group differences in the training volume load accrued per session (TRAD: 1,089.85 kg ± 480.16; CLUS: 1,162.95 kg ± 383.58 kg; p = 0.703, g = 0.16, 95% CI = −0.66 to 0.99). There were no significant between-group differences in the average relative load (i.e., %1RM) used throughout the intervention (TRAD: 84.50% ± 1.16%; CLUS: 85.06% ± 1.07%; p = 0.268, g = 0.48, 95% CI = −0.35 to 1.32). The conflicting observations between repetitions performed and volume accrued are likely because of the large standard deviations calculated in absolute load used during the training program. When training volume load was calculated using relative load compared with absolute load (i.e., relative load [%] multiplied by average repetitions performed per session), there was a significantly greater relative volume load for CLUS compared with TRAD (TRAD: 16.09 ± 0.87; CLUS: 16.84 ± 0.37; p = 0.017, g = 1.10, 95% CI = 0.82 to 1.38). All measurement protocols proved to have good reliability, except for the measure of muscular endurance, which demonstrated moderate reliability (Table 2).

Velocity Loss

The magnitude of velocity loss at midpoint of the training intervention is presented in Figure 1. TRAD showed significantly greater loss in velocity than that of CLUS in sets one (p = 0.001, g = −1.71, 95% CI = 0.74 to 2.68), 2 (p = 0.001, g = −1.75, 95% CI = 0.78 to 2.73), and 4 (p = 0.032, g = −0.97, 95% CI = 0.10 to 1.84) with no significant differences in set 3 (p = 0.401, g = −0.42, 95% CI = −0.41 to 1.26). When all sets were averaged, TRAD also showed significantly greater velocity loss than that of CLUS (p = 0.002, g = 1.50, 95% CI = 0.56 to 2.44).

Body Composition and Site-Specific Muscular Thickness

Data for measures of body composition and site-specific muscular thickness are presented in Table 3. Total skeletal muscle mass significantly increased over time (p = 0.043) with no significant differences between groups (p = 0.353). CLUS significantly increased body mass compared with TRAD after the intervention period, although the effect size was trivial (p = 0.031, g = 0.11, 95% CI = −0.75 to 0.96). There were no significant time, group, or group by time interactions for all remaining measures of body composition. Regarding site-specific muscle thickness, there were no significant between-group differences in the change in triceps brachii muscle thickness (p = 0.833, g = −0.07, 95% CI = −0.92 to 0.79). TRAD led to significantly greater pectoralis major PROX-MT compared with CLUS (p = 0.029, g = −0.34, 95% CI = −1.20 to 0.52), whereas greater increases in pectoralis major MID-MT narrowly missed significance, although a small effect was observed favoring TRAD (p = 0.061, g = −0.25, 95% CI = −1.11 to 0.61). There was a time effect for increases in pectoralis major DIS-MT from baseline (p = 0.047) with no significant differences between groups (p = 0.786, g = −0.03, 95% CI = −0.89 to 0.82). Example images of the triceps brachii and pectoralis major from a single subject preintervention and postintervention are presented in Figure 2.

Table 3 - Changes in muscular strength, body composition, and site-specific muscle thickness during and after the intervention period.^*†

Measure	CLUS (n = 11)		TRAD (n = 10)		ANOVA (p)			Between-group ES (g)‡
	Pre	Post	Pre	Post	T	G	G × T	ES	95% CI of ES
Bench press 1RM (kg)	66.59 ± 23.16	72.95 ± 24.44	64.00 ± 29.28	70.50 ± 29.53	<0.001‡	0.830	0.923	−0.01	−0.86 to 0.85
Bench press 1RMBM (kg·kg−1)	0.88 ± 0.24	0.96 ± 0.25	0.84 ± 0.33	0.92 ± 0.33	<0.001‡	0.751	0.421	0.00	−0.86 to 0.86
Bench press 1RMLM (kg·kg−1)	1.19 ± 0.25	1.29 ± 0.26	1.15 ± 0.33	1.28 ± 0.32	<0.001‡	0.908	0.659	−0.10	−0.96 to 0.76
Relative muscular endurance (repetitions)	13.91 ± 2.21	13.82 ± 2.09	15.00 ± 2.79	13.50 ± 1.51	0.072§	0.660	0.108	0.54	−0.33 to 1.41
Relative muscular endurance (volume, kg)	625.91 ± 175.68	682.59 ± 188.98	681.95 ± 346.42	673.20 ± 299.09	0.289	0.836	0.153	0.23	−0.63 to 1.09
Body mass (kg)	74.24 ± 9.99	75.20 ± 10.41	75.57 ± 9.73	75.45 ± 9.82	0.086§	0.928	0.031‡	0.11	−0.75 to 0.96
Body fat percentage (%)	22.62 ± 9.93	22.76 ± 9.68	25.67 ± 11.33	25.47 ± 11.95	0.791	0.562	0.205	0.03	−0.81 to 0.86
Total fat mass (kg)	15.95 ± 7.87	16.30 ± 7.90	18.52 ± 8.92	18.36 ± 9.55	0.681	0.543	0.267	0.06	−0.78 to 0.89
Total skeletal muscle mass (kg)	29.71 ± 6.47	30.28 ± 6.55	29.88 ± 7.33	30.10 ± 7.69	0.043‡	0.999	0.353	0.05	−0.81 to 0.91
Triceps brachii MT (cm)	3.38 ± 0.82	3.31 ± 0.83	3.29 ± 0.55	3.27 ± 0.83	0.723	0.839	0.833	−0.07	−0.92 to 0.79
Pectoralis major PROX-MT (cm)	1.41 ± 0.40	1.37 ± 0.38	1.21 ± 0.33	1.30 ± 0.38	0.397	0.417	0.029‡	−0.34	−1.20 to 0.52
Pectoralis major MID-MT (cm)	1.82 ± 0.72	1.82 ± 0.64	1.69 ± 0.54	1.86 ± 0.66	0.067§	0.872	0.061§	−0.25	−1.11 to 0.61
Pectoralis major DIS-MT (cm)	1.85 ± 0.75	1.96 ± 0.73	1.88 ± 0.91	2.02 ± 1.02	0.047‡	0.902	0.786	−0.03	−0.89 to 0.82

^*1RM = one repetition maximum; 1RM_BM = kilograms per kilogram of body mass; 1RM_LM = kilograms per kilogram of lean mass; ANOVA = analysis of variance; CLUS = cluster-set group; cm = centimeters; DIS-MT = distal muscle thickness; G × T = group by time interaction; G = group effect; kg = kilograms; MID-MT = middle muscle thickness; MT = muscle thickness; p = p value; Pre = pretraining/baseline testing; PROX-MT = proximal muscle thickness; Post = posttraining testing; TRAD = traditional-set group; T = time effect.

^†Data presented as mean ± SD.

^‡Significant (<0.05).

^§Trend for significance (≥0.05 to <0.1).

Figure 2.: Example ultrasound images of the triceps brachii (A) and pectoralis major (B) muscles including measurements of a single subject preintervention (I) and postintervention (II). cm = centimeters; DIS-MT = distal muscle thickness; MID-MT = middle muscle thickness; MT = muscle thickness; PROX-MT = proximal muscle thickness.

Muscular Strength

After the familiarization phase, there was a significant increase (p < 0.001) in bench press 1RM by 4.95% ± 3.49% (g = 0.07, 95% CI = −0.50 to 0.64) in TRAD and 6.25% ± 4.78% (g = 0.12, 95% CI = −0.42 to 0.67) in CLUS, with no significant differences between groups (p = 0.405, g = 0.03, 95% CI = −0.82 to 0.87); however, the effect sizes of these increases were trivial. The time course of muscular strength development is presented in Figure 3. After pretesting, bench press 1RM significantly increased (p < 0.001) by week 3 of the intervention in both groups by 4.36% ± 3.76% (g = 0.07, 95% CI = −0.50 to 0.64) in TRAD and 3.80% ± 2.74% (g = 0.10, 95% CI = −0.45 to 0.65) in CLUS, although the effect sizes were trivial. TRAD demonstrated significant increases in bench press 1RM across all time points throughout the intervention while CLUS showed significant increases after week 5 of the intervention (Figure 3).

Figure 3.: Time course of muscular strength development from preintervention testing. *Significantly different (p < 0.05) from previous time point. CLUS = cluster-set group; TRAD = traditional-set group.

Compared with pretesting scores, TRAD significantly increased bench press 1RM by 12.16% ± 7.07% (p < 0.001, g = 0.20, 95% CI = −0.37 to 0.78), bench press 1RM_BM by 12.16% ± 7.07% (p < 0.001, g = 0.22, 95% CI = −0.35 to 0.80), and bench press 1RM_LM by 12.22% ± 7.95% (p < 0.001, g = 0.37, 95% CI = −0.23 to 0.95). CLUS significantly increased bench press 1RM by 9.90% ± 4.60% (p < 0.001, g = 0.25, 95% CI = −0.31 to 0.80), bench press 1RM_BM by 8.55% ± 4.48% (p < 0.001, g = 0.30, 95% CI = −0.26 to 0.86), and bench press 1RM_LM by 8.68% ± 4.70% (p < 0.001, g = 0.36, 95% CI = −0.20 to 0.93). There were no significant between-group differences for each measure of muscular strength (Table 3).

Relative Muscular Endurance

Results and ES data for relative muscular endurance are presented in Table 3. There was a time effect found that was near significance for the number of repetitions performed (p = 0.072); however, there was no significant group effect (p = 0.660). There was no significant group by time interaction, although a small between-group effect was observed, indicating CLUS maintained the number of repetitions performed to a greater extent compared with TRAD (p = 0.108, g = 0.54, 95% CI = −0.33 to 1.41). There was no significant time effect (p = 0.289) or group effect (p = 0.836) found for the volume accrued during the assessment. There was no significant group by time interaction; however, a small effect was observed favoring CLUS, which indicated that a higher volume load was performed during the assessment (p = 0.153, g = 0.24, 95% CI = −0.63 to 1.09).

Discussion

The current study aimed to compare the effects of altering set structure during an 8-week volume-equated (i.e., matched for total repetitions per session at the same relative load) high-load bench press training program on the development of muscle thickness, body composition, muscular strength, and endurance. Site-specific muscular hypertrophy of the proximal (g = −0.34) and middle (g = −0.25) regions of the pectoralis major improved to a greater extent over the intervention period in TRAD compared with CLUS. CLUS was found to significantly increase body mass compared with TRAD (g = 0.11), with no between-group differences being observed for remaining outcomes of site-specific muscular hypertrophy or body composition. Muscular strength significantly increased from baseline in both groups (p < 0.001) but there were no significant differences between groups. When investigating the time course of muscular strength development, CLUS ceased developing muscular strength after week 5 while TRAD continued to significantly improve throughout the intervention period. Changes in relative muscular endurance were unaffected by the set structure performed during the intervention (repetitions: g = 0.54; volume: g = 0.23). Because of the nature of the 2 set structures, velocity loss magnitude at the intervention midpoint was found to be greater for TRAD compared with CLUS (g = 1.50). All measures except for muscular endurance (moderate reliability) proved to have good reliability. The findings of this study indicate that different set structures may influence site-specific muscular hypertrophy but are unlikely to affect performance measures, such as muscular strength endurance.

The implementation of cluster-set structures seems to impair site-specific muscular hypertrophy compared with traditional-set structures. This was evident by TRAD showing a greater velocity loss magnitude during the intervention and experienced augmented site-specific muscular hypertrophy compared with CLUS. These findings are supported by Pareja-Blanco et al. (⁴⁰), where larger velocity loss magnitudes promoted greater muscle hypertrophy, as well as Goto et al. (17) and Karsten et al. (27), which demonstrated greater site-specific muscle hypertrophy within traditional-set structures. Building on this finding, it seems that altering set structures in the bench press may alter the hypertrophy response of the pectoralis major. The greater hypertrophy in the proximal and middle regions of the pectoralis major in TRAD compared with CLUS may derive from differing muscle activations during fatiguing resistance training. Morton et al. (³⁵) observed that performing sets to failure at a similar load to the current study (80% 1RM) led to greater activation of muscle fibers that correlated with reductions in peak torque during sets (i.e., fatigue) and phosphorylation of signaling proteins, such as p70S6K. Phosphorylation of key signaling proteins is critical to protein synthesis and thus, muscular hypertrophy (⁵⁵). The greater hypertrophy observed in the proximal and middle regions of the pectoralis major in TRAD compared with CLUS may have occurred because of increases in activation and phosphorylation of important signaling proteins in these areas. It must be noted that the findings in the current study produced small-to-moderate effect sizes; therefore, any conclusive findings and any mechanistic underpinnings will require confirmation in future research.

The results of the current study suggest that similar increases in muscular strength can be achieved when implementing cluster-set and traditional-set structures in conjunction with a full-body resistance training program. The program was progressed using a 1RM assessment where loads were adjusted according to individual rates of progression. This method also allowed for relative load to remain constant within the intervention and between groups. Given the high-load nature of the study, long rest periods were prescribed based on the recommendations for exercise that involves short efforts at high intensity (⁴⁹). Although these findings have been observed in previous studies on the topic (^{24,25,33,36,40}), the time course of strength adaptation has yet to be fully examined when resistance training is performed with different set structures. Both groups significantly increased muscular strength from baseline by week 3 of the intervention, which agrees with previous work on the topic (¹). However, CLUS ceased significant increases in muscular strength after week 5 of the intervention period, whereas TRAD continued to significantly increase muscular strength across all measured time points (see Figure 3). It seems that the training stimulus for CLUS lost potency by week 5 of the intervention period, yet the stimulus for TRAD continued to be potent until the intervention ceased. The underlying mechanisms for muscular strength development stem from improvements in neuromuscular (i.e., voluntary muscular activation, skill coordination, synergist/stabilizer activation etc.) and morphological (i.e., fiber hypertrophy, fiber type alteration, and connective tissue stiffness) sources (¹¹). Therefore, the greater site-specific muscle hypertrophy observed in TRAD compared with CLUS may have also led to the continued strength adaptation observed throughout the intervention. Regular measurements of muscle thickness to establish a time course would have provided greater insights into whether changes in muscle thickness of the pectoralis major coincided with strength development during the later weeks of the intervention, thus providing some evidence of muscular hypertrophy influencing the continued strength response observed for TRAD (^1,34).

To the authors' knowledge, the current study is the first to assess changes in local muscular endurance using the same relative load after resistance training of different set structures. We conducted the local muscular endurance test at preintervention and postintervention keeping the relative load constant (i.e., 70% 1RM) to assess local muscular endurance capacity (i.e., resistance to fatigue). This removes the confounding variable of muscular strength that is observed in absolute muscular endurance assessments (³). For example, if 70% of the pretraining 1RM was kept constant into posttesting in the current study, as with absolute muscular endurance testing protocols, subjects would have used 57.65–68.37% 1RM in the posttraining assessment. As more repetitions can be performed with lower relative loads until concentric muscular failure (³¹) (i.e., the load-repetition relationship), the underlying mechanism supporting the higher number of repetitions performed would be unknown. Theoretically, this may derive from an improvement in fatigue resistance that has been reported in endurance athletes (⁴³) or a reduction in relative load as a consequence of muscular strength development. In the current study, CLUS seemed to produce greater improvements in muscular development through a better ability to maintain the repetitions performed at 70% 1RM (g = 0.54) and increase the volume load performed in the set (g = 0.23), although it must be stated that these results were not statistically significant. This result may have been observed because of a better maintenance of movement skill with cluster-set structures that has been presented in the power clean (²⁰). Given that there is a decrease in movement efficiency as a set to concentric muscular failure is performed (⁹), the implementation of cluster-set structures may enhance fatigue resistance through efficient bar paths of each repetition that is practiced within each cluster. However, future studies will need to confirm these findings.

Certain limitations may have affected the results of this study. First, although panoramic imaging has been validated for estimating muscle thickness and cross-sectional area of the quadriceps (⁵⁰), further scrutiny is warranted when determining pectoralis major muscle thickness. Second, although our measurement of local muscular endurance was based on a constant relative load (i.e., 70% 1RM), the assessment of this outcome would have been strengthened through the addition of a traditional absolute muscular endurance assessment to ascertain any differences (if any) between the 2 measurements. Third, we recruited subjects who had at least 6 months of resistance training experience; therefore, caution should be used when extrapolating the results to athletic populations. There was also a difference in the total rest accumulated between groups with the CLUS group accumulating 17 minutes of rest while TRAD accumulated 15 minutes of rest. TRAD, therefore, performed the same work in a shorter time (i.e., greater external power output) and likely increased the magnitude of neuromuscular fatigue experienced compared with CLUS, which facilitates a more considerable difference in velocity loss between groups. Although subjects were instructed not to change their diet and to limit their physical activity external to the intervention (subjects reported diet and physical practices preintervention and postintervention), adherence was poor. As CLUS significantly increased body mass compared with TRAD, CLUS would likely have been in caloric surplus during the intervention that may have affected the response. Finally, the cohort of recruited subjects encompassed men and women. Although there may have been differences in the response between sexes, the sample size of men and women in each group was very small and likely did not provide enough power for true differences to be observed.

Practical Applications

The prescription of a cluster-set or traditional-set structure is likely to influence the magnitude of velocity loss experienced during sets that has been demonstrated to be an indicator of neuromuscular fatigue. When seeking muscular strength development, the current study suggests that set structure is likely to be unimportant within the context of a high-load training block. However, when seeking muscular hypertrophy, the prescription of traditional-set structures, in which significant velocity losses occur, is likely to lead to more favorable outcomes. However, cluster-set structures, in which small velocity losses occur during sets, do not seem to reduce muscle size and, therefore, can be safely implemented in certain phases of training without fears of detraining effects. These observations may not occur in athletic populations where greater training loads are required to induce an adaptation; therefore, caution must be warranted when extrapolating these recommendations to this cohort. Coaches can also be made aware that set structures that manipulate velocity loss with high loads do not impact endurance performance. As a final note, for those seeking increases in muscular strength without gains in muscle mass, such as athletes in weight-restricted sports, high-load cluster-set structures with upper-body exercises may allow practice of heavy loads to facilitate neuromuscular performance adaptations in upper-body exercises. This would consequently reduce the influence of some factors that would lead to hypertrophy, such as fatigue accumulation. This investigation is relevant for athletes and resistance trainers alike seeking improvement in muscle thickness and performance, especially when muscular strength is an essential outcome of their success.