Review Articles On Muscular Adaptations
Overview of Resistance Training Volume
Resistance training is the main exercise mode used to increase individuals muscle mass. It has been hypothesised that the volume of training performed per session has a significant role in chronic adaptation responses in both muscle strength and size (Kraemer and Ratamess, 2004). There is evidence that supports the use of multiple sets compared to single set training as multiple sets increase the phosphorylation of p70S6 kinase and muscle protein synthesis (MPS). This suggested that increased volume of training may maximise hypertrophic responses (Burd et al. 2010; Terzis et al. 2010). That said, when evaluating the results of longitudinal research, several studies failed to demonstrate significant differences in hypertrophy at lower and higher volumes. This is potentially due to the low sample sizes that raise the potential of type II errors, making it difficult in drawing inferences about training volume.
An excellent meta-analysis [meta-regression] by Schoenfeld et al. (2016) assessed the effects of total weekly resistance training volume on muscle mass. Fifteen studies met the inclusion criteria that comprised of 34 treatment groups. Schoenfeld and colleagues reported that weekly sets (continuous variable) showed a significant effect on muscle size changes (P = 0.002). They reported a dose-response relationship between the total number of weekly sets and increase in muscle hypertrophy. With each additional set being associated with an increase percentage size gain of 0.37% (effect size [ES] = 0.023). When assessing ES differences (ES = 0.241) between lower and higher volume a percentage gain difference of 3.9% for higher volumes were reported. However, Schoenfeld et al. stated that they could only determine a dose-effect response up to 10 weekly sets due to the limited available evidence.
Schoenfeld et al. (2018) conducted a further study that assessed muscular adaptations (strength and hypertrophy) between low, moderate and high volume resistance training protocols in resistance-trained men. The study involved 34 resistance-trained males who were randomly assigned to one of three groups (low volume [1-set per exercise]; moderate volume [3-sets per exercise]; and high volume [five sets per exercise]. All subjects trained three times per week on non-consecutive days for eight weeks. Muscular assessments consisted of 1RM squat and bench press testing for strength, upper body endurance by evaluating subjects 50%1RM bench press to failure. Muscle hypertrophy was evaluated using B-mode ultrasonography on the thighs (mid-thigh/ lateral thigh) and elbow flexors/extensors.
Pre-to post strength and endurance differences were significant for all groups with no differences between groups. However, when examining muscle hypertrophy all groups increased in muscle size in most measures, significant differences were reported with the higher volume routine for elbow flexors, mid thigh, and lateral thigh but not elbow extensors. This study reports that resistance-trained individuals can increase muscle strength with just 3x 13-minute session per week when training with a loading range of 8-to-12 repetitions per set. However, for increasing muscle size greater training volume may be required to achieve this goal.
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Overview of Resistance Training Loading
Resistance training (RT) has been recognised as an effective method for strengthening muscles, connective tissue, and enhancing physical functioning associated with sporting performance [ACSM, 2009]. These adaptive responses are a result of careful manipulation of acute training programme variables including load, volume, frequency, inter-set recovery, contraction velocity of movement, the range of motion, selection and order of exercise [ACSM, 2009]. However, our current state of knowledge concerning RT loading that results in ‘optimum’ strength adaptations are equivocal [Smith & Bruce-Low, 2004]. It has been largely assumed that performing a low number of repetitions per set (< six) with increased resistance loading is best for increasing muscular strength, and that increased number of repetitions (> 20) with low resistance loading (LL) will increase muscular endurance [Fleck & Kramer, 1997]. Campos et al.  stated that RT with three-to-five repetitions is an appropriate stimulus to increase skeletal muscle strength. However, Smith and Bruce- Low  state that a low to a moderate number of repetitions has also been shown to generate increases in muscular strength, with no repetition range proving superior.
Previous recommendations seem to favour high loading (HL) based on the findings of others [Aagaard et al. 1996; Fink et al. 2016; Jenkins et al. 2015; Mitchell et al. 2012; Schuenke et al. 2012; Tanimoto & Ishii. 1985]. The use of heavier RT loading to induce post-exercise muscle strength adaptations, have often been accepted and recommended as the superior mode to maximise muscular strength [ACSM, 2009, Kraemer & Ratamess, 2004; Baechle & Earle, 2008]. This has led to several professional exercise bodies and organisations recommending that HL is needed to maximise muscular strength. These organisations including the American College of Sports Medicine [ACSM, 2009] and the National Strength and Conditioning Association (NSCA)[Baechle & Earle, 2008] recommend that individuals use RT loading of ≥ 85% of the individuals one repetition maximum (1RM) to stimulate muscular strength development. However, the specific RT load and training variables that determine muscle strength development remain a contentious subject and one of continuous exploration [Jenkins et al. 2015]. Existing RT guidelines advocate that training loads of more than 70% 1RM are necessary to stimulate muscular-skeletal strength responses [ACSM, 2009]. These recommendations and standpoints are founded on the credence that HL initiate higher-threshold motor unit activation that aid in producing maximal muscular adaptations.
Recent evidence, however, demonstrates that LL RT performed to volitional concentric failure can be as effective as HL RT for muscular hypertrophy [Mitchell et al. 2012; Ogasawara et al. 2013; Schoenfeld et al. 2015; Van Roie et al. 2013]. Observations on previously untrained individuals demonstrated that training with LL might be equally an effective stimulus for muscular adaptations as HL RT [Schoenfeld, 2013; Raymond et al. 2013;Burd et al. 2010]. Moreover, Burd et al.  demonstrated that a single bout of RT exercise performed at 30% of subjects 1RM to the point of fatigue was equally effective in stimulating myofibril protein synthesis rates as loads lifted at 90% 1RM. Additionally, myofibril protein synthesis rates were greater, with 30% 1RM than the 90% 1RM after 24 hours. Some researchers [Leger et al. 2006; Alegre et al. 2015] provided evidence that when mechanical work is matched, both lighter and heavier RT loading provoke substantial gains in muscle strength and mass. However, some studies have reported that RT trainees have a diminished muscle protein synthetic response to RT [25-26], and may be less adaptable compared to untrained participant’s phenotypic adaptations [Mitchell et al. 2012; Burd et al. 2010].
Several meta-analyses support that training at HL is associated with increased muscle strength [Rhea et al.2003; Peterson et al. 2004; Peterson et al. 2005; Steib et al. 2010; Schoenfeld et al. 2016; Schoenfeld et al. 2017]. A meta-analysis by Rhea et al.  of 140 studies suggested that trained and untrained population groups responded differently with varying RT loading intensities (40-to-90% 1RM). Untrained participant’s effect size ([ES] 2.8 ± 2.3 standard deviation [SD]) suggested that a mean loading of 60% 1RM elicits maximal gains, whereas, trained individuals should use 80% 1RM (ES = 1.8 ± 1.3 SD). However, some limitations and variances could skew the findings. For example, both population groups had a different total number of ES, with 15ES reported at 40% 1RM compared with 484 ES at 75% 1RM. Furthermore, no comparisons can be drawn for trained participants at training loadings of between 40-to-60% 1RM.
Another meta-analysis [in part] conducted by Peterson, Rhea and Alvar  examined the dose-response continuum for intensity, frequency, and volume. The analysis reported that trained and untrained population groups respond differently to various RT loading. For untrained individuals, maximal strength gains were suggested at a loading of 60% 1RM, three d×wk-1, and four sets per muscle group. For the recreationally trained group, maximal strength was reported to be at an RT load of 80% of 1RM, two days per week (d×wk-1) and four sets per muscle group. Peterson and colleagues  suggested that athletes should use an RT loading of 85% of 1RM, two d×wk-1, and with eight sets performed per muscle group to develop maximal strength. However, the reported ES data was almost twice as high with 85% 1RM (ES = 1.12) compared with 80% 1RM (ES = 0.57). They also stated that training at 75% 1RM (ES = 0.07) was ten times as effective as 70% 1RM (ES = 0.07) at producing maximal strength gains. Peterson, Rhea and Alvar  did not state how a five per cent variance in resistance loading could generate such a substantial difference in strength regardless of training status.
A recent meta-analysis by Schoenfeld et al.  probed the effects of RT loading using either ≤ 60% 1RM vs ≥ 65% 1RM, concluding that both ≤ 60 and ≥ 65% 1RM loading produced significant increases in muscle strength and hypertrophy. The strength analysis from nine studies suggested a trend for strength outcomes to be greater with ≥ 65% 1RM compared to ≤ 60 (ES = 1.07 ± 0.60 SD; confidence intervals (CI): -0.18 – 2.32; P = 0.09). However, Schoenfeld, Ogborn and Krieger  performed a further meta-analysis as they indicated issues with statistical power due to the inclusion of only nine studies in the previous meta-analysis. As a result, a follow-up meta-analysis was conducted by Schoenfeld, Ogborn and Krieger  due to the emergence of further studies in this area (84 ESs from 14 studies). In this revised study, Schoenfeld and colleagues reported significantly 1RM strength gains with ≤ 60% 1RM loading compared to > 60% 1RM loading. The mean ES across all studies was 1.50 ± 0.23 SD (CI: 1.01 – 1.99), with a mean per cent change of 31.6 ± 4.5 SD (CI: 22.0 – 41.2). A significant difference emerged between > 60 and ≤ 60% 1RM loading (ES = -0.37 ± 0.10 SD: CI: -0.59 – 0.16; P = 0.003), in favour of > 60% 1RM in terms of superior ES and percentage 1RM strength gain. In terms of isometric strength 23 ESs from eight studies indicated that there was no significant difference.
Future Development & Research
The existing dogma that HL strength development has primarily been indefinable and contentious. The assertion that HL is necessary has limited empirical evidence to support this belief, and it is unclear as to the physiological mechanisms. Some researchers have challenged the view that only HL might serve as the primary mechanism of RT-associated muscular adaptations [Schuenke et al. 2012; Burd et al. 2013]. This reproach relates to the absence of methodological control and consideration towards studies that compare different RT procedures and the differences in the total mechanical work performed or the degree of fatigue subjects experience by the RT treatment [Raymond et al. 2013; Fisher, Steel & Bruce-Low, 2011]. For example, some argue that a lighter load lifted to the point of fatigue would result in many muscle fibres being recruited, similarly to that of heavier training loads.
Although there is a supporting body of evidence for performing HL to maximise strength adaptations some recent research [Burd et al. ] has suggested that low RT loading (30% 1RM) performed to volitional muscular fatigue provides complete MU recruitment. Indeed, a critically extensive review by Carpinelli  questioned the burden of evidence that supports the contention that maximal or near maximal resistance loading is required to achieve maximal strength development. Carpinelli debated that the underlying mechanisms are the degree of effort that an individual works too. This paper partially agrees with this assertion that an individual’s intensity of effort can generate improvements in strength rather than just specific loading. Marginal strength differences were reported in this paper between LL and HL when trained and untrained groups were combined on multi-joint and single-joint exercise. However, with limited studies available, it is difficult to conclude.
Current RT loading recommendations are unfortunately founded on a paucity of scientific evidence. There has been a body of research that has investigated LL vs HL, but there appears to be inadequate evidence to support the use of loading between 60-to-80% 1RM. Unfortunately, limited evidence was available that allowed a detailed analysis of ML which most recommendations are established upon. There is a need for a larger body of scientific evidence from studies that investigate ML and HL if we are to understand potential differences in RT loading on strength. Considerable attention should be placed upon the study design using larger homogenous sample pools (similar biological characteristics and training histories).
Recommendations for the applicable resistance loading for maximal strength development are contentious due to the complex process of manipulating training variables. It would be invaluable if a known association between RT loading and strength improvement. This paper presents additional evidence on RT loading ranges and details that both LL and HL RT can increase muscle strength with marginally greater gains present with HL compared to LL. However, there is a drift towards HL compared to LL as a marginally more effective method to increase strength on multi-joint and single-joint combined exercise and training to failure. Subgroup analysis showed that HL was significantly more effective in producing strength on multi-joint only exercise but reported no difference in strength gains between HL and LL on single-joint only exercise.
Consequently, LL training to failure appears to be an effective strategy to increase muscle strength during the early-stage of individual training. This may have relevance for specific population groups, including individuals who may not be able to perform resistance exercise at higher intensities. The effect of load on RT may generate unique adaptations in different aspects of morphology or muscle function. Individuals should consider their specific training objectives and decide whether the manipulation of RT load might impact on these goals. Effective RT may not necessarily require the use of HL to produce significant increases in strength. Strength training literature at present does not identify what specific mechanisms contribute to the optimum loading stimulus for strength development. Training to maximal or near maximal intensity (effort) at the end of every set may regardless of training load produce similar strength outcomes. Results from this meta-analysis indicate the necessity for future research that explores appropriate RT loading in both females and trained population groups. Future research should seek to clarify the extent of strength effects along the loading continuum using realistic strength programmes, as well as exposing these effects in individuals with considerable RT experience.
Overview of Inter-set Recovery Recommendations & Strength Development
Coaches and exercise practitioners need to manipulate several training variables (i.e. loading, training volume, frequency, exercise selection, exercise order, and inter-set rest intervals) to augment the resistance training benefits (Kraemer, Ratamess, & French, 2002). Of all these variables, evidence-based guidelines for rest intervals are the most limited. Rest intervals represent the time dedicated to recovery between sets and exercises (Baechle & Earle, 2000) with the emphasis being essentially on the inter-set rest periods. Inter-set rest periods may be considered as an important variable of resistance training, as they directly effect fatigue, muscle recovery, the training goal, and training length (Willardson, 2008). Training recommendations suggest that by restricting the rest periods to 60 seconds would maximise the hypertrophic effects (Willardson, 2008). However, it is important to consider that short rest periods have been shown to acutely increase levels of the catabolic hormones (i.e. corticotropin and cortisol) (De Salles et al. 2009).
It has been conjectured that increasing the duration of inter-set recovery significantly affects training adaptations (ACSM, 2009; ACSM, 2011). The amount of recovery time between sets has been considered an essential aspect that can be manipulated to develop an effective strength programme (Willardson & Burkett, 2005). More extended recovery periods reportedly allow for maximal activation of motor units and maintenance of RT intensity (De Salles et al. 2009). Research has identified that an appropriate period of recovery between sets is necessary since RT accentuates anaerobic metabolism with muscles fatiguing quickly (Willardson, 2006). Initial research on inter-set recovery focused primarily on the acute effects of short vs long duration recovery periods. Kramer et al., (1990) demonstrated that a reduction in recovery intervals to 60-seconds in a whole-body training session resulted in a greater post-exercise growth hormone concentration. Therefore, the recovery interval must be of an adequate length to recover the energy source (i.e. adenosine triphosphate and phosphocreatine), clear fatigue-generating substances, and regenerate force production (Matuszak et al., 2003). Individual trainees’ time to fatigue is determined by the resistance exercise loading ranging between 30-to-100% of 1RM (ACSM, 2009; Baechle & Earle, 2000).
Acute responses & the inter-set recovery period
Several studies that have examined the acute responses and inter-set recovery periods have established that extended recovery periods between multiple-sets improves repetitions duration. Some authors have stated that a recovery rate less than three-minutes can result in a significant decrease in the performance of RT repetitions (Kramer 1997; Richmond & Godard, 2004; Willardson & Burkett, 2005; Willardson & Burkett, 2006). Kraemer (1997) examined the effects of one vs three-minutes recovery periods on the total number of repetitions completed with three-sets of 10RM on the bench press and leg press. Twenty national collegiate division one American football players (21 ± 1.3 years) who had a minimum of two years’ experience in RT volunteered. When subjects had a recovery period of three-minutes, they could achieve three-sets of 10RM in both the bench press and leg press. However, when the recovery period was reduced to one-minute a significant reduction in the number of repetitions was observed (set one = 10 ± 0, set two = 8 ± 1.4, set three = 7.1 ± 3.5).
Richmond & Godard, 2004 examined the recovery rates between two-sets for recreational weightlifters. Twenty-eight subjects (21.5 ± 3.2 years) performed two-sets of the bench press at 75% 1RM with rest periods of one, three, or five minutes between sets. Richmond and Godard stated that a reduced number of repetitions were achieved during the second set compared to the first set at all rest periods (one, three, or five minutes). The total work completed with a one-minute rest period was significantly less than with three-and five-minute rest intervals. Additionally, when a recovery period of three and five minutes was applied, subjects completed eight-to-12 repetitions for the first and second sets. However, when a one-minute rest period was implemented subjects performed fewer repetitions. The authors concluded that a rest period of between three-to-five minutes per exercise could provide an adequate recovery rate for trainees to maintain eight-to-twelve repetitions. Further support of a three-to-five-minute recovery period was provided by Willardson and Burkett (2005 and 2006). However, both studies by Willardson and Burkett demonstrated that a three-to-five-minute recovery period was not adequate to maintain consistent repetitions in recreationally trained males.
Willardson and Burkett (2005) compared three different rest (one, two, or five minutes) periods on squat and bench press volume on 15 college-age men (20.73 ± 2.6 years). Subjects completed four-sets of squats with an 8RM load. The results for volume completed (total number of repetitions) for squats was significantly different between one and five-minute rest (17.13 ± 4.42 vs 25.73 ± 4.23) and two- and five-minute rest intervals (21.60 ± 4.52 vs 25.73 ± 4.23). Conversely, the volume completed was not significant between one and two-minute rest periods (17.13 ± 4.42 vs 21.60 ± 4.52). The bench press results were comparable to the squat exercise. The results for the volume completed for the bench press were significantly different between all rest intervals. When the volume was analysed, the five-minute recovery interval allowed subjects to complete the highest volume (28.80 ± 3.08), followed by two (25.53 ± 4.29), and one-minute (22.47 ± 4.79), rest intervals.
Willardson and Burkett (2006) investigated the effects of three different rest periods on multiple-sets of the bench press with heavy (80% 1RM) vs light loads (50% 1RM). Sixteen resistance trained male subjects performed five-sets of bench press for three weeks. Two sessions were performed each week consisting of subjects performing 80% of 1RM with one, two, or three-minute rest periods between sets. Session two repeated the same testing procedure; however, subjects performed at 50% of 1RM. Results for maximal strength suggested that three-minute rest between sets avoid a decline in repetitions. A final study by Willardson and Burkett (2006) compared 30-second, one-minute, and two-minute rest periods of the number of completed repetitions for bench press and squat exercises. Fifteen young trained male subjects (24.87 ± 4.07 years) completed five-sets with a 15RM load. A significant reduction in the number of repetitions occurred between the first and fifth sets irrespective of the recovery period. For the bench press, significant differences occurred between the one-minute and two-minute conditions and between 30-second and two-minute rest periods. However, no significant differences were observed between the 30-second and one-minute rest periods. For the squat exercise, a significant difference in repetition volume occurred between 30-second and two-minute conditions. However, no significant differences were reported between 30-second and one-minute rest periods and between one-minute and two-minute recovery conditions.
Chronic responses and the rest period between sets
ACSM (2009 & 2011) have provided recommendations on recovery periods between sets. ACSM claims that a minimum of two-to-three-minutes recovery period is necessary between sets for structural/core exercises (i.e. squat and bench press). ACSM has cited ten studies that contributed to the development of these inter-set recovery recommendations. However, several studies that were cited did not support the claim made in the position statement. For example, a study by Robinson et al. (1995) compared the different recovery periods between sets (30, 90, and 180-seconds) in moderately training males (n = 33). The subjects were divided into three groups (n = 11) but were not randomised and had no control group. Robinson and colleagues reported that the 1RM squat was significantly greater in the 180-second group compared to the 30-second rest group (7% vs 2%). However, no significant difference in muscular strength gain was present between 180 and 90 second rest period group (7% vs 6%).
Another study cited to support the ACSM (2009) position statement of extended recovery periods was the work of Pincivero, Lephart and Karunakara, (1997). Pincivero and colleagues compared 40-seconds (group one) vs 160-seconds (group two) recovery period in previously untrained subjects who engaged in training three d×wk-1 for four-weeks. The subjects (n = 15) were randomly located into two groups, but no control group was used. Subjects performed four-sets during the first three sessions, with an additional set performed for the further nine sessions. Subjects were assessed for both quadriceps and hamstring isokinetic dynamometry strength. No significant variances were observed between groups for 12 out of the 14 dynamometer variables and no significant difference in functional performance. Pincivero, Lephart and Karunakara (1997) concluded that longer recovery periods promote hamstring strength compared to shorter rest periods. However, they also concluded that isokinetic quadriceps torque and functional performance improved regardless of recovery rate manipulation.
The published work by Antiainen et al. (2005) that was cited by ACSM (2009) compared the acute and long-term hormonal and neuromuscular adaptations to RT in 13 recreationally trained young males (6.6 ± 2.8 years). A cross-over design was applied with subjects performing two separate three-month training periods with a recovery period of two-minutes (short) compared with five minutes (long) between sets. It was stated that seven out of 20 subjects (35%) withdrew during the experimental period because of training-induced pain in the back and knees. It is therefore questionable whether the RT protocols were physically too demanding for this population group. Antiainen and colleagues reported no significant differences between recovery protocols. However, a significant 8% increase in maximal isometric strength was reported for the right leg 1RM during the six-month RT period. It was concluded that the length of recovery between sets did not influence long-term strength adaptation in previously strength-trained men. Unfortunately, these studies summarised above failed to support the recommendations declared by ACSM (2009).
Conclusion on the inter-set recovery period
There is a scarcity of studies, systematic reviews, or meta-analytical research produced that have investigated inter-set recovery periods on muscular strength. The current recommendations do not provide enough evidence to suggest that a three-to-five-minute inter-set recovery period is necessary to maximise strength development. When considerations are made towards the chronic effects of inter-set recovery and strength gain, there is insufficient evidence cited that could support the recommendations made. There is a need to further investigate this area by either seeking to perform a series of systematic reviews or meta-analytical research with specific population groups, rather than pooling data to investigate pre-to-post strength effects.
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