The frequency of resistance training refers to how often a specific muscle group is stimulated within a given week. The debate between training a muscle group once a week, often called a “bro-split,” and training it twice a week is common in fitness discussions. The core question is whether distributing the workload over multiple sessions provides a superior stimulus for muscle growth (hypertrophy) and strength development compared to consolidating it into one intense weekly session. This article examines the underlying biological processes and the role of total work performed to compare the effectiveness of training a muscle group once versus twice per week, focusing on scientific evidence.
The Biological Mechanism Driving Training Frequency
The primary biological driver of muscle growth following resistance exercise is the elevation of muscle protein synthesis (MPS). MPS is the process by which muscle cells build new proteins necessary for repair and growth. A single, challenging resistance training session stimulates a significant increase in MPS rates, but this elevated state does not last indefinitely. It typically remains high for approximately 24 to 48 hours in trained individuals before returning to baseline levels.
For a muscle group trained only once a week, the muscle spends the next five days in a non-elevated state, representing a lost opportunity for maximizing the anabolic signal. Training a muscle group twice per week, with sessions spaced 48 to 72 hours apart, allows a new stimulus to be applied just as the MPS rate from the previous session declines. This strategy creates two separate periods of elevated MPS within the same week, maintaining a higher weekly average MPS rate for consistent muscle adaptation and growth.
The Essential Role of Training Volume and Intensity
Training frequency cannot be analyzed in isolation because the total amount of work performed, known as training volume, is the most influential variable for muscle growth. Volume is generally calculated as the total number of sets, repetitions, and weight lifted over a week. For a fair comparison between training schedules, researchers use “volume matching,” ensuring both groups perform the exact same total number of weekly sets and repetitions.
The intensity of the training session is also a major factor, referring to how close a set is taken to momentary muscular failure. Training sets must be performed with sufficient intensity to recruit high-threshold motor units and provide an effective stimulus for growth.
The issue with a once-per-week schedule is the physiological limit on how much effective volume can be accumulated in a single session before fatigue and diminishing returns set in. Research suggests that performing too many sets for one muscle group in a single workout yields progressively less return after 6 to 8 hard sets. Trying to complete 15 or 20 sets in one session leads to excessive fatigue that compromises the quality and effectiveness of the later sets. Splitting that same total volume into two or more sessions per week allows the lifter to perform all sets with a higher level of force production and focus, making the total volume more productive overall.
Comparing Strength and Hypertrophy Outcomes
When studies compare the effects of training a muscle group once versus twice per week while ensuring the total weekly volume is identical, the results for muscle hypertrophy favor the higher frequency. Meta-analyses indicate that training a muscle group two times per week results in greater gains in muscle size compared to one time per week. This benefit is attributed to re-stimulating the muscle protein synthesis pathway more frequently throughout the week.
For strength gains, the influence of frequency is less pronounced when volume is equated. Multiple analyses show no significant difference in maximum strength improvements between training a muscle group once or twice per week if the total weekly volume is the same. Strength is highly dependent on neurological adaptations and the total volume of high-intensity work performed.
However, the twice-per-week frequency offers a practical benefit for strength development by providing more opportunities for skill practice, especially with complex, multi-joint movements like the squat or deadlift. This increased practice frequency allows for better motor unit recruitment and technical refinement, which are important drivers of strength improvement.
Practical Application: Choosing the Right Frequency for You
The optimal training frequency depends heavily on an individual’s training experience. Beginner lifters see significant progress training a muscle group only once per week because their muscles are highly sensitive to growth signals. Their longer-lasting muscle protein synthesis response also means the five-day gap is less detrimental to their progress.
As a lifter gains experience and moves into intermediate or advanced stages, their body becomes more efficient, and the elevated MPS response becomes shorter. For these individuals, training a muscle group twice per week is more advantageous to maximize the number of growth signals within the week. This higher frequency also aids in recovery by distributing the accumulated fatigue from a high weekly volume across two separate sessions.
The choice between once or twice a week should align with personal preference, recovery capacity, and scheduling constraints. A twice-per-week frequency is optimal for maximizing both strength and size, particularly for those performing high weekly volumes. Training a muscle group once per week remains adequate for maintaining muscle mass or for individuals whose schedule does not allow for a higher frequency.