It has been well established that regular resistance training is an effective means to increase skeletal muscle mass, and findings from previous research suggest that a wide range of movement tempos can be used during resistance training to stimulate muscular hypertrophy . Evidence suggests that intentionally slowing down the movement tempo of a single repetition increases the TUT and can increase muscle activation for a given number of repetitions [8, 30]. Hypothetically, increasing the activity of the muscle combined with longer TUT could positively mediate intracellular anabolic signaling, promoting a greater hypertrophic response . However, the anabolic process is a complex phenomenon, and a change in one variable (e.g., tempo) can directly or indirectly affect other variables that also play a significant role in anabolic processes (e.g., load, number of performed repetitions, TUT, fatigue, etc.) . Thus, increasing the TUT, combined with other neuromuscular and metabolic factors, could reduce not only the total number of possible repetitions, but also the dynamic inertia during a lift. Subsequently, less dynamic inertia likely requires less force output during the eccentric–concentric transition and during the concentric phase [8, 22, 31, 32]. As a result, performing fewer repetitions with lower force requirements may diminish any positive effects that increasing individual TUT and muscular activity may provide. Therefore, skeletal muscle growth could be positively or negatively affected by changing the movement tempo, depending on a myriad of other interrelated factors.
Although research has extensively investigated the effects of different intensities, volumes, and rest intervals on muscle growth [1,2,3,4], many of the present hypertrophy guidelines do not account for different movement tempos, likely only applying to volitional movement tempo. To further elaborate on the already unclear anecdotal evidence of different movement tempos on muscle hypertrophy, the research on the topic does not provide equivocal evidence. As previously mentioned, the ACSM  recommends a moderate or slower tempo of movement for novice- and intermediate-trained individuals, but a combination of slow, moderate, and fast tempos for advanced training, depending on the load and the repetition number [23, 28, 33]. Regarding hypertrophy, these guidelines generally concur with one fairly recent meta-analysis  which indicates that similar hypertrophic responses occur when the repetition duration ranges from 0.5 to 8 s, which is a very wide range, whereby acute exercise stress could largely vary . However, it must be noted that neither the meta-analysis  nor the ACSM  recommendations accounted for the duration of particular movement phases (eccentric vs. concentric), thus making it difficult to draw definitive conclusions in terms of resistance-training recommendations. Considering the favorable hypertrophic effects of traditional eccentric exercise [6, 7, 34,35,36,37] compared against the favorable hypertrophic findings of traditional concentric exercise [38, 39] (i.e., normal dynamic constant external resistance training, not accompanied by blood flow restriction, etc.), it is important to differentiate concentric and eccentric durations during different movement tempos under standard resistance-training conditions.
To determine the impact of movement tempo with changes in the duration of both the eccentric and concentric phases on muscular hypertrophy, Tanimoto and Ishii  compared training programs with medium tempo (MED) with a light load (3/1/3/0; 50%1RM), fast tempo (FAS) with a heavy load (1/1/1/0; 80%1RM), and FAS tempo with a lighter load (1/1/1/0; 50%1RM) . All three protocols included 3 sets of knee extensions performed to muscular fatigue with 60-s rest intervals for a period of 12 weeks. When using the same load (50%1RM), the MED tempo resulted in significantly greater hypertrophy based on cross-sectional area (CSA) than the FAS tempo. Since the intensity and the amount of total work were the same for the MED and FAS tempos, the slower tempo during MED can be considered as the primary factor responsible for the greater hypertrophic effect of the MED tempo. However, when comparing the effect of training with a MED tempo with a lighter load (50%1RM) to the FAS tempo with a heavier load (80%1RM), there was no difference in the hypertrophic response. Therefore, the study by Tanimoto and Ishii  indicates that slowing down the tempo may compensate for decreasing the load used when the training goal is hypertrophy. However, it should be noted that event if the same tempo is used, lighter loads can still induce similar hypertrophic responses as heavier loads as long as there is sufficient muscle fatigue . Nevertheless, the results of Tanimoto and Ishii  showed that a constant load of 50%1RM with MED tempo resulted in greater hypertrophy than the FAS tempo. However, this study compared the effect of a single resistance exercise, which may not necessarily translate into a whole-body training program. To determine the effects of movement tempo during a whole-body resistance-training program (squat, chest press, latissimus dorsi pull-down, abdominal crunches, and back extensions), Tanimoto et al.  investigated MED (3/0/3/0; 55–60%1RM) and FAS (1/0/1/1; 80–90%1RM) tempos over 13 weeks using 3 sets of repetitions performed to muscular fatigue and 60-s rest intervals. They showed that despite using a lighter load with a MED tempo, similar hypertrophic effects were observed compared to the FAS tempo with a heavier load, which is consistent with the single-joint individual exercise results presented previously .
Although the load, or the mechanical stimulus, has been suggested to be of critical importance for inducing hypertrophic adaptations [36, 41, 42], most studies use different external loads for the faster and slower tempos [18, 25, 40, 43], which results in different mechanical stimuli. Furthermore, the meta-analysis made by Schoenfeld et al.  showed that not only the external load but also the point of muscle failure during the set is an important factor affecting hypertrophy. Therefore, the results of the two previously described studies [18, 40] indicate that movement tempo is an important variable that also plays a significant role in the anabolic process, and slower movements can be useful to compensate for any decreases in the load used as long as the exercises are performed to muscular failure. Although the data of Tanimoto and Ishii  and Tanimoto et al.  indicate that hypertrophy can be similar, or greater, with a MED movement tempo at 50–60%1RM compared to a FAS one at (80–90%1RM), not only differences in the load used, but also the volume of exercise should also be considered. Exercise volume is most often determined using the load and the number of repetitions, which can, at times, be overly simplistic [10, 30, 44].
When calculating training volume using sets*repetitions with the same load and exercise, a slower movement tempo decreases the maximum number of repetitions one is able to perform compared to a faster tempo [8, 9, 14, 45]. Thus, in these cases, a greater number of performed repetitions in a faster protocol, and thus volume, is not synonymous with greater TUT (i.e., the product of the number of repetitions and the TUT of each repetition). This becomes even more important to consider when factoring in the load used, as a single 3/0/3/0 repetition at 40%1RM may not result in the same stimulus as a 1.5/0/1.5/0 repetition at 80%1RM. In fact, multiple studies have shown that performing exercises using a slower tempo consisting of 5- or 6-s eccentric and concentric phases results in a significantly longer total TUT compared to a faster movement tempo [10, 14, 30, 46], but these studies did not consider the load, volume, and TUT all together. Since heavier loads result in greater motor unit recruitment and tension is one of the major stimulants of muscular hypertrophy and changes in muscle architecture , if the TUT is extended, greater hypertrophy adaptations could be achieved [22, 30]. Therefore, it can be assumed that the greater  or comparable  hypertrophy effect after a MED tempo with lighter load compared to a FAS tempo with heavier load in the aforementioned studies could have been partially associated with a greater total TUT.
Although longer TUT seems to play an important role in the hypertrophic responses, Schuenke et al.  reached partially contradictory findings. Schuenke et al.  found a statistically significant increase in total mean fiber area (CSA) after training (6–10RM; 3 sets; repetitions performed to muscular fatigue; 2-min rest interval; exercises: squat, leg press, leg extension; 2–3 times per week; 6 weeks) with a FAS (2/0/1/0; 80–85%1RM) when compared to slow (SLO) (4/0/10/0; 40–60%1RM) movement tempo (38.8% vs 10.6%; ES = 1.54 vs 0.65, respectively). The result of this study is contrary to others who suggest that a longer TUT may be beneficial for inducing muscle hypertrophy [30, 47]. However, according to Schoenfeld , hypertrophy occurs preferentially when the duration of the eccentric phase is increased; while in the study of Schuenke et al. , the duration of the concentric contraction was significantly extended.
It should be noted that limited evidence suggests that training at volitionally very slow durations (10 s per repetition) is superior or inferior from a hypertrophy standpoint, although a lack of controlled studies on the topic makes it difficult to draw definitive conclusions. One train of thought is that extremely slow tempos require lighter loads whereby the athlete can voluntarily control the tempo, which may be suboptimal for maximizing gains in muscle hypertrophy presumably as a result of inadequate motor unit recruitment and stimulation . Furthermore, consideration should be given to determining not only the TUT for the entire movement but also independently for particular phases of the movement: TUT concentric (TUT-C) and TUT eccentric (TUT-E). Specifically, a slower eccentric movement increases the TUT-E, the level of metabolic stress, the hormonal responses [14, 46], and muscle fiber damage and protein degradation [49,50,51], inducing a stronger anabolic signal with the muscle and connective tissue. In contrast to a slower eccentric contraction, it seems beneficial to use faster concentric contractions for muscle hypertrophy by increasing muscle activation  and the rate of fatigue , which is more effective for stimulating the highest threshold motor units associated with type II fibers . Folland and Williams  have suggested that these muscle fibers have a greater potential for muscle growth, even though this suggestion remains controversial [2, 55, 56]. This leads to the conclusion that not only is the TUT of the entire movement important to consider, but the relationship between the duration of the eccentric and concentric phases could also impact hypertrophy responses following resistance exercise.
Only one study compared resistance training using two conditions that were matched for total TUT (6/0/2/0 vs. 2/0/6/0), but with each condition including different TUT-C and TUT-E (9 weeks; 6–8RM repetitions performed to muscular fatigue; 2.5-min rest interval; lower body exercise: bilateral incline leg press, parallel squat, bilateral leg extension and leg extension; upper body exercise: bench press and one of three—bilateral bicep curl, lateral pull-down or seated row) . The muscle biopsy analyses demonstrated that both type I and IIA vastus lateralis fiber areas significantly increased following the slower concentric contraction, while only type I fiber area increased following the slower eccentric contraction, but differences between groups were not significant. However, in a study by Gillies et al. , a simultaneous change in the duration of both the eccentric and concentric phases was induced. Under such conditions, it was impossible to accurately determine whether the beneficial effects of hypertrophy occurred as a result of changes in only the concentric duration, only the eccentric duration, or a combination of changes in both phases of movement.
The effects of extending only the duration of the concentric contraction on muscle hypertrophy were examined by Nogueira et al.  who showed that faster concentric contractions (3/0/1/0) resulted in greater muscle hypertrophy (CSA) compared to slower concentric contractions (3/0/3/0), but only in the biceps brachii, as there was no such effect in the rectus femoris (10 weeks; 20 training sessions; load: 40%1RM for the first two sessions, 50%1RM for the third and fourth sessions, and 60%1RM for the subsequent sessions; 8–10 repetitions; 90-s rest interval; exercise: horizontal leg press, knee extension, knee flexion, chest press, seated row, elbow extension, elbow flexion). The lack of hypertrophic benefits by extending the duration of the concentric phase has also been demonstrated in a study by Keeler et al. , where the authors compared resistance training (1 set; 8–12 repetitions to muscular fatigue; 60–90-s rest interval; exercises: leg press, leg curl, leg extension, anterior lateral pull-down, bench press, seated row, biceps curl, triceps extension) with a 5/0/10/0 tempo with 50%1RM, to 4/0/2/0 tempo performed at 80%1RM. However, their assessments were made in relation to fat-free mass (FFM), and although FFM measures provide a general estimate of hypertrophic gains over the course of a resistance-training program, they lack the sensitivity to evaluate subtle changes in muscle mass . Compared to lengthening the concentric phase, a contrary effect was observed when the eccentric duration was extended in the study of Pereira et al. , where the participants performed 2 training sessions per week of the arm curl exercise, for 12 weeks with MED (4/0/1/0) or FAS (1/0/1/0) movement tempo. During each training session, 3 sets of 8RM were performed to muscular fatigue. The CSA in the biceps brachii was greater after training with the slower eccentric tempo compared to the faster one, which suggested that the extension of only the eccentric phase can be beneficial in the development of hypertrophy. However, more research should isolate the effects of eccentric durations to determine that these findings hold true in other exercises, populations, and eccentric durations.
Although this review focuses on the effects of different tempos during training, it is important to not overlook the fact that the 1RM procedures for the studies discussed above used volitional movement tempos. In fact, research has shown that slower tempos decrease the load lifted during 1RM tests when compared to faster tempos [11,12,13], meaning that if the same load is used during different tempos during training (based on a volitional tempo 1RM test), the intensity of the effort will not be the same and slower tempos increase the intensity of effort for a given load. Thus, the movement tempo used during training also impacts the rating of perceived exertion (RPE). Diniz et al.  demonstrated that strength training protocols matched for the number of sets and repetitions, load, and rest interval (3 sets; 6 repetitions; 60%1RM; 3-min rest intervals) but with different tempos (4/0/2/0, 2/0/2/0, V/0/V/0) (V represents volitional tempo of movement) produced different responses in RPE. Resistance training with a tempo of 4/0/2/0 yielded greater RPE compared to 2/0/2/0 and V/0/V/0. It is likely that due to the constrictive nature of muscle contractions, the physiological effect of slower movement tempo during resistance exercise can be similar to what occurs during resistance exercise with external occlusion in which the manipulation of blood flow restriction results in greater ratings of perceived exertion [62, 63]. Therefore, researchers and practitioners should consider how the tempo of the 1RM test can affect the subsequent program choices, as the principle of (testing) specificity reigns supreme when considering one’s relative maximum compared to their maximal performance abilities.
Another factor to consider when interpreting research is how changes in muscle tissue are assessed. Some authors assess regional specific hypertrophy and changes in fiber-type distribution via muscle biopsy [43, 64], while others have assessed changes in muscle CSA or thickness by either MRI or ultrasound [18, 40, 65]. Additionally, some studies indirectly assess site-specific changes in muscle growth and employ measures of overall fat-free mass (FFM) (i.e., DXA and densitometry) [25, 29, 66]. Although FFM measures provide a general estimate of hypertrophic gains over the course of a resistance-training program, they lack the sensitivity to assess subtle changes in muscle tissue . In addition to skeletal muscle, FFM also includes such components as body fluids, bone, collagen, and other non-fatty tissues. Thus, it cannot be concluded that changes in FFM are specific to muscle hypertrophy.
Considering the data discussed throughout this section, movement tempo should be taken into consideration when planning and executing resistance-training programs to increase hypertrophy. Specifically, the results of studies indicate that neither isolated slow nor isolated fast movement tempos are effective for muscle hypertrophy, but it seems that the most favorable is a combination of slower movement in the eccentric phase with a faster movement during the concentric phase. However, the optimal use of variable movement tempos to increase muscle hypertrophy cannot be analyzed independently of other training variables (especially load, number of repetitions, TUT, etc.). Using a slower tempo during resistance exercises requires decreasing the external load compared to using a faster tempo [10,11,12], but can simultaneously increase the TUT during particular sets , possibly providing an adequate stimulus to induce hypertrophy [18, 40, 67], especially if the exercise is performed to muscle failure . Therefore, if low-load resistance training is not carried out to muscular failure, high-load training appears to provide a superior hypertrophic stimulus, and thus greater growth of all muscle fibers . However, exercise carried out to concentric failure with a longer TUT but a lighter load can be more effective in the development of hypertrophy than heavier loads with shorter TUT .