Muscular failure and muscle hypertrophy: Taking the hard road to Gainzville Original paper

In this randomized controlled trial, resistance-trained participants who trained with high-velocity loss thresholds (i.e., closer to muscular failure) increased their vastus lateralis thickness more than participants who trained with lower-velocity thresholds.

The effect of training with low-velocity vs. high-velocity loss thresholds in the squat and bench press exercises on resistance exercise-induced adaptations.

The specific outcomes assessed were the following:

• Strength: assessed via 1-repetition maximum (1RM) in the free weight squat and bench press exercises
• Power: assessed via leg press, 1 m/s load test (i.e., the percentage of 1RM that can be lifted in the squat and bench press exercises at a velocity of 1 meter per second) and vertical jump
• Body composition and muscle hypertrophy: assessed via DXA and ultrasound scans of the vastus lateralis and triceps brachii lateral head

16 resistance-trained adults (average age of 27; 9 men, 7 women) who were considered “advanced” or “highly advanced”.

The men had a relative strength ratio (1RM divided by body weight) of 1.8 in the squat and 1.4 in the bench press, whereas the women had relative strength ratios of 1.3 and 0.8.

In this 6-week randomized controlled trial, the participants performed the squat and bench press exercises three times per week and were assigned to one of two groups:

• Low-velocity loss (LVL): Each set of squats and bench press was terminated when a velocity loss of 20% and 30%, respectively, was achieved.
• High-velocity loss (HVL): Each set of squats and bench press was terminated when a velocity loss of 40% and 60%, respectively, was achieved.

Because a greater velocity loss threshold corresponds with terminating a set closer to (or at) muscular failure (i.e., the point at which another concentric repetition cannot be completed) and thus the performance of a greater number of repetitions (reps) per set, the participants in LVL performed more sets in order to equalize training volume between groups.

The participants had to be body-weight stable for at least 3 months before the study and were asked to continue their regular diet.

Squat and bench press 1RM increased in both groups, with no difference between groups. Vastus lateralis thickness increased in HVL compared to LVL.

The results of the summarized study are in agreement with those of previous studies, in that muscle hypertrophy was superior with higher-velocity compared to lower-velocity loss thresholds.^[1][2][3] However, unlike the summarized study, the previous studies did not equalize training volume between groups, meaning that a significantly greater number of reps were performed in the higher-velocity threshold groups. This is a notable limitation because there seems to be an inverted U-shaped relationship between volume and hypertrophy,^[4] meaning that higher volumes lead to greater muscle hypertrophy up to a point, after which more volume does not result in further benefit and may even impair muscle hypertrophy.

During the intervention, HVL completed an average of approximately 3 sets of 8 reps for both exercises per session, whereas the average session for LVL involved approximately 6 sets of 4 reps in the squat and 5 sets of 5 reps in the bench press. Considering the critical role of volume in eliciting hypertrophy, it could be expected that hypertrophy would have been similar between groups because training volume was equalized.

Previous studies that equalized training volume between groups and had participants perform a different number of reps per set found the following:

• 4 sets of a 3–5RM and 3 sets of a 9–11RM produced similar hypertrophy in the vastus lateralis.^[5]
• 6 sets of a 4RM and 3 sets of a 10RM produced similar hypertrophy in the arm (i.e., the combined cross-sectional area of the triceps brachii, biceps brachii, and brachialis).^[6]
• 7 sets of a 2–4RM and 3 sets of an 8–12RM produced similar hypertrophy in the biceps brachii.^[7]

In each of these studies, each set was reportedly performed to muscular failure. Therefore, the primary variable manipulated was the number of reps performed per set. Collectively, the results suggest that low-rep sets provide a hypertrophy stimulus similar to that of moderate-rep sets, as long as more sets are performed to equalize training volume.

With this in mind, the difference in the average number of reps performed per set likely doesn’t explain the difference in muscle hypertrophy observed in the summarized study. The remaining variable to consider is proximity to failure, which was controlled for in the aforementioned studies (i.e., every set was taken to muscular failure), but not in the summarized study (i.e., HVL trained closer to muscular failure than LVL).

Meta-analyses have reported that training to failure and not to failure produce similar muscle hypertrophy when volume is equalized between groups.^[8][9] In more granular analyses that compared different velocity loss thresholds, it was shown that training with a higher-velocity loss threshold (>25%) is superior to training with a lower-velocity loss threshold (<20%) for muscle hypertrophy.^[10][11] This suggests that although taking every set to muscular failure may not be necessary to maximize muscle hypertrophy, there is a minimum threshold of effort that needs to be met each set to obtain a sufficient hypertrophic stimulus.

The relationship between velocity loss and muscle hypertrophy

Low velocity loss = <20%, moderate velocity loss = 20%–25%, high velocity loss = >25%. Adapted from ^[10].

Although it’s critical to perform enough volume to maximize muscle hypertrophy, volume is not the primary driver of muscle hypertrophy — mechanical tension is.^[12]

During resistance exercise, motor units (which consist of a motor neuron and all of the muscle fibers it innervates) are recruited by the central nervous system in a set pattern based on the level of force that needs to be produced. Smaller, less powerful motor units (i.e., low-threshold motor units) composed of type I (slow-twitch) muscle fibers are recruited first to undertake muscular contractions involving low forces, whereas larger, more powerful motor units (i.e., high-threshold motor units) composed of type II (fast-twitch) muscle fibers are recruited at high forces. During a set of resistance exercise, low-threshold motor units are initially recruited to meet force demands, and as muscular failure is approached, muscle fiber activation progressively increases and high-threshold motor units are recruited to meet force demands.^[13]

Therefore, for type II muscle fibers — which are capable of greater hypertrophy than type I fibers^[14] — to experience the mechanical tension required to induce hypertrophy, sets need to be performed close to muscular failure, otherwise high-threshold motor units won’t be recruited.

Without sufficient mechanical tension, it doesn’t matter how much volume is performed because high-threshold motor units will not be recruited, and thus, hypertrophy of type II muscle fibers will not occur. This explains why volume is not the primary driver of muscle hypertrophy and why aerobic exercise has little effect on muscle hypertrophy. Instead, volume can be thought of as a dial for the overall magnitude of mechanical tension experienced by the muscle fibers once a sufficient proximity to failure is in place. In sum, a sufficient proximity to failure needs to be coupled with sufficient training volume to maximize muscle hypertrophy.^[10]

The current evidence suggests that approximately 12–20 sets per muscle group per week maximizes muscle hypertrophy,^[4] but it’s important to highlight that this recommendation is based on the average response, and the response to resistance exercise is highly individual.^[15] As such, the optimal amount of volume will be significantly less for some individuals and possibly higher for others. However, the available evidence is insufficient to provide a clear cutoff for when it’s appropriate to terminate a set. It’s recommended that sets be performed “with a close proximity-to-failure”^[10] or “near to failure”,^[16] and that “some sets” (but not all) be performed to muscular failure.^[17]

Although further research is needed to clarify the proximity to failure necessary to achieve a sufficient hypertrophic stimulus within a set, as a general rule of thumb, sets should be performed within approximately 4 repetitions of muscular failure. Following this guideline, a combination of failure and nonfailure sets may be used within a hypertrophy-oriented resistance training session. It’s recommended that failure sets be used sparingly to prevent the generation of excessive amounts of fatigue.^[10][17] Moreover, the most appropriate times to apply failure sets are the following:

• During the last set of a given exercise or muscle group
• While performing less taxing and complex movements (e.g., a dumbbell biceps curl, as opposed to a barbell back squat)
• While using lower loads (e.g., 30%–40% 1RM, as opposed to 80% 1RM)
• In the context of low-volume, low-frequency training programs (i.e., <5 sets per muscle group per session; each muscle group is directly trained approximately once a week).^[10][17]

The researchers speculated that the lack of improvement in power may have been a consequence of the training volume being too high. Additionally, the sample size was based on the number needed to detect statistically significant changes in strength and hypertrophy, not power. Therefore, the sample size may have been too small to detect statistically significant changes in power-related outcomes.