The impact of repetition mechanics on the adaptations resulting from strength-, hypertrophy- and cluster-type resistance training

Resistance training (RT) is considered the primary modality when maximal strength development is the training goal (Baechle and Earle 2008) and the magnitude of adaptations resulting from an RT programme is known to be dependent on the specific combination of training-session components [load (mass lifted), volume, contraction velocity] in addition to other possible confounding variables (e.g. nutritional status). From a scientific perspective, evidence suggests that different combinations of training-session components result in different external mechanical stimuli (e.g. time under tension, Crewther et al. 2005) and that these stimuli interact to produce different acute physiological (e.g. metabolic) responses (Crewther et al. 2006). When applied over a period of time, these mechanical and physiological stimuli influence the type and magnitude of training adaptations (Kraemer et al. 1990; Goto et al. 2005).

Although ‘light’ and ‘maximal’ RT workouts may not be mutually exclusive (Burd et al. 2012), coaches typically distinguish between strength- (STR), hypertrophy- (HYP) and endurance-type RT. Based on the current terminology, it would appear that STR regimens involving high loads [≥85 % of one repetition maximum (1RM)], low volumes (2–6 sets, ≤6 repetitions) and long inter-set rest intervals (3–5 min) represent the optimal method of improving maximal strength (Baechle and Earle 2008) with the current recommendations reflecting the prevailing view (Carpinelli 2008) that high loads (≥85 % 1RM) need to be utilised to maximise strength development. The main basis for the prescription of high loads is based on the size principle of motor unit recruitment (Henneman 1957) and research which supports the need for progressively higher forces to enable the recruitment of the higher threshold motor units (Gordon et al. 2004). Although the prescription of high loads during STR regimens would appear to be based on a sound scientific basis, studies exist which suggest that higher threshold motor units can also be recruited in the absence of high loads when blood flow restriction is present (Moore et al. 2004) or in the latter stages of submaximal exercise when fatigue levels are higher (Houtman et al. 2003). These observations may explain why the longitudinal evidence in support of STR regimens is far from unequivocal (Carpinelli 2008).

Considering the conflicting literature, it is perhaps not surprising that there is an emerging body of evidence which questions the importance of high loads for maximal strength development with metabolic stress and the associated muscle ischemia also being put forward as key stimuli (Goto et al. 2005). Despite the fact that metabolite accumulation as indicated by blood lactate and hydrogen ion accumulation is known to be maximised following HYP regimens that involve high volumes (3–6 sets, 8–12 repetitions), moderate loads (67–80 % 1RM) and short rest intervals (30–90 s) (Nicholson et al. 2014), the need for metabolite accumulation has often been argued based on the effectiveness of other training practices since low-load RT workouts involving blood flow restriction have shown to be effective for enhancing strength and muscle hypertrophy (Takarada et al. 2002). Although it has been suggested that metabolic stress may be an important driver of the intracellular protein synthesis signalling pathway (Schoenfeld 2010), recent research suggests that regimens involving high metabolic stress may not be as effective for enhancing the neural contributions to strength gains (Moore et al. 2004; Sakamaki et al. 2012). Furthermore, the role(s) of metabolic stress and circulating hormones in the hypertrophic response is not fully understood (West et al. 2009) and evidence actually exists which suggests that the metabolic stimulus is unlikely to be the primary and only stimulus for strength development (Yasuda et al. 2011). Instead, evidence suggests that elevated metabolic responses in combination with high loads may have an additive effect on the development of maximal strength (Goto et al. 2005). Despite the scientific underpinning, a poor understanding exists regarding the magnitude of metabolic stress required to optimise strength adaptations.

In addition to research which supports the need for metabolic stress, evidence exists to suggest that high-threshold motor unit recruitment is also possible when the acceleration of a load is optimised (Linnamo et al. 2000). Whilst it is generally agreed that elevating lifting velocity through the use of low loads will result in smaller strength improvements than high-load training (Kaneko et al. 1983), recent research presents the possibility that lifting velocity may be elevated without sacrificing the load lifted. More specifically, the inclusion of short (30–90 s) rest intervals in between small groups of repetitions has been termed ‘cluster set’ training (CL). Compared to traditional STR workouts, CL workouts have been shown to allow greater velocities and power outputs to be maintained over multiple sets (Haff et al. 2003) albeit with a lower level of metabolic stress (Goto et al. 2005). Whilst the ability to optimise repetition kinematics without sacrificing training load represents an exciting proposition, a poor understanding exists concerning the relative importance of these kinematic responses for maximal strength development (Crewther et al. 2005). Furthermore, it has been theorised that kinetic responses such as force, time under tension and the product of the two (impulse) may play a key role in the strength training stimulus (Crewther et al. 2005; Neils et al. 2005). Whilst these theories appear logical, there is a lack of research that has examined the relative importance of repetition mechanics, metabolic stress and chronic adaptations within the same investigation. It is, therefore, not surprising that a lack of consensus exists regarding the best ways to utilise the CL concept.

At present, studies exist both for (Oliver et al. 2013) and against (Goto et al. 2005) the effectiveness of CL when the goal is maximal strength development. A major limitation of previous CL studies is that they have failed to consider that more frequent rest allows higher loads to be utilised without sacrificing repetition number (Iglesias et al. 2010). Although the ability to enhance volume load in this manner may enhance motor unit recruitment, it is at odds with the so-called ‘repetition maximum continuum’ (Baechle and Earle 2008). Despite evidence which supports greater neural adaptations following explosive (Linnamo et al. 2000) and heavy (Hakkinen et al. 1987) RT, there is a lack of information regarding the neural responses to CL regimens (Iglesias-Soler et al. 2015). Research which examines the neural responses to CL training will assist practitioners in designing RT programmes for sports in which relative strength is important.

The aim of this study was to compare the acute (metabolic and mechanical) and chronic responses to STR, HYP, and two novel CL regimens involving the back-squat exercise. This approach was intended to answer key questions regarding the magnitude and type (i.e. neural) of adaptations resulting from workouts which set out to emphasise contrasting mechanical and metabolic responses. We hypothesised that the CL regimens would optimise the acute kinematic and kinetic responses with an attenuated metabolic response and that a CL regimen which permits a higher load would result in the largest increases in strength and muscle activity.

This is the first study designed to investigate the chronic responses to traditional STR and HYP regimens and CL regimens which have intended to elevate repetition velocity and volume load. Our findings demonstrate that both types of CL regimen do not offer clear benefits for the development of maximal dynamic strength over STR regimens following a 6-week training period. The STR and higher volume load CL regimen did, however, elicit significantly greater improvements in maximal strength than the HYP regimen. From a scientific perspective, the findings enhance the understanding of the mechanical stimuli underlying strength adaptations indicating that the superiority of the STR and CL-2 regimens may have been associated with the optimisation of concentric impulse and TUT within each repetition performed during RT. In contrast, the smaller improvements demonstrated by the HYP and CL-1 regimens underlines that metabolic stress and repetition velocity are of secondary importance for the development of maximal strength.

The significantly greater improvements in 1RM for the STR and CL-2 regimens when compared to the HYP regimen are consistent with previous studies (Campos et al. 2002) which support the superiority of high-versus moderate-load regimens. In addition, the elevation of total resting time offered no significant benefits for maximal strength development which is consistent with the balance of previous research (Folland et al. 2002; Hansen et al. 2011). Although no significant differences were observed between the STR and CL-1 regimens in 1RM, the ES in STR (1.161) was larger than the ES in CL-1 (0.588). Whilst this observation is somewhat consistent with previous studies that have compared traditional and CL regimens equated by volume load (Goto et al. 2005), one of the key findings of the present investigation is that CL regimens which permit higher volume loads offer no benefits over STR regimens for the development of maximal strength. This implies that elevations in training-session duration (using CL workouts) may not be worthwhile when designing strength training programmes.

In line with the intended design of the workouts, metabolic stress was manipulated on a continuum type basis with the HYP workout resulting in the largest post-workout increases in BL concentration, the STR workout resulting in a smaller yet significant increase whilst the CL allowed the complete removal of any metabolic stimulus even when volume load was elevated. Enhanced metabolic accumulation as indicated by increases in BL concentration following the STR (244.84 %) and HYP (422.88 %) workouts is consistent with previous investigations (Kraemer et al. 1990; Nicholson et al. 2014). The present findings are in contrast to previous research (Denton and Cronin 2006) which reported significant increases in BL concentration following CL workouts. The heightened metabolic responses in the research most likely resulted from the equation of resting time between the regimens and the inclusion of less frequent rest intervals in the CL regimens which is often overlooked when interpreting the previous literature. From a scientific perspective, this is one of few studies that provide information regarding the relative importance of metabolic stress using exercises and workouts that are commonly used in training practice. Importantly, the post-training improvements for the CL groups suggest that metabolic stress is not the primary mechanism underpinning maximal strength adaptations. Whilst this finding is at odds with some previous research into blood-flow restriction (Takarada et al. 2002), questions still remain regarding the role of metabolic stress and circulating hormones in mediating muscle hypertrophy (West et al. 2009). Furthermore, evidence exists (Sakamaki et al. 2012) to suggest that light-load training with higher levels of metabolic stress may favour hypertrophy-specific strength gains which may not be desirable in sports where relative strength is important.

Since the large strength improvements in the CL-2 regimen suggest that metabolic stress is not paramount for maximal strength development, closer inspection of the acute mechanical characteristics may provide more information regarding the stimuli underlying strength adaptations. The mechanical measurements made during the four regimens demonstrated that the cumulative mechanical performance (over the course of a training session) was greatest during the HYP workout. Despite the fact that a greater cumulative performance during high-volume workouts has been previously used to explain studies which have not observed differences between high- and low-load regimens (Crewther et al. 2005), very few studies have examined acute mechanical performances alongside chronic changes in strength. The smaller 1RM improvements in the HYP regimen, therefore, provide important evidence against the importance of cumulative mechanical performance for maximal strength development. Instead, the greatest 1RM improvements occurred in the STR and CL-2 regimens which were characterised by greater peak mechanical responses, as has been previously suggested (Crewther et al. 2005). Crucially, the present findings, therefore, support the need to optimise the mechanics associated with each individual repetition when strength development is the training goal.

In terms of the importance of specific mechanical variables, it would appear that the CL-1 regimen was not optimally designed for strength development since the ability to offset fatigue-induced reductions in repetition velocity and power did not translate to greater strength improvements. In some respects, this observation supports the specificity of training adaptations (Kaneko et al. 1983) since maximal strength assessments are not typically associated with higher velocities and power outputs. Instead, it seems logical that the slower repetitions in the STR and CL-2 regimens closely resembled the 1RM assessment and contributed to the larger strength improvements in these regimens. More specifically, the higher load in the CL-2 workout and the fatigue-induced changes during the STR workout resulted in longer TUT and higher force outputs. The present findings actually support the product of these two variables (impulse) for strength development since impulse was greatest in the STR and CL-2 regimens. The notion that impulse, produced by elevating both TUT and force output and not by augmenting only one of the two variables (i.e. as in HYP regimens), may represent the critical stimulus underpinning strength development is consistent with previous investigations (Schuenke et al. 2012) which have demonstrated that the ability to maximise TUT may yield superior strength improvements as long as force output is not compromised. From a physiological perspective, it is possible that the higher impulse generation in the STR and CL-2 regimens may have resulted in the inhibition of force-feedback reflex mechanisms (McDonagh and Davies 1984) and/or an increased level of MU recruitment (Gordon et al. 2004). The stimulus-tension theory states that training loads need to be near-maximal and of sufficient duration if motor unit recruitment is to be maximised (Komi and Buskirk 1972); however, evidence exists on the contrary with similar neural responses being observed between different loading zones when the intensity of effort is maximised (Houtman et al. 2003; Carpinelli 2008). Whilst the present study does not provide firm evidence in support of any of the proposed mechanisms, the findings do support the positive effects of load elevation during CL regimens and the load–fatigue interaction during STR regimens to maximise impulse generation.

Although the dynamic strength improvements were also accompanied by mean increases in isometric strength measured during the isometric back-squat assessment, the improvements in isometric back-squat strength (5–9 %) were smaller than the 1RM increases (9–15 %) and no differences were observed between the regimens in the isometric strength improvements. This coheres with prior research (Coyle et al. 1981) which suggests that for a valid assessment of strength gain from a resistance training regimen, the testing modality should closely resemble the training conditions (i.e. type of muscle action, etc.). Given that the isometric back-squat protocol previously demonstrated high between-session reproducibility and significant correlations with the 1RM back squat (Nicholson and Bissas 2015), this highlights the need for coaches to closely consider the sensitivity of multi-joint isometric assessments for monitoring dynamic training improvements.

Metabolite accumulation has been proposed to mediate muscle hypertrophy via a range of mechanisms and although different levels of metabolic stress were observed, no measure of muscle hypertrophy was included in the present study. Importantly, however, the increases in isometric strength were accompanied by increases in sEMG of the key leg muscles along with significant main effects of time. Increases in sEMG activity have been widely used in support of neural contributions to increases in force output following a period of RT (Aagaard et al. 2000). It is important to consider, however, that increases in sEMG activity may not be entirely attributable to neural factors because a number of non-neural factors (e.g. blood flow, subcutaneous tissue) are also known to influence the recorded signal (De Luca 1997). Furthermore, the limitations of this technique do not allow specific changes (e.g. increased MU recruitment/firing rate) to be identified. Whilst the increases in neural drive are consistent with research which supports neural contributions during the initial stages of training (Chilibeck et al. 1998), the study provides further support to previous research (Hakkinen et al. 1987) which has highlighted a possible role for neural adaptations beyond the initial weeks of training. Whilst the findings are in agreement with previous studies that have observed increases in sEMG following heavy RT (Aagaard et al. 2000), there was no evidence to suggest that the STR or CL regimens resulted in greater neural stimuli despite the fact that the elevation of velocity (Linnamo et al. 2000) and load (Hakkinen et al. 1987) has been previously linked to increased neural contributions. In this respect, the present findings are in agreement with previous research (Iglesias-Soler et al. 2015) which has not observed differences in neural adaptations between traditional and CL regimens even using more clinical techniques (e.g. Interpolated Twitch Technique).

Despite the relatively short duration of the training intervention, the strength gains were transferred to explosive performance improvements as demonstrated by increases in isometric RFD, isokinetic peak torque and vertical jump height which are consistent with some previous investigations (Folland et al. 2002; Cormie et al. 2010). The fact that the increases in isokinetic peak torque at 30^°/s were confined to the STR, CL-1 and CL-2 regimens was expected (Coyle et al. 1981) since the aforementioned regimens involved higher loads and slower velocities than the HYP regimen. Limited additional support for the concept of velocity specificity can be gained; however, no differences were observed in the magnitude of the improvements in isokinetic peak torque measured at 180^°/s, isometric RFD or jump performance despite the HYP and CL-1 workouts emphasising repetition velocity to a greater extent than the STR and CL-2 regimens. In this respect, the findings are not at odds with the theory that it is the intention to move a resistance as fast as possible that is the key stimulus for improvements in explosive performance (Behm and Sale 1993). The lack of support for the concept of velocity specificity perhaps results from the use of the back squat as the training exercise, the examination of a relatively short training period or the failure to include a low-load ballistic training group.

Based on the present findings, it would appear that CL regimens do not offer clear benefits over STR regimens for the development of maximal strength although there exists the possibility that differences may have been observed if a longer block of training (e.g. >2 months) had been examined. The present findings, however, provide a valuable insight into the adaptations resulting from short blocks of training (6–8 weeks) that are commonly employed by coaches. From a practitioner’s perspective, it was interesting to note that the CL-1 regimen was associated with a significantly lower average RPE over the training intervention than the HYP regimen. Whilst some authors have concluded that perceived exertion is unaffected by rest interval length (Pincivero et al. 1999), the findings are consistent with research (Hardee et al. 2012) which supports lower RPE values when RT is performed with intra-set rest intervals. In this respect, the findings suggest that CL regimens provide an effective means of increasing maximum strength with lower levels of perceived exertion which may have implications for training adherence, motivation and the avoidance of overtraining. Interestingly, participants in the CL-1 regimen reported that the intra-set rest intervals initially enabled them to focus on the ‘quality’ of their back-squat repetitions, which is supported by the acute kinematic performance. On the contrary, it is important to note that the monotony of the CL regimens also negatively impacted on participants’ motivation at times and that the RPE and RPS of the CL-2 regimen increased sharply after the mid-point of training. Although more research is needed which includes more scientific markers (e.g. salivary IgA levels), the findings provide some evidence to suggest that CL regimens may be more suited to shorter, more targeted periods of training (<3 weeks).

In conclusion, CL regimens did not provide a more effective alternative to STR regimens for the development of maximal strength, explosive performance or the level of neural adaptations. Importantly, however, CL regimens were effective at reducing the metabolic demands of RT, limiting fatigue-induced reductions in repetition mechanics and allowed the utilisation of load-volume combinations that do not conform to the typical repetition maximum continuum (Baechle and Earle 2008). Based on these findings coaches should be mindful that CL regimens may result in contrasting acute responses to traditional STR regimens and as a result, rest interval frequency should be considered alongside rest interval duration when designing strength training interventions. From a scientific perspective, it would appear that the optimisation of repetition kinematics does not lead to greater strength adaptations but may be effective at reducing the level of perceived exertion. Instead, the use of STR regimens or CL regimens which elevate training load would seem to elevate the kinetic responses (i.e. impulse) that are central to the strength training stimulus. Furthermore, the optimisation of metabolic stress did not seem paramount to the strength improvements observed which is contrary to some recent suggestions. The findings highlight that intra-set as well as inter-set rest intervals are key considerations when designing resistance training regimens aimed at developing maximal strength.