An 8-week intervention with cluster training: The effects of two different inter repetition rest periods on maximal strength in the back squat.

Introduction

Cluster training is a system where the use of short rest periods between repetitions are used (Haff et al., 2003; Tufano, Brown & Haff, 2017). These pre-planned rest periods typically occur between 15-45 seconds (Haff, Burgess & Stone, 2008). The use of short rest periods between sets allow for a positive effect upon repetition performance whilst minimising fatigue which is associated with the use of traditional set structures (Tufano, Halaj, Kampmiller, Novosad, & Buzgo, 2018). During traditional sets, movement velocity and power output decreases to a greater extent than when using cluster set structures (Tufano et al., 2018). This is interesting and supports the use for cluster set protocols for when seeking adaptations for power development. It has been hypothesised that 15-30 seconds of rest periods between repetitions in a set allows for partial replenishment of phosphocreatine (PCr) stores and adenosine triphosphate (ATP) whilst traditional sets increase the production of lactate and lactic acid due to a greater demand for the use of muscle glycogen (Haff et al., 2003). Furthermore, with an elevation of lactate concentration there is a significant decrease in the amount of force generating ability (Sahlin & Ren, 1989). The authors suggest there is a positive increase in maximal force generating capacity with additional rest periods during a set. This provides evidence to support the use of pre-planned rest periods during a set when seeking improvements in strength and power. Some studies suggest cluster set training may be most beneficial to use for the improvements of explosiveness and ballistic methods such as weightlifting methods (Lawton, Cronin, & Lindsell, 2006). The current literature also supports this as most of the exercises investigated during cluster protocols have been on weightlifting movements (Haff et al., 2008). Rooney, Herbert, and Balnave (1994) suggests that the use of cluster sets do reduce the amount of fatigue however do not result in the same strength improvements when compared to traditional set configurations. This is based on the theory that when using traditional sets, an increased activation of high threshold motor units and the production of metabolic fatigue induces adaptations for developing strength (Lawton et al., 2006; Rooney et al., 1994). Studies have suggested that cluster set training offers no long-term strength gain benefit however not many of these studies to date have long enough training studies to ascertain this. For the development of muscular strength, it has been thought that the importance of fatigue should be present (Krieger, 2009). However, training to failure is not regarded as a necessary means when trying to develop maximal strength (Drinkwater et al., 2007; Folland, Irish, Roberts, Tarr, & Jones, 2002). When trying to develop maximal strength, studies suggest that cluster set training may be beneficial because of the better maintenance of movement velocity as opposed to traditional set training whereby velocity decreases (Iglesias et al., 2016; Oliver et al., 2013). Not only does cluster set training allow for greater maintenance of movement velocity and quality of repetitions, the use of them also allow for a greater number of repetitions to be performed with a given load which therefore results in a greater volume load (Denton & Cronin, 2006; Iglesias et al., 2014). Increases in volume load have been shown to be a way to create a stimulus for the improvement of maximal strength (Krieger 2009; Sooneste, Tanimoto, Kakigi, Saga, & Katamoto, 2013). Lower body strength training with heavy loads at 85% of 1 repetition maximum (1RM) is certainly a way to increase maximal strength, this essentially is a result of neural and coordination adaptations such as motor unit recruitment, synchronization, activation of muscles and increased recruitment of muscle units (Behm & Sale, 1993). Training at these intensities therefore also allow for an increase in athletic performance along with performance markers (Nimphius & McGuigan, 2010). The main premise for  85% of 1RM to be used for developing maximal strength is primarily based on the size principle of motor unit recruitment (Henneman, 1957) and how progressively higher forces enable the recruitment of higher threshold motor units. There are currently two studies that provide supportive evidence for the use of cluster training when trying to improve maximal strength (Nicholson, Ispoglou, & Bissas, 2016; Oliver et al., 2013). Although there are other studies surrounding the use of cluster set training as already suggested earlier in these writings, most of them fail to consider that a more frequent rest can allow higher intensities to be used without sacrificing repetition number (Iglesias, Boullosa, Dopico, & Carballeira, 2010). Interestingly, the study from Nicholson et al. (2016) investigated the acute and chronic responses to strength, hypertrophy and cluster resistance training and used various intensities. For the cluster set group, 90% of 1RM was used on the back squat and improvements were seen using 25 second inter repetition rest periods. Within the literature, there are two main types of rest periods associated with cluster training; intra set rest and inter repetition rest (Tufano et al., 2017). Intra set rest describes the rest periods associated between repetitions within cluster sets of more than 1 repetition whereas inter repetition rest describes the rest periods associated between single repetitions only. An example is shown in figure 1 along with how a traditional set structure is set up. Set structure is of huge importance not only for this study to have an outcome, but also because of the need for strength and conditioning coaches and sport scientists to not confuse terminology as has often been the case within the cluster training literature.

Fig. 1 Two sets of 4 repetitions with 120 seconds of inter set rest using 3 different set configurations. Arrows indicate number of repetitions performed in sequence, triangles indicate intra set or inter repetition rest periods, and quadrilateral shapes indicate inter set rest periods. (A) Traditional sets with neither intra set nor inter repetition rest. (B) Cluster sets doubles with intra set rest periods. (C) Cluster sets single with inter repetition rest periods. (Tufano, Brown & Haff, 2017).

Hypothesis/Aims

Much of the literature discussed and observed seems to mostly justify the effects cluster set training has on power. Whereas for the development of maximal strength, it seems that only acute variables are used within studies such as force production, training volume and muscle activity. These acute studies do provide supporting evidence to positively support the use of cluster set training and therefore allow scientists to extrapolate further hypotheses on maximal strength development. However, acute studies should not be used to determine the effect that chronic studies have on maximal strength development protocols (Tufano et al., 2017). This is a key issue within the current literature and why the current study being proposed will explore this area. The purpose of this investigation will therefore be to investigate the chronic effects of two different inter repetition rest periods during a cluster set protocol on back squat 1RM. Specifically, subjects will be tested on their 1 repetition maximum back squat strength at pre, mid and post intervention. The secondary investigation that will be examined will be the acute effects of the workouts on mean concentric velocity throughout all repetitions on every workout. The independent variable within this study will be the rest period, this is the variable that will be varied in both research groups. The dependant variable will be the maximal strength of the back squat, this will be the response that will be measured. Controlled variables held constant will be total volume load, intensity, the exercise type and order of exercises performed for both groups (Fig 2). The investigation will hypothesise that the cluster group 1 with longer inter repetition rest periods (30 seconds) will increase maximal strength greater than that of cluster group 2 (15 seconds) and from an acute perspective, will also maintain concentric barbell velocity greater than that of cluster group 2.

Cluster Group 1 (CG1)

3 sets of 5/1 repetitions @ 90% 1RM

30s inter-repetition rest

4 min inter-set rest

Pre-conditioning

Pre-testing

Mid-testing

Post-testing

Cluster Group 2 (CG2)

5 sets of 3/1 repetitions @ 90% 1RM

15s inter-repetition rest

4 min inter-set rest

Fig. 2 Schematic representation of the experimental groups and design. 5/1 and 3/1 denotes repetitions performed as singles.

Methods

Subjects

14 males aged between 25-40 years old will participate in this study. All participants will have at least 2 years of strength training experience using the barbell back squat and will all be experienced training with regular maximum or near maximum loads ( 6RM). Participants will all be able to back squat a minimum of 150% of their bodyweight for 1RM. To qualify for inclusion in the study, all subjects will be screened using a physical activity readiness questionnaire form (PAR-Q) which will be the same one used by St Marys University (Fig 3). Participants will be excluded if they report any musculoskeletal injuries. Participants will be able to continue with their usual upper body strength training throughout the intervention. During the 8-week training period, participants will not be allowed to participate in any other lower body strength training outside of the experiment. The only supplementation subjects will be allowed to consume will be protein supplementation, all other nutritional and/or ergogenic aids will be banned from use during the 8-week study. All participants will also need to give written consent prior to participation.

Study design and procedures

A longitudinal research design will be devised to compare the effects of two different inter repetition rest periods during cluster set training in a program designed to develop maximal strength in the back squat over 8 weeks. The 14 subjects will be randomly put into one of two groups. To eliminate any possible cofounding factors, the type and order of exercises performed and volume load will all be equal between both groups. This is critical to the design of the study as others have shown variations between these variables impact training adaptations (Campos et al., 2002). The volume, loads and rest intervals involved are shown in figure 2 and the training programme for the 8-week intervention is shown in table 2. Both groups will begin to work at an intensity of 90% of 1RM, group 1 will do 3 sets of 5 single repetitions whilst group 2 will do 5 sets of 3 single repetitions. The program will be split into two 4 week cycles with weeks 4 and 8 being a deloading week where subjects will reduce volume by 40% whilst maintaining or increasing training intensity. A reduction such as this has been shown as an effective way to enhance maximal muscular strength (Pritchard, Keogh, Barnes, & McGuigan, 2015). During the deload weeks, participants 1RM will be tested. The reason for a mid-intervention test will be to then adjust the new 90% of 1RM for the final 4-week training phase. The strategy used for progressive overload will be simply to only allow subjects to increase their weight if all repetitions are executed with success. Participants will perform their testing and workouts whilst being supervised by a certified strength and conditioning coach who will be part of the research project. Prior to baseline testing, all participants will undergo a familiarization session with a certified strength and conditioning coach to establish correct exercise technique for testing and training. A standardised 2-week pre-conditioning period will take place prior to baseline testing so that all participants are not only proficient at the back squat, but are also prepared psychologically for working at intensities at  90% of 1RM. During this 2-week period, participants will also have a group meeting and be advised on sleep, nutrition and recovery. Regards to sleep, participants will be advised to sleep 6-8 hours per day and try their best to maintain a regular sleep pattern. Nutritional advice for participants will be simply to maintain their regular eating habits and consider a regular 3-4 meals per day and adequate amounts of protein, calories and water. Recovery advice will be reflected in both sleep and nutrition advice, as well as keeping stress minimal and getting post workout nutrition on-board in the form of protein and carbohydrates. Prior to testing, baseline group characteristics will be measured for both groups and will be presented in Table 1.

CG1

CG2

Combined

p

Age (y)

Height (cm)

Body mass (kg)

Years trained

No. days trained per week

CG1 = cluster group 1; CG2 = cluster group 2; Combined = collapsed across time.

Data are mean ± SD

Table 1. Baseline group characteristics.

1 repetition maximum testing

Back squat 1RM will be performed as a testing measure for lower body maximal strength. Subjects will be asked to report to the training facility after having done no exercise for 48 hours before testing on week 1, week 4 and week 8 of testing. Training logs and self-reporting estimations of 1RM will allow loading on the barbell throughout the warm up to be as safe as possible during week 1. However, on weeks 4 and 8 of 1RM testing, training logs throughout the intervention will be used for warm up loading strategy. The warm up will consist of 5 minutes on an ergo rower (Indoor Concept 2, Model E). Once the subjects have done this, warm up sets will commence using an Olympic barbell (Klokov, IWF standardised 2.2m length, 20kg weighted barbell). The warm up will consist of a protocol used by McBride et al. (2009). 8-10 repetitions at an estimated 30% of 1RM, 4-6 repetitions at an estimated 50% of 1RM, 2-4 repetitions at an estimated 70% of 1RM and then 1 repetition at an estimated 90% of 1RM. Following this, weight will continue to increase until subjects’ 1 repetition maximum has been reached. All back-squat depth will be measured to a point of where the knee angle is at 70. This angle will be determined with the use of a goniometer during the familiarization session as previously mentioned. Subjects will be given up to 4 attempts to reach maximal 1RM and rest periods will be between 3-5 minutes. Foot and hand placement will be recorded at baseline to keep testing conditionings consistent as well as bar placement along with footwear used. All tests will be supervised by a certified strength and conditioning coach. The same method for testing will be used for all testing days; pre, mid and post intervention.

Intervention training

Throughout the training intervention, the use of an accelerometer (PUSH, Inc., Canada) will be used to monitor mean concentric velocity throughout. The device will be attached to either one of the arms on the subjects. Research has shown the PUSH device to have reliability (Balsalobre-Fernandez, Kuzdub, Poveda-Ortiz, & Campo-Vecino, 2016). The typical speed that experienced squatters maintain at 90% of 1RM is 0.34m/s, whereas the typical average squatter should be able to maintain a slightly faster velocity at about 0.46m/s (Zourdos et al., 2016). Helms et al. (2017) showed that powerlifters who are strong (squat 1RM average 202.5kg at 87.9kg BW) typically have an average velocity score of 0.44m/s when training the back squat at 90% of 1RM. This data suggests that the stronger more experienced squatters should be able to move heavy loads at slower velocities because of technical mastery and the ability to grind out loads at higher intensities. Average concentric velocity scores will be measured over the 2 training phases to establish acute effects of inter repetition rest periods on repetition quality. The use of the push band will be to also provide augmented feedback for the subjects as feedback can help increase force output (Kellis & Baltzopoulos, 1996). Because feedback can help maximise movement intent which is a critical component to strength adaptations the use of the push band to monitor mean concentric velocity will therefore be used. Ratings of perceived exertion (RPE) will be recorded after each training session using the 10-point scale (Borg, 1982). From this data, the mean RPE for the training phase will be established for each group and compared.

Data Analysis

The use of SPSS software will be used for data analysis. One-way analysis of variance (ANOVA) will be used to firstly determine if there are any differences in baseline characteristics (Table 1). Overall total volume load and training intensity will be analysed by independent t-test as well as overall 1RM mean differences between groups pre and post test. Independent t-test will also be used to determine the mean differences between concentric velocity of both groups pre and post test. A sample size of 14 subjects will be used, this was determined from ensuring a confidence level of 95% and a margin of error of 5%. A big limitation with this study will be the small sample size as this can affect the true effect. Because of this limitation, it would be beneficial to conduct a larger study in the future to provide further evidence. For all tests, statistical power will be of at least 80% and statistical significance will be defined as p 0.05.

Fig 3. Physical Activity Readiness Questionnaire.

Weeks 1-4

Lower Body 1

Sets

Reps

Rest

Back Squat

Cluster Protocol

240

DB Split Squat

4

8

90

Lying Leg Curl

4

6

90

Leg Press

3

10

90

BB Goodmorning

3

10

90

Ab Rollout

3

15

60

Lower Body 2

Sets

Reps

Rest

Back Squat

Cluster Protocol

240

Above The Knee Deadlift

4

6

180

45 Degree Back Extension

3

12

90

calf raise

3

12

90

Pallof press isometrics

3

5

60

Weeks 5-8

Lower Body 1

Sets

Reps

Rest

Back Squat

Cluster Protocol

240

Alternating DB Lunge

4

6

90

Seated Leg Curl

4

6

90

Romanian Deadlift

3

8

90

Reverse Hyperextension

3

12

90

Garhammer Raise

3

15

60

Lower Body 2

Sets

Reps

Rest

Back Squat

Cluster Protocol

240

Hex Bar Deadlift

4

4

180

BB Step Up

3

10

90

Standing Calf Raise

3

12

90

Decline Sit Up

3

15

60

BB = Barbell, DB = Dumbbell.

Table 2. Training program intervention, 2 workouts per week, weeks 1-4 & weeks 5-8.

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