Sets must be taken close to failure to result in optimal muscle growth. Exercise must reach a certain threshold to stimulate muscle growth. Light weight training stopping short of failure results in sub-optimal muscle growth.
EXERCISE INTENSITY AND MUSCLE GROWTH. AT WHAT POINT DO YOU NEED TO STOP A SET FOR MUSCLE GROWTH SUMMARY
- Junk volume refers to adding sets but not meeting the minimum expectations for exercise intensity.
- Sets must be taken close to failure to result in optimal muscle growth.
- Exercise must reach a certain threshold to stimulate muscle growth.
- Light weight training stopping short of failure results in sub-optimal muscle growth.
- Heavy weight resistance exercises may increase androgen receptors.
UPDATE: 4/18/2022: FATIGUE DURING EXERCISE RELATED TO MUSCLE GROWTH
For muscle growth to occur, fatigue must occur with the intensity of effort with each set. Velocity-based training monitors the magnitude of velocity loss during a set, which is an indicator of increased muscle fatigue. As fatigue increases, the velocity or how fast the bar slows down.
Researchers had subjects trained for eight weeks and divided subjects into four different velocity loss programs: 0%, 10%, 20%, and 40%. 40% velocity loss represents training will high fatigue close to or at failure.
At the end of the study, only the 20% and 40% velocity loss groups had increases in muscle growth of the legs. The 40% velocity loss had a +7.0 increase in the cross-sectional area of muscle growth, while the 20% velocity loss had a +5.3% increase in muscle cross-sectional area.
Training at a low level of fatigue or intensity of effort was not shown to increase muscle growth, as indicated by the absence of muscle growth in these groups (0% or 10% velocity loss). This reinforces that a certain amount of effort or fatigue must take place for muscle growth to occur. 
4/28: GREATER VELOCITY LOSS IS BETTER FOR MUSCLE GROWTH AND WORSE FOR STRENGTH GAINS.
Velocity loss is the percentage of decrement that occurs over a set. It tracks the bar velocity over a set. A velocity loss of 40% results in more repetitions being performed than a 20% velocity loss. As fatigue sets in, the bar moves slower, and a greater velocity loss occurs.
A recent meta-analysis compared velocity loss <25% (i.e., training further away from failure) to velocity loss >25% (i.e., training closer to failure). The studies’ review found that those with greater velocity loss (>25% velocity loss) and trained closer to failure had greater muscle gains but less strength gains. Conversely, those that trained with a lesser velocity loss (<25% velocity loss) had greater strength gains but less muscle growth.
The greater fatigue and metabolic stress associated with training with reps closer to failure have been suggested to be a contributing factor to muscle growth. Those training with a greater velocity loss are also performing more repetitions than those with a lower velocity loss. The author of the paper suggested that the higher volume associated with greater velocity loss could be driving muscle hypertrophy rather than fatigue.
The lower amount of fatigue with lower velocity loss is suggested to provide favorable neuromuscular adaptations to promote strength gains. Training at velocity loss thresholds of 0–25% and lowering fatigue enables the utilization of higher percentages of 1RM more frequently to train the high-force component of the force-velocity profile for 1RM strength adaptations.(12)
9/22 GREATER VELOCITY LOSS ASSOCIATED WITH GREATER GAINS IN MUSCLE MASS
Training closer to muscular failure has been associated with a greater increase in muscle mass than training further away from failure. A velocity loss of 40-50% implies that a muscle is close to or near muscular failure. Previous studies have found that training with more velocity loss or closer to failure results in greater muscle mass than training further away from failure or with less velocity loss.
However, if strength is your primary goal, training further away from failure results in better neurological adaptations, possibly because of lesser fatigue. For example, training with 40% velocity loss resulted in greater muscle mass than 20% velocity loss, but the 20% velocity loss group had greater strength gains. (13) Researchers once again found that training closer to failure results in greater muscle mass, whereas training further from failure resulted in greater strength gains.
Velocity Loss Studies
Researchers randomized 50 resistance-trained young men to one of four groups in a counterbalanced sequence based on their bench press 1RM: 0% (VL0), 15% (VL15), 25% (VL25), 50% (VL50) velocity loss thresholds. They measured muscle mass, bench press strength, and fatigue. The training intervention lasted eight weeks, during which they bench pressed twice per week with an intensity of 55-70%, increasing linearly. Three sets were performed, each session with four minutes of rest between sets.
At the end of eight weeks, only the Only VL50 showed significant increases in muscle growth (i.e., group training closer to failure).VL15, VL25, and VL50 significantly improved 1RM strength, but VL0 did not. Only VL0 and VL15 showed a significant increase in maximal unloaded velocity.L50 performed significantly more maximum reps post-training during the fatigue test than VL0. If you look at the total reps completed, the V50 group performed significantly more reps, training closer to failure. The repetitions completed were:
VL0= 48 reps
VL15= 189 reps
VL25= 310 reps
VL50= 491 reps
If you are looking to build muscle, the study suggests that sets should be performed closer to muscular failure, whereas if strength is your primary focus, train further away from failure. The VL25 group attained the greatest gains in 1RM strength. (14)
HIGH INTENSE DEFINITION: HOW INTENSE SHOULD EXERCISE BE?
Mike Mentzer was a bodybuilder in the ’80s who recommended High-Intensity Training, and advocated brief, intense workouts. Mentzer believed that a set had to reach a certain intensity threshold to stimulate muscle growth. Some concepts, such as training one set per body part, were misguided, but he understood the relationship between exercise intensity and recovery.
Mentzer advocated low reps (6-9 reps), heavy weights, training to complete muscular failure, and adequate recovery (i.e., brief, infrequent workouts). He also advocated one to two sets per body part because he understood that high-intensity training resulted in fatigue and, therefore, very few sets could be performed without overtraining. This was before there was any research on intensity and muscle growth. Still, he understood that exercise must reach a certain intensity or effort threshold to stimulate muscle growth, and sets that stopped short of an intensity threshold were not as effective for muscle growth.
He also understood that taking muscle groups to failure resulted in increased muscle damage and fatigue, so workouts were limited to one bodypart per week. He realized that training a muscle before it was fully recovered resulted in impaired workout performance the next workout. Although, it’s known that all sets do not need to be taken to failure for muscle growth. Both light weight taken to failure and heavy weight stopping a few reps short of failure stimulates muscle growth equally. Mentzer was correct that exercise intensity must reach a certain threshold for muscle growth to occur.
HIGH-INTENSITY EXERCISE MAY BOOST ANDROGEN RECEPTORS
Since we are on the topic of intensity, although this is highly controversial, high-intensity exercise or heavy weight may boost androgen receptors. The androgen receptor is what circulating testosterone binds to and activates protein synthesis. Think of a store looking to get as many people as possible in the store for sales (i.e., muscle growth). The store will not have a very successful business if the parking lot is too small. If the store’s parking lot has a max capacity of ten parking spots (few androgen receptors), fewer people (i.e., testosterone) will go into the store.
Now think of the same parking lot with 50 parking spots (more androgen receptors). More cars can be parked in those spots, and more business will occur. The same concept applies to testosterone and androgen receptors on the muscle; more androgen receptors can result in more testosterone having a greater physiological response.
Androgen Receptor Studies
Studies have found that people with more androgen receptors have greater muscle mass.  One study found that heavy weight resistance exercises increased androgen receptors post-exercise. In contrast, the light weight group had no effect.  Although we know that both heavy and light weight training can produce similar muscle growth, it points out that different training intensities can have different molecular effects on the muscle. Your chest may grow well with a certain number of sets, and your calves may not, despite doing the same sets. It could be because of more androgen receptors in the chest, causing greater muscle growth.
It’s been found that there is a direct correlation between increases in androgen receptors and muscle growth. In contrast, post-exercise changes in testosterone were not found to have an effect.  Others have found that overtraining reduces androgen receptor concentrations in muscle.  Tension overload on the muscle influences increasing androgen receptors.  This again suggests that a combination of heavy and light weight can optimize muscle growth.