Pros of Training to FailureMotor unit recruitment is maximally recruited when training to failure. Simply put, a motor unit is your motor neuron and all the muscle fibers it innervates. The size principle of recruitment dictates that as training intensity increases, larger motor units containing fast twitch (Type 2) muscle fibers are progressively recruited to maintain the level of force to lift the weight. The closer you train to the point of failure or 1RM the higher number of fast twitch muscle fibers are recruited and it is these high threshold fibers that have the most potential for growth.

This simplification of muscle fiber recruitment patterns shows that by taking sets to failure you are exhausting more muscle fibers than if you stopped the set short of failure – and is strong support for the practice of training to the point of momentary muscular failure.

In addition, training to failure also heightens metabolic stress. Metabolic stress refers to the buildup of various metabolites like lactate and hydrogen ions which are thought to spur the muscle cell to grow. The mechanisms that contribute to muscle growth resulting from metabolic stress include increased fiber recruitment, elevated systemic hormonal production, alterations in local myokines, heightened production of reactive oxygen species and cell swelling. This metabolic stress in the muscle signals adaptation, and can increase satellite cell activation as well as activation of the mTOR pathway which in turn means increased protein synthesis and muscle size.

Cons of Training to FailureOn the other side of the coin, there is some evidence to indicate that training to failure on every set significantly increases resting levels of the catabolic hormone cortisol and suppresses anabolic growth factors such as IGF-1. In this 2006 study led by Dr. Mikel Izquierdo, it was demonstrated that subjects who avoided to failure training had lower testing cortisol levels, who those who did train to failure had reductions in resting IGF-1.¹ This may mean that bodybuilders who take every set to absolute failure may be putting themselves at risk for slower long-term muscle growth and strength gains.

Training to failure also has a high impact on the Central Nervous System (CNS). The CNS takes as much as five to six times longer than the muscles to recover from intense training. So by constantly going to muscle failure, you can overload the CNS so much that it becomes impossible to train with a high frequency. Fatigue occurs in part due to the depletion of the neurotransmitters dopamine and acetylcholine. Each neuron must release acetylcholine every time that it fires (or ‘twitches’) a motor unit. A decrease in acetylcholine levels is associated with a decrease in the efficiency of the neuromuscular transmission – eventually muscle contraction becomes markedly slower and weaker. Eventually the nerve cell will assume a state of inhibition to protect itself from further stimuli.

Just like every other mechanism in the body, CNS fatigue has a negative feedback loop. When the body is overstressed peripherally, the CNS inhibits its excitatory procedures causing a state of fatigue and recovery resulting in a loss of performance, coordination, motivation to train, fatigue, muscle tremors and eventually if over training continues, depression.

ConclusionSo at the end of the day, what should you do? Train to failure or not?

You should do both. Going to failure or not should be an exercise-dependant variable. The more demanding an exercise is on the CNS, the farther away from failure you should stop the set. So for exercises like squats, deadlifts, free weight pressing or other “full body” compound movements, you should stop short of failure. Leave some in the tank, and complete another set rather than taking your current set to failure.

However, in exercises where the CNS is less involved, like machine pressing, and  chest, quad, hamstring, lower back, ab, calf and arm isolation work you should go to failure on at least one set per exercise to maximize motor unit recruitment.

Clearly, personal recovery patterns have to be considered and monitored when a training approach is adopted – you may be able to take more sets to failure than your training partner, however this guideline should help you get you started!