A Comparison of the Braking and Propulsion Phase Characteristics of Traditional and Accentuated Eccentric Loaded Back Squats in Resistance-Trained Women

Abstract

The aim of this study was to compare the braking and propulsion force–time and barbell velocity characteristics between back squat sets performed using traditional (TRAD) or accentuated eccentric loading (AEL) in resistance-trained women. In total, 14 participants completed four separate testing sessions that included a one repetition maximum (1RM) back squat and three squat testing sessions that used either TRAD or AEL. During the squat testing sessions, participants performed sets of three back squat repetitions using TRAD loads with 50, 60, 70, and 80% 1RM or performed the same loads with the addition of weight releasers that equated the total load to 100% (AEL-MAX) or 110% (AEL-SUPRA) 1RM during the eccentric phase of the first repetition of each set. Braking and propulsion mean force, duration, and impulse as well as mean and peak barbell velocity were examined across each back squat set. Significantly greater braking impulses were produced during the AEL conditions across all loads (p < 0.02), while greater braking mean force during AEL-SUPRA was produced compared to TRAD during with 50 and 60% 1RM (p < 0.02). There were no other significant differences in braking, propulsion, or barbell velocity that existed between different conditions (p > 0.05). AEL-MAX and AEL-SUPRA provide a greater braking stimulus compared to TRAD squats, while the propulsion phase may not be significantly impacted. Rapid and maximal force production may be favored by larger and smaller load spreads, respectively.

1. Introduction

The use of eccentric training methods within resistance training programs has increased in popularity over the past decade. Likewise, the amount of research focused on eccentric training has increased likely due to the force production potential of eccentric muscle actions [1] and their contribution to an athlete’s hypertrophy, strength, and power characteristics [2]. Strength and conditioning professionals may use a wide variety of eccentric training methods including tempo training, flywheel inertial training, accentuated eccentric loading (AEL), accelerated eccentric exercises, and plyometrics [3]. Although some general training recommendations for each eccentric training method have been made [4,5], further research is needed to provide more concrete programming recommendations. Specifically, further information on how different eccentric training methods should be loaded may provide unique insight into their potential for strength-power adaptations for athletes.

AEL is a form of eccentric training that has three primary requirements: a greater load during the eccentric phase compared to the concentric phase, coupled eccentric and concentric muscle actions within a stretch-shortening cycle action, and minimal disruption of the natural movement mechanics of the exercise [6]. AEL is unique in that it can be used with ballistic exercises such as dumbbell jumps [7,8,9] and jump squats [10] but also with strength-based movements like the back squat [11,12,13] and bench press [14,15,16,17,18,19]. Beyond the obvious benefits to the braking (eccentric) due to greater loads, AEL may provide a within-set potentiation effect to benefit the concentric movement as well [14,15,16]. In fact, researchers have shown that 5–10 weeks of AEL training may lead to improvements in maximal strength [20,21,22,23,24,25,26] and power output [7,25,26]. Despite the existing literature, limited information exists on how best to implement AEL with strength-based movements from a loading standpoint and thus, further research is warranted.

Suchomel and colleagues [13] examined the braking and propulsion force–time characteristics and barbell velocity differences between traditional (TRAD) (i.e., using the same load during the eccentric and concentric movement), maximal AEL (AEL-MAX), and supramaximal AEL (AEL-SUPRA) back squats performed with 50, 60, 70, and 80% 1RM in resistance-trained men. The authors showed that moderate–large (g = 0.99–3.52) and large (g = 1.67–2.24) braking impulses were produced during AEL-MAX (i.e., 100% 1RM) and AEL-SUPRA (i.e., 110% 1RM) back squats compared to TRAD squats across the loads, respectively. Moreover, these impulses were underpinned by moderate (g = 0.77–1.16) and large (g = 1.67–2.24) differences in braking mean force while only trivial–small (g = 0.01–0.57) differences in braking duration existed across conditions. It should, however, be noted that the previous authors also found trivial–small effects for all propulsion and barbell velocity variables, indicating little to no difference between conditions. Although the previous study provides insight into how different loading methods can impact the braking and propulsion back squat stimulus, it was performed on a sample that included only resistance-trained men. Like much of the strength and conditioning literature [27], there is a limited amount of AEL research that has examined its effects in female populations. Therefore, the purpose of this study was to compare the braking and propulsion force–time and barbell velocity characteristics between back squat sets performed using TRAD, AEL-MAX, and AEL-SUPRA loading in resistance-trained women. Based on the existing literature, it was hypothesized that the AEL conditions would display a greater braking stimulus compared to TRAD but would not negatively impact the propulsion phase characteristics of the movement.

2. Materials and Methods

2.1. Participants

The current study included 14 resistance-trained women (age = 23.6 ± 2.6 years, height = 166.5 ± 6.5 cm, body mass = 70.3 ± 8.4 kg, relative one repetition maximum [1RM] back squat strength = 1.50 ± 0.19 kg/kg). The recruited participants needed to be free of injuries for six months prior to the study, between the ages of 18 and 35 years old, and resistance trained at least three times per week for at least one year prior to the study and regularly included the back squat in their training sessions. Participants were recruited from the hosting university and local fitness facilities in the surrounding area via word of mouth. If potential participants did not meet these criteria, or they could not complete all the testing sessions for any reason, they were excluded from the study. Although 15 participants were recruited for this study, one could not complete the exercise sets within the testing sessions and was thus removed from the study. This study was conducted in accordance with the Declaration of Helsinki and was approved by the Institutional Review Board at Carroll University (protocol code: #21-044, approval date: 30 November 2021). Written informed consent was obtained from each participant involved in the study prior to their participation.

2.2. Design

The force production and barbell velocity characteristics of TRAD, AEL-MAX, and AEL-SUPRA back squat sets were compared using a randomized, repeated measures design. Participants completed four separate testing sessions over a 10-day period and attended each session within a two-hour period to account for changes in Circadian rhythms. Force production characteristics including braking and propulsion mean force, duration, and impulse were measured using force plates while a linear position transducer was used to measure mean (MBV) and peak barbell velocity (PBV) during each back squat set. The loading conditions included using 50, 60, 70, and 80% of the participants’ 1RM back squat during the propulsion (concentric) phase of the squat and each variable was compared between the TRAD and AEL conditions.

2.3. 1RM Back Squat and Familiarization

The 1RM back squat testing session began with the participants completing the informed consent process and providing their age and predicted 1RM before their body mass and height were measured. A standardized warm-up that included three minutes of light–moderate stationary cycling and dynamic stretching (e.g., lunges, hurdle step-overs, zombie walks, walking quadriceps stretch, etc.) followed before the participants completed a 1RM back squat warm-up that included empty barbell repetitions as well as and back squat sets using 30% (five repetitions), 50% (five repetitions), 70% (three repetitions), and 90% (one repetition) of their predicted 1RM. After completing the warm-up, the principal investigator and research assistants increased the load for the first 1RM attempt. After successfully completing 1RM attempts, the load was increased at least 2.5 kg between attempts. To be considered a successful squat attempt, the top of the participants’ thighs needed to (at a minimum) be parallel to the floor. The depth of each squat repetition was visually monitored by the principal investigator and the research assistants. Participants were given two minutes of rest between the first two warm-up sets and 3–5 min between the final two warm-up sets and 1RM attempts. The 1RM of each participant was achieved within five or fewer attempts.

After the 1RM was determined, a self-selected rest period was provided before the participants performed familiarization repetitions using weight releasers (Monster Grips, Columbus, OH, USA) based on previous procedures [13]. To ensure the proper height of the weight releasers, empty 20 kg barbell repetitions were performed with weight releasers (5 kg each) placed on each sleeve of the barbell. Weight releaser height was adjusted by the investigators to ensure that the weight releaser contacted the ground and fell off (released) the barbell before the participant reached their deepest squat position. After the empty barbell repetitions, the participant completed a set of three repetitions with 50% of their 1RM back squat on the barbell and empty weight releasers. The weight releasers fell off during the first repetition and the participant performed the final two repetitions with only the barbell weight. The final familiarization repetitions included performing a set of three squat repetitions with 50% of the participants’ 1RM on the barbell paired with weight releaser loads that each represented 10% of the participants’ 1RM, totaling 70% 1RM. Within this set, the eccentric (braking) phase of first repetition used 70% 1RM while the concentric (propulsion) phase used performed with 50% 1RM, and the subsequent two repetitions (eccentric and concentric) used 50% 1RM.

2.4. Traditional and Accentuated Eccentric Loading Testing Sessions

The first of three testing sessions took place 72 h following the 1RM and familiarization session. The order of the remaining sessions was randomized for each participant and each separated by 72 h. Once participants arrived for the testing sessions, their body mass was collected before completing the general and dynamic warm-up previously described. Participants then completed a self-selected number of squats with an empty 20 kg barbell before performing three back squat repetitions with the 5 kg weight releasers that fell off after the first repetition. This served as a last familiarization trial and allowed the investigators to confirm the weight releaser length for each participant. The warm-up protocol then required the participants to perform back squat sets using 30% (five repetitions), 50% (three repetitions), and 70% (three repetitions) of their 1RM. The participants had two minutes of rest between warm-up sets. During the AEL sessions, a “walk-out” with the load of the first testing set (i.e., 50% 1RM and weight releaser load) was performed to acclimatize them to the feeling of the potential weight releaser swing as well as the maximal or supramaximal load that they would be eccentrically lowering during the squat. After the warm-up, three back squat repetitions were performed using 50, 60, 70, and 80% of the participants’ 1RM. The sets were performed in a progressive order while three minutes of rest was provided between each set. Readers should note that the weight releasers were used on the first repetition of each back squat set during the AEL conditions but not during the subsequent repetitions. In addition, the participants were asked to complete the eccentric phase of each back squat repetition using their normal squatting tempo but were encouraged to perform the concentric phase with maximal intent (i.e., as fast as possible). Briefly, each set required participants to pick up the barbell from the squat stands, step back onto the force platform, and stand motionless for at least one second before receiving a countdown of “3, 2, 1, Go!”. After the countdown, the participants performed the back squat set with maximal effort before re-racking the barbell on the squat stands. While the same load was used throughout each repetition during the sets performed during the TRAD session, the loading conditions differed during the AEL sessions. For example, 100 or 110% of the participants’ 1RM was used during the eccentric (braking) phase of the first repetition of each back squat set during the AEL-MAX and AEL-SUPRA sessions, respectively; however, the concentric loads were equivalent to those performed during the TRAD session [11,12]. As an example of this loading method, the AEL-SUPRA session had participants perform the braking phase of the first repetition with 50% 1RM on the barbell and 60% 1RM added by the weight releasers on either end of the barbell (i.e., 30% 1RM on each weight releaser), totaling 110% 1RM. During the first squat repetition, the weight releaser load (i.e., 60% 1RM) would fall off at the bottom of the squat before the participant performed the concentric phase and following two repetitions with only 50% 1RM. Thus, the braking phase of the first repetition within the set was performed with a supramaximal load while the propulsion phase of the first repetition and the braking and propulsion phases of the following two repetitions were performed with only the barbell load.

2.5. Data Analyses

Testing repetitions were performed on a force platform (Model 6090-06, Bertec Corporation, Columbus, OH, USA), while a linear position transducer (GymAware Powertool, Kinetic Performance Technology, Braddon, Australia) was attached to the shaft of the barbell (Figure 1). The force platform sampled at 1000 Hz, while a variable sampling rate with level crossing detection was used by the linear position transducer. Raw force–time data were exported to a custom spreadsheet (Microsoft Inc., Redmond, WA, USA) that was used to calculate the braking and propulsion phase net mean force, duration, and net impulse of each back squat repetition [13]. Briefly, the braking phase started when the force produced was above system mass (participant body mass + concentric barbell load) after the unweighting phase of each squat repetition and ended at the bottom position of the squat (identified by the linear position transducer) and where the peak braking force occurred. The start of the propulsion phase of the squat occurred immediately following the peak braking force and ended when the final force value was produced above the system mass. The average force produced during the braking and propulsion phases was identified as the net mean force of each phase. Phase duration was then calculated as the length of time of each phase. Finally, the braking and propulsion net impulses were calculated as the product of net mean force and duration of each phase. Relative braking and propulsion mean force was determined by dividing the net mean force of each phase by the body mass of the participant. The average performance during each squat set was used for statistical comparison. A tablet (iPad 2, Apple Inc., Cupertino, CA, USA) that had the GymAware application (version 4.0) was connected to the linear position transducer. Barbell displacement was divided by the movement time to calculate barbell velocity–time data. The average and peak velocity values calculated during the propulsion phase of each squat repetition were determined as MBV and PBV, respectively. Like the force–time variables, the average MBV and PBV values produced across each squat set were used for statistical comparison.

2.6. Statistical Analyses

The normal distribution of the data was assessed using the Shapiro–Wilks test. Two-way, mixed intraclass correlation coefficients (ICCs) were used to examine the relative reliability of each force production and barbell velocity variable. Test–retest reliability of each variable was determined during the TRAD squat sets since AEL may influence the performance of subsequent repetitions [12]. ICCs were classified as poor, moderate, good, and excellent when ICC data with the lower bound 95% confidence intervals were <0.50, 0.50–0.74, 0.75–0.90, and >0.90, respectively [28]. In addition, 3 (condition) × 4 (load) repeated measures ANOVA tests were used to determine the differences in force production and barbell velocity characteristics between the TRAD and AEL back squat conditions. Post hoc comparisons were performed using Bonferroni corrections. Greenhouse–Geisser adjusted p-values were reported if the assumption of sphericity was violated. Finally, Hedge’s g effect sizes were calculated to determine the magnitude of the differences between conditions and loads. Effect size magnitudes were considered trivial, small, moderate, large, very large, and nearly perfect when the Hedge’s g values were 0.00–0.19, 0.20–0.59, 0.60–1.19, 1.20–1.99, 2.00–3.99, and ≥4.00, respectively [29]. All statistical tests were completed using SPSS 28 (IBM, Chicago, IL, USA) and the criteria for statistical significance was set at p ≤ 0.05.

3. Results

All the braking and propulsion force–time and barbell velocity data were normally distributed. ICC reliability data are displayed in

Table 1. Intraclass correlation coefficients (ICCs) with 95% confidence intervals (CIs) for braking and propulsion force–time characteristics and barbell velocity variables.

Variable	ICC (95% CI)
	50%	60%	70%	80%
BMF (N/kg)	0.94 (0.85–0.98)	0.88 (0.71–0.96)	0.95 (0.95–0.87)	0.93 (0.83–0.97)
BDur (s)	0.93 (0.82–0.97)	0.92 (0.81–0.97)	0.95 0.90–0.98)	0.92 (0.81–0.97)
BImp (Ns)	0.84 (0.61–0.94)	0.87 (0.68–0.95)	0.93 (0.84–0.98)	0.83 (0.59–0.94)
PMF (N/kg)	0.97 (0.93–0.99)	0.94 (0.85–0.98)	0.94 (0.85–0.98)	0.89 (0.74–0.96)
PDur (s)	0.93 (0.84–0.98)	0.95 (0.87–0.98)	0.96 (0.90–0.99)	0.87 (0.69–0.95)
PImp (Ns)	0.91 (0.78–0.97)	0.91 (0.80–0.97)	0.93 (0.84–0.98)	0.76 (0.44–0.91)
MBV (m/s)	0.94 (0.85–0.98)	0.87 (0.67–0.95)	0.97 (0.93–0.99)	0.79 (0.50–0.93)
PBV (m/s)	0.96 (0.91–0.99)	0.96 (0.89–0.99)	0.94 (0.84–0.98)	0.91 (0.77–0.97)

BMF = net braking mean force; BDur = braking duration; BImp = net braking impulse; PMF = net propulsion mean force; PDur = propulsion duration; PImp = net propulsion impulse; MBV = mean barbell velocity; PBV = peak barbell velocity.

, while the mean and standard deviation of each variable produced during the TRAD and AEL back squat conditions are displayed in

Table 2. Mean and standard deviation of braking performance variables for traditional (TRAD), maximal accentuated eccentric loading (AEL-MAX), and supramaximal AEL (AEL-SUPRA) back squats.

Condition	BMF (N/kg)	g (95% CI)	BDur (s)	g (95% CI)	BImp (Ns)	g (95% CI)
		50% 1RM a,b,c
TRAD	4.3 ± 1.1	-	0.46 ± 0.10	-	130.2 ± 15.9	-
AEL-MAX	5.3 ± 1.2	0.78 (0.02–1.55)	0.50 ± 0.10	0.38 (−0.37–1.12)	180.6 ± 20.5 *	2.67 (1.65–3.69)
AEL-SUPRA	5.7 ± 1.4 *	1.03 (0.24–1.81)	0.50 ± 0.10	0.40 (−0.35–1.14)	195.7 ± 22.6 *	3.25 (2.13–4.38)
		60% 1RM a,b
TRAD	3.9 ± 1.1	-	0.51 ± 0.09	-	131.6 ± 14.4	-
AEL-MAX	4.9 ± 1.1	0.92 (0.14–1.70)	0.54 ± 0.09	0.35 (−0.40–1.10)	184.9 ± 21.4 *	2.84 (1.79–3.89)
AEL-SUPRA	5.2 ± 1.1 *	1.11 (0.32–1.91)	0.52 ± 0.07	0.14 (−0.61–0.88)	187.3 ± 18.3 *	3.27 (2.14–4.41)
		70% 1RM a
TRAD	3.8 ± 1.2	-	0.54 ± 0.12	-	134.7 ± 20.2	-
AEL-MAX	4.4 ± 1.2	0.49 (−0.26–1.24)	0.58 ± 0.09	0.39 (−0.36–1.14)	172.4 ± 18.3 *	1.90 (1.01–2.80)
AEL-SUPRA	4.7 ± 1.0	0.78 (0.01–1.54)	0.56 ± 0.07	0.28 (−0.46–1.03)	183.9 ± 13.0 *	2.82 (1.77–3.86)
		80% 1RM
TRAD	3.1 ± 1.1	-	0.64 ± 0.16	-	130.9 ± 17.7	-
AEL-MAX	3.7 ± 1.0	0.48 (−0.27–1.23)	0.64 ± 0.09	−0.01 (−0.75–0.73)	159.1 ± 24.9 *	1.27 (0.46–2.08)
AEL-SUPRA	3.9 ± 0.9	0.71 (−0.05–1.47)	0.64 ± 0.10	−0.02 (−0.76–0.72)	167.5 ± 18.0 *	1.99 (1.08–2.90)

% 1RM based on the concentric load used during the movement; BMF = net braking mean force; BDur = braking duration; BImp = net braking impulse; g = Hedge’s g effect sizes with 95% confidence intervals indicating the differences between the TRAD and AEL conditions; * = significantly greater than TRAD (p < 0.02); italics = practically meaningful difference based on Hedge’s g effect size confidence intervals; ^a = significantly different from values at 80% 1RM (p < 0.001); ^b = significantly different from values at 70% 1RM (p < 0.001); ^c = significantly different from values at 60% 1RM (p < 0.05).

,

Table 3. Mean and standard deviation of propulsion performance variables for traditional (TRAD), maximal accentuated eccentric loading (AEL-MAX), and supramaximal AEL (AEL-SUPRA) back squats.

Condition	PMF (N/kg)	g (95% CI)	PDur (s)	g (95% CI)	PImp (Ns)	g (95% CI)
		50% 1RM a,b,c
TRAD	3.9 ± 0.7	-	0.57 ± 0.07	-	150.6 ± 23.5	-
AEL-MAX	3.7 ± 0.6	−0.18 (−0.92–0.57)	0.60 ± 0.06	0.53 (−0.23–1.28)	155.6 ± 27.4	0.19 (−0.55–0.93)
AEL-SUPRA	3.8 ± 0.6	−0.13 (−0.87–0.61)	0.63 ± 0.08	0.81 (0.04–1.58)	163.0 ± 23.0	0.52 (−0.23–1.27)
		60% 1RM a,b
TRAD	3.4 ± 0.4	-	0.66 ± 0.08	-	157.2 ± 18.1	-
AEL-MAX	3.4 ± 0.5	−0.10 (−0.66–0.82)	0.69 ± 0.09	0.36 (−0.39–1.10)	162.0 ± 21.5	0.23 (−0.51–0.98)
AEL-SUPRA	3.5 ± 0.6	0.08 (−0.66–0.82)	0.71 ± 0.09	0.57 (−0.18–1.33)	171.1 ± 24.4	0.63 (−0.13–1.39)
		70% 1RM a
TRAD	3.1 ± 0.4	-	0.81 ± 0.09	-	172.4 ± 21.3	-
AEL-MAX	2.9 ± 0.4	−0.45 (−1.21–0.30)	0.88 ± 0.14	0.57 (−0.19–1.32)	176.0 ± 29.0	0.14 (−0.61–0.88)
AEL-SUPRA	3.1 ± 0.4	−0.09 (−0.83–0.65)	0.84 ± 0.11	0.32 (−0.42–1.07)	177.6 ± 24.6	0.22 (−0.52–0.96)
		80% 1RM
TRAD	2.5 ± 0.4	-	1.06 ± 0.16	-	179.5 ± 17.8	-
AEL-MAX	2.3 ± 0.3	−0.56 (−1.32–0.19)	1.16 ± 0.17	0.60 (−0.16–1.35	182.7 ± 20.2	0.16 (−0.58–0.91)
AEL-SUPRA	2.4 ± 0.5	−0.28 (−1.03–0.46)	1.17 ± 0.26	0.51 (−0.25–1.26)	187.9 ± 25.6	0.37 (−0.38–1.12)

% 1RM based on the concentric load used during the movement; PMF = net propulsion mean force; PDur = propulsion duration; PImp = net propulsion impulse; g = Hedge’s g effect sizes with 95% confidence intervals indicating the differences between the TRAD and AEL conditions; italics = practically meaningful difference based on Hedge’s g effect size confidence intervals; ^a = significantly different from values at 80% 1RM (p < 0.001); ^b = significantly different from values at 70% 1RM (p < 0.001); ^c = significantly different from values at 60% 1RM (p < 0.001).

and

Table 4. Descriptive barbell velocity data for traditional (TRAD), maximal accentuated eccentric loading (AEL-MAX), and supramaximal AEL (AEL-SUPRA) back squats.

Condition	MBV (m/s)	g (95% CI)	PBV (m/s)	g (95% CI)
	50% 1RM a,b,c
TRAD	0.80 ± 0.07	-	1.27 ± 0.14	-
AEL-MAX	0.78 ± 0.07	−0.18 (−0.93–0.56)	1.25 ± 0.14	−0.22 (−0.97–0.52)
AEL-SUPRA	0.77 ± 0.07	−0.17 (−0.91–0.58)	1.24 ± 0.15	−0.07 (−0.81–0.67)
	60% 1RM a,b
TRAD	0.70 ± 0.07	-	1.18 ± 0.12	-
AEL-MAX	0.69 ± 0.07	−0.16 (−0.90–0.58)	1.18 ± 0.12	0.01 (−0.73–0.75)
AEL-SUPRA	0.70 ± 0.07	0.07 (−0.67–0.82)	1.17 ± 0.13	−0.08 (−0.82–0.66)
	70% 1RM a
TRAD	0.62 ± 0.07	-	1.12 ± 0.11	-
AEL-MAX	0.59 ± 0.08	0.05 (−0.69–0.79)	1.09 ± 0.14	−0.26 (−1.00–0.48)
AEL-SUPRA	0.61 ± 0.06	0.19 (−0.55–0.93)	1.10 ± 0.13	−0.12 (−0.86–0.62)
	80% 1RM
TRAD	0.49 ± 0.06	-	1.02 ± 0.12	-
AEL-MAX	0.47 ± 0.07	−0.11 (−0.85–0.63)	0.99 ± 0.12	−0.19 (−0.93–0.55)
AEL-SUPRA	0.47 ± 0.08	0.19 (−0.56–0.93)	1.00 ± 0.11	−0.14 (−0.88–0.60)

% 1RM based on the concentric load used during the movement; MBV = mean barbell velocity; PBV = peak barbell velocity; ^a = significantly different from values at 80% 1RM (p < 0.001); ^b = significantly different from values at 70% 1RM (p < 0.001); ^c = significantly different from values at 60% 1RM (p < 0.001).

.

Condition × Load Interaction Effects

Significant condition × load interaction effects were present for braking mean force (p = 0.048) and impulse (p < 0.001); however, the interaction effects for braking duration (p = 0.559), propulsion mean force (p = 0.583), duration (p = 0.182), and impulse (p = 0.647) were not statistically significant. In addition, there were no significant condition × load interaction effects present for MBV (p = 0.376) or PBV (p = 0.654). Post hoc significant differences are displayed in Table 2.

4. Discussion

The objectives of this study were to examine force–time and barbell velocity characteristics of traditional, AEL-MAX, and AEL-SUPRA back squats using various loading schemes in resistance-trained women. The main findings of this study were that significantly greater braking mean force and impulses were produced during the AEL conditions across all loads. Conversely, there were no other statistically or practically meaningful differences in the braking, propulsion, or barbell velocity characteristics between the TRAD and AEL conditions. Finally, the external load had a significant impact on braking and propulsion force, duration, and impulse and barbell velocity characteristics.

Similar to the findings of previous AEL studies on resistance-trained men [11,12,13], the women in the current study showed the greatest overall braking stimulus during the AEL-SUPRA condition. Specifically, the differences in braking mean force and impulse were moderate and large–very large compared to TRAD back squats across the examined loads, respectively. In addition, the braking mean forces and impulses produced during the AEL-MAX condition also displayed small–moderate and large–very large differences compared to the TRAD condition, respectively. In contrast to braking mean force and impulse, there were no differences in braking duration between the AEL and TRAD squats (trivial–small effect sizes). These findings agree with those of Suchomel and colleagues who used the same protocol with resistance-trained men [13] and suggest that, combined with greater braking mean forces, AEL may allow individuals to create a larger “peaked” impulse (i.e., a greater rate of force development). However, given that there were generally only small differences between the AEL-SUPRA and AEL-MAX conditions for braking mean force and impulse, it could be argued that supramaximal loads may not always be needed to provide an enhanced braking stimulus. However, if athletes are able to actively resist supramaximal loads, the additional load may benefit the development of eccentric strength during the back squat.

With the exception of a shorter propulsion phase duration produced during TRAD squats performed with 50% 1RM (moderate difference), there were no significant or practically meaningful differences in propulsion force–time characteristics between squat conditions. These findings are supplemented by non-significant, trivial–small differences between squat conditions for MBV and PBV across the examined loads. The lack of significant differences between conditions in the current study are similar to the findings found in men during the back squat [11,12,13], but in contrast to those during the bench press [14,15,16]. As mentioned previously, limited research has examined the force–time characteristics of AEL in resistance-trained women; thus, it is unclear if the positive propulsion effects shown during the bench press in previous studies would carry-over into this population. The lack of acute enhancement in propulsion force–time characteristics shown in the current study may be explained by the comparison of the average performance across all the repetitions performed in each exercise set. Researchers have shown that including AEL on the first repetition of a five-repetition set subsequently enhanced the eccentric rate of force development during the subsequent three repetitions, but performance returned to baseline during the 4th and 5th repetitions [12]. Thus, there may be carry-over effects from the first repetition to subsequent repetitions in a set; however, a further analysis of individual repetitions is needed to determine if these effects existed in the current population.

Suchomel and colleagues [13] indicated that a lack of acute benefits to the propulsion phase of AEL squats may be due to the wide range of relative strength characteristics within a subject population. Moreover, Merrigan et al. [30] indicated that only those with greater relative strength appeared to show within-set potentiation effects during AEL bench press sets. These findings, combined with additional back squat potentiation literature [31], support the notion that greater relative strength is needed to realize potentiation benefits. The relative strength of the women in the current study ranged from 1.10 to 1.77 times their body weight; however, it is unclear if this range contributed to a lack of within-set potentiation effects. While investigating injury rates in collegiate athletes, Case et al. [32] suggested that a relative back squat of 1.6 times body weight may serve as a strength standard indicative of a lower likelihood of injury in collegiate female athletes. Thus, future research is needed to examine the differences in AEL performance between resistance-trained women using the previous strength standard as a criterion for stronger and weaker participants. Given that AEL has been classified as an advanced training method [4], determining the potential differences would benefit practitioners looking to implement AEL within resistance training programs.

Similar to the previous study examining the force–time characteristics of various AEL-MAX and AEL-SUPRA loading combinations in men [13], it was determined that the largest braking net mean forces were produced during short durations when the load spread (i.e., the difference in load between AEL total load and barbell load) was greater. These findings support the notion that larger load spreads during AEL may serve as an effective stimulus for developing rapid braking force characteristics. In contrast, provided there is a large enough load on the barbell, a smaller load spread during AEL may serve as a more effective stimulus for maximal force production and strength adaptations. While this study supports the findings from the previous study investigating AEL during the back squat with men [13], only one other study used wide load spreads during the bench press (i.e., eccentric loads of 100 and 110% 1RM combined with 30–80% 1RM concentric loads) [15]. Thus, researchers are encouraged to investigate wide load spreads with more exercises to provide insight on AEL stimuli and benefit implementation strategies.

There are several limitations that should be acknowledged within the current study. First, the sample size in the current study was relatively small and thus, the conclusions drawn may need to be interpreted with caution. While the results from this study are valuable, larger samples may provide a more comprehensive understanding of AEL. Second, only resistance-trained women were included as participants; however, to the authors’ knowledge, this is the first study to examine the differences in force–time and barbell velocity characteristics between TRAD and AEL back squats in this population. While previous AEL research included female participants [7,33], it is the authors’ hope that the current study serves as the foundation for further AEL research with a population exclusively made up of women. Finally, only the first repetition of each set was performed with each AEL load combination. While this study replicated the methodology of previous research [13], further research is needed using AEL on multiple repetitions as well as over multiple sets to determine to what extent the training stimulus is modified.

5. Conclusions

Resistance-trained women displayed a greater overall braking stimulus (mean force and impulse) during AEL-SUPRA and AEL-MAX back squats compared to TRAD squats. However, there were no notable differences between conditions during the propulsion phase of the back squat repetitions regardless of the concentric load. Strength and conditioning practitioners should consider using larger or smaller load spreads during AEL to benefit rapid braking force production characteristics and maximal strength characteristics, respectively. Due to the potential for within-set potentiation effects, practitioners should consider AEL as an advanced training strategy that may not be implemented with weaker athletes.