How Many Reps Do You Need? - Strength, Hypertrophy, Endurance

One of the most frequently asked questions among individuals who engage in resistance training is how many repetitions and sets should be done for an exercise . The answers given in gyms, training programs, and especially on digital platforms vary considerably. Often, a simple model is presented suggesting that specific repetition ranges lead to specific adaptations: low repetitions promote strength development, medium repetitions promote muscle hypertrophy, and high repetitions improve muscle endurance.

This approach is referred to in the literature as the "repetition continuum" model . According to this model, there is a systematic relationship between the load used and the number of repetitions that can be performed, and this relationship triggers different physiological adaptations. For example, it has been suggested that low repetitions performed with high loads increase maximal strength, repetitions performed with moderate loads are ideal for muscle growth, and high repetitions performed with low loads improve muscle endurance. This view has been considered a fundamental point of reference in the planning of resistance training for many years.

However, numerous studies published in recent years indicate that this model may not have as precise limitations as we thought. In particular, there is a growing body of evidence suggesting that adaptations such as muscle hypertrophy are not limited to specific repetition ranges, but that similar results can be achieved across a wide range of loads. This necessitates a more comprehensive assessment of how load and repetition count in resistance training affect adaptations.

The aim of this paper is to address the relationship between load magnitude and repetition rate used in resistance training and to examine the effects of these variables on three fundamental adaptations in light of scientific data:

• maximal muscle strength

• muscle hypertrophy

• local muscle endurance

The aim is not to propose a single “ideal repetition range”; rather, it is to explain the physiological mechanisms through which these adaptations arise and to present the scientific basis of resistance training within a more objective framework.

ADAPTATION IN WEIGHT TRAINING

Resistance training is considered a powerful stimulus that creates both acute and chronic physiological changes in skeletal muscle. The main adaptations observed in muscle tissue as a result of this type of training include increased maximal strength, muscle hypertrophy, and improvements in muscle endurance . These adaptations occur because the mechanical and metabolic stress applied during training triggers a series of biological processes in muscle cells.

Adaptive responses to resistance training depend largely on several key variables of the training program. The following factors stand out in particular:

• magnitude of the load used (training load)

• number of sets and repetitions performed (training volume)

• intensity of exercise and effort level

• training frequency

Among these variables , load magnitude , that is, the ratio of the weight lifted to the individual's maximal capacity, is considered one of the most critical parameters. Load magnitude not only determines how much the muscle is mechanically stressed; it also significantly influences the metabolic, hormonal, neuromuscular, and cardiovascular responses that occur during exercise . These acute physiological responses, combined with repeated training stimuli over time, transform into long-term muscle adaptations.

An important concept here is 1-repetition maximum (1RM) . 1RM refers to the maximum weight that can be lifted for a single repetition in an exercise and is a commonly used reference value to define the magnitude of the load in resistance training. For example, performing an exercise at 80% 1RM means that the weight used corresponds to approximately eighty percent of the individual's maximum capacity.

Another critical element in understanding muscle adaptations is motor unit activation . Motor units consist of a motor neuron and the muscle fibers it controls. As the load used during resistance training increases, motor units with higher threshold values are activated, and this plays a significant role in the development of neuromuscular adaptations, particularly in increasing force production capacity.

Therefore, the adaptations observed in resistance training are not solely a result of the number of repetitions performed. Rather, these adaptations arise from the complex interplay of load magnitude, applied mechanical stress, metabolic stress, and nervous system adaptations .

REPETITION CONTINUUM: TRADITIONAL MODEL

For many years, the most common theoretical framework used in planning resistance training has been the "repetition continuum" model . This model suggests a linear relationship between the load used and the number of repetitions that can be performed, and that different repetition ranges optimize different physiological adaptations.

According to this model, adaptations in resistance training are explained through three main load zones:

1. Strength zone

• 1-5 repetitions

• approximately 80–100% 1RM

• It aims for maximum strength development.

2. Hypertrophy zone

• 8–12 repetitions

• approximately 60–80% 1RM

• considered ideal for muscle growth

3. Endurance zone

• 15 or more repetitions

• Loads below 60% 1RM

• It is thought to improve muscle endurance.

This approach originates from early strength training studies conducted in the mid-20th century. Specifically, the model developed by DeLorme suggested that high loads increased strength development, while low-resistance exercises improved muscle endurance. Subsequent studies supported this approach, contributing to the repetition continuity model forming the basis of modern resistance training programs.

The appeal of this model stems largely from its simplicity and applicability . It offers a practical method for determining the repetition range based on target adaptation when designing training programs. Therefore, the repetition continuity model has been considered an almost undisputed point of reference in sports science and the fitness industry for many years.

However, recent research suggests that some assumptions in this model may be overly reductionist . In particular, the classic view that muscle hypertrophy occurs only in intermediate repetition ranges is increasingly being questioned. Current literature indicates that muscle adaptations can occur over a wider load range than previously thought.

Therefore, while the repetition continuity model still offers a useful framework for understanding resistance training, it is considered a paradigm that needs to be re-evaluated in light of current scientific data .

POWER DEVELOPMENT

Muscle strength is defined as the capacity of a muscle or muscle group to generate maximum force against external resistance . In sports science, this capacity is often measured by 1-repetition maximum (1RM) ; that is, the highest weight that can be lifted for a single repetition in an exercise.

Meta-analyses in the literature show that training with high loads provides a significant advantage in terms of maximal strength increase. These results demonstrate that the repetition continuum model is largely valid for strength development.

Jenkins et al. 's study can be given as an example of this situation . In this study, participants performed the leg extension exercise with two different loads:

• 80% 1RM

• 30% 1RM

After six weeks of training, the group working at 80% 1RM showed greater increases in both voluntary muscle activation and electromyographic (EMG) activity . This result indicates that heavier loads stimulate nervous system adaptations more strongly.

The underlying mechanism behind these findings is largely explained by neuromuscular adaptations . During exercise with heavy loads, the nervous system is forced to activate motor units with higher threshold values. Motor units are functional units consisting of a motor neuron and the muscle fibers it controls. In situations requiring higher force output, more and larger motor units are activated. Over time, this process improves coordination between the nervous system and muscle tissue, increasing force production capacity.

However, research shows that training with light loads can also provide a certain degree of strength increase. For example, some studies have shown that training with low loads can generate strength increase, but this increase is generally more limited compared to training with heavy loads . In particular, as training experience increases, maximal strength development becomes increasingly dependent on training with higher loads .

Therefore, when the available evidence is considered together, the repetition continuum model is strongly supported for maximal strength development . Training with heavy loads and low repetitions provides a significant advantage for maximal strength development by most effectively stimulating neuromuscular adaptations that increase force production capacity.

MUSCLE HYPERTROPHY

Muscle hypertrophy is defined as muscle tissue growth characterized by an increase in the cross-sectional area of muscle fibers . According to the traditional repetition continuum model, the optimal repetition range for hypertrophy is considered to be 8–12 repetitions . This approach has been used as a standard recommendation in sports science and the fitness world for many years.

One of the main pillars of this view is early studies showing that training in moderate repetition ranges leads to larger acute increases in post-exercise anabolic hormones . However, later research has shown that these hormonal changes do not strongly correlate with long-term muscle growth. Therefore, the idea that hypertrophy is solely dependent on a specific repetition range has increasingly come under scrutiny.

Numerous long-term studies in recent years have shown that muscle hypertrophy can be achieved over a much wider load range .

For example, in the study by Mitchell et al. (2012), participants were trained using three different protocols:

• 80% 1RM (high load)

• 30% 1RM (low load)

• low volume 80% 1RM

  
  

When quadriceps hypertrophy was examined after 10 weeks of training, it was observed that even training at 30% 1RM (high reps, low weight) resulted in similar muscle growth to training at 80% 1RM (low reps, high weight) .

Similarly, Morton et al. (2016) compared two different training protocols in a 12-week study conducted on individuals with experience in resistance training:

• 8–12 repetitions

• 20–25 repetitions

  
  

At the end of the study , no significant difference was found between the two groups in either the cross-sectional area of muscle fibers or lean body mass . This indicates that training with higher repetitions was also effective in terms of muscle hypertrophy.

These findings significantly weaken the idea that there is a single “ideal repetition interval” for hypertrophy.

The fundamental mechanism underlying these results is related to motor unit activation . Muscles function through motor units, and different motor units have different threshold values. In exercises with heavy loads, high-threshold motor units are directly activated. In contrast, in exercises with light loads, initially only low-threshold motor units are engaged; however, as the sets progress, fatigue increases, and high-threshold motor units are also activated to produce more force .

Therefore, even training with low loads can generate hypertrophic stimulation when sets are performed with sufficient effort.

However, there is an important limitation here. Lasevicius et al (2018) compared different load levels:

• 80% 1RM

• 60% 1RM

• 40% 1RM

• 20% 1RM

The study concluded that training within the 40–80% 1RM range resulted in similar hypertrophy, while training at 20% 1RM led to significantly less muscle growth . This suggests there may be a specific minimum load threshold for hypertrophy .

Current literature suggests that this threshold value may be approximately 30% 1RM .

In light of these findings, the effect of different repetition ranges on muscle hypertrophy can be summarized as follows:

Low repetitions – heavy load (1–5 repetitions)

• can cause muscle hypertrophy

• ideal for strength development

• However, it usually requires more sets.

• can create a high load on the joints and nervous system.

Moderate repetitions – moderate load (6–12 repetitions)

• It is a highly effective range for hypertrophy.

• Power development is low compared to heavy load.

• From a practical point of view, it is considered the most efficient range for hypertrophy.

High repetition – low load (15–30 repetitions)

• It can promote muscle growth.

• However, sets should generally be made close to failure.

• Higher metabolic stress and longer set times occur.

Therefore, current scientific data show that hypertrophy is not merely an “adaptation specific to the 8–12 repetition range.” Muscle growth can occur across a wide load spectrum when sufficient mechanical tension and adequate effort are provided .

Therefore, instead of recommending a single repetition range for hypertrophy in modern sports science, a more flexible training approach is suggested , using different load ranges strategically .

DURABILITY

Muscular endurance is defined as a muscle's capacity to resist fatigue under a submaximal load . In other words, this adaptation refers to the muscle's ability to work under a given resistance for as long as possible.

From a physiological perspective, improvements in muscle endurance are generally associated with the following adaptations:

• increased mitochondrial density

• dilation of the capillary network in muscle tissue

• Increased activity of enzymes involved in oxidative metabolism

These adaptations are thought to be particularly pronounced in exercises performed with high repetitions. Therefore, according to the traditional repetition continuum model, muscle endurance is best developed with low-load training performed with 15 or more repetitions .

However, research in the literature suggests that this relationship is not as clear-cut as we might think.

STUDIES SUPPORTING REPETITION CONTINUUM (TRADITIONAL MODEL)

Some early studies have shown that lower loads may offer an advantage in terms of muscle endurance.

For example, in the study by Anderson and Kearney (1982), participants were trained using three different training protocols:

• High load: 6–8 repetitions

• Moderate load: 30–40 repetitions

• Very low load: 100–150 repetitions

Muscle endurance was examined in bench press tests after 9 weeks of training. The results showed greater increases in endurance in groups working with lower loads. For example, endurance increased by approximately 41% in the low-load group, while this increase was approximately 28% in the high-load group .

Similarly, in the study by Campos et al. (2002) , the group that performed 20–28 repetitions in squat and leg press exercises showed greater improvement in muscle endurance compared to the groups that performed lower repetitions.

These findings suggest that the repetition continuum model may be valid to some extent in terms of muscle endurance.

STUDIES THAT CONTRADICT REPETITION CONTINUUM

However, more recent studies have yielded different results.

For example, in the study Jessee et al. (2018), participants trained with two different loads:

• 70% 1RM (high load)

• 15% 1RM (very low load)

After 8 weeks of training, muscle endurance was tested and no significant difference was found between the two groups .

These results suggest that the development of muscle endurance may not be explained solely by the load used.

WHAT CAUSES THE DIFFERENCE? - THE EFFECT OF MEASUREMENT METHODS

One of the most important methodological problems in muscle endurance research is how endurance is measured .

Muscle endurance is generally assessed in two different ways:

• Absolute endurance → the maximum number of repetitions that can be performed with a fixed weight.

• Relative endurance → the maximum number of repetitions performed with a given 1RM percentage.

These two measurement methods can yield different results. For example, when a person increases their 1RM value after training, the same weight now represents a lower percentage load, and this can affect test results. Therefore, one of the main reasons for inconsistencies in results across different studies is the differences in measurement methods .

When the existing literature is evaluated together, the following picture emerges:

• Some studies show that lower loads improve muscle endurance more.

• However, some studies show that load magnitude does not make a significant difference.

Therefore, the repetition continuum model is partially supported in terms of muscle endurance , but this relationship cannot be said to be as strong and consistent as in strength development. In other words, high-repetition training with low loads may provide an advantage for muscle endurance; however, current scientific data show that this relationship is not a universal and definitive rule .

WHAT SHOULD A REAL MODEL LOOK LIKE?

Modern research on resistance training reveals that the repetition continuity model is not equally valid for all adaptations. While the traditional approach suggests that specific repetition ranges optimize certain physiological adaptations, current literature indicates that this relationship is not as rigid as previously thought.

Current scientific data generally reveals the following picture:

• Maximal Strength: Training with heavy loads provides a significant advantage in terms of maximal strength development. This is largely explained by neuromuscular adaptations and the principle of specificity.

• Muscle hypertrophy: Muscle growth can occur over a wide range of loads. Both low and high-load training can produce similar hypertrophic results when sufficient mechanical stress and effort are provided.

• Muscle endurance: There is evidence that low-load training with high repetitions may provide an advantage in muscle endurance. However, the results in the literature are not entirely consistent, and the effect of load magnitude on this adaptation has not been definitively established.

These findings demonstrate that adaptations in resistance training cannot be explained solely by the number of repetitions. Muscle growth and performance improvement result from the interaction of many variables, including the magnitude of the load used, the level of effort applied, the total training volume, and program design.

CONCLUSION: THERE IS NO SINGLE CORRECT REPETITION INTERVAL.

One of the most frequently asked questions in the resistance training literature is "how many repetitions should be done?" . The traditional repetition continuum model offers a simple answer: low repetitions are ideal for strength, medium repetitions for hypertrophy, and high repetitions for muscle endurance. However, current scientific data shows that this relationship is not as rigid as we thought.

Research indicates that different load ranges can stimulate different physiological mechanisms, and therefore muscle adaptations cannot be reduced to specific repetition ranges. In terms of maximal strength development, low-repetition training with heavy loads offers a significant advantage. However, muscle hypertrophy can occur over a wide load range, and similar muscle growth can be achieved with both low and high loads when sufficient mechanical stress and effort are provided. Regarding muscle endurance, while high repetitions may offer an advantage in some cases, findings in the literature suggest that this relationship is inconsistent.

These findings reveal that adaptations in resistance training cannot be explained solely by the number of repetitions. Muscle growth and performance improvement;

• the size of the load used

• training effort level

• total training volume

• general structure of the program

It arises as a result of the interaction of many variables, such as those mentioned above.

Therefore, modern training approaches often prefer hybrid programming models that strategically combine different load ranges . Low repetitions with heavy loads particularly support neuromuscular adaptations and maximal strength development, while high repetitions with lighter loads create greater metabolic stress within the muscle, activating different adaptation pathways. Thanks to this approach, different muscle fibers and different physiological mechanisms can be stimulated within the same training cycle.

In conclusion, reducing resistance training solely to specific repetition ranges oversimplifies the complex nature of muscle physiology and training adaptations. Current scientific approaches demonstrate that muscle adaptations can be achieved across a broad load spectrum, and that an effective training program can be constructed by using different load and repetition ranges in a balanced manner.

References:

• Schoenfeld BJ, Grgic J, Van Every DW, Plotkin DL. Loading Recommendations for Muscle Strength, Hypertrophy, and Local Endurance: A Re-Examination of the Repetition Continuum. Sports (Basel). 2021 Feb 22;9(2):32. doi: 10.3390/sports9020032. PMID: 33671664; PMCID: PMC7927075