Why training to failure is important for hypertrophy.
Mechanical tension is the force generated by muscles as they contract against resistance. It's one of the key factors that drives muscle growth, along with metabolic stress and muscle damage. When muscles are subjected to mechanical tension, it activates a variety of signaling pathways that promote muscle hypertrophy, including the activation of mTOR and the upregulation of myogenic transcription factors.
Research has shown that mechanical tension is a primary driver of muscle hypertrophy. A 2010 study published in the Journal of Strength and Conditioning Research reviewed multiple lines of evidence that suggest mechanical tension plays a critical role in muscle growth. The study's author, Brad Schoenfeld, concluded that mechanical tension is the primary driver of hypertrophy. The study found that mechanical tension induces a variety of signaling pathways that promote muscle growth, including the activation of mTOR and the upregulation of myogenic transcription factors. The PMID number for this study is 20847704.
Another study published in the Journal of Physiology in 2010 investigated the effects of different loads on muscle growth in human subjects. The study found that lifting lighter loads to failure, which resulted in the same number of total reps as lifting heavier loads to failure, produced similar hypertrophic responses in the muscles. This suggests that mechanical tension, rather than the specific load lifted, is the key driver of muscle growth. The study concluded that "the stimulus for muscle hypertrophy appears to be total work performed, with the load being secondary, as long as the load is sufficient to achieve the desired level of fatigue." The PMID number for this study is 20711498.
So why is mechanical tension so important for muscle growth? The answer lies in the fact that muscles adapt to the stresses placed upon them. When you lift weights, you create tension in your muscles, which triggers a process of adaptation that results in muscle growth. Over time, as you continue to subject your muscles to mechanical tension through progressive overload, they will continue to adapt and grow.
One of the best ways to create significant amounts of mechanical tension is through training to failure. When you train to failure, you activate the highest threshold motor units in your muscles, which are responsible for generating the most force. These motor units are typically only recruited during high-intensity, high-force activities, such as lifting heavy weights. By training to failure, you ensure that these motor units are fully activated, creating a high degree of mechanical tension in your muscles.
However, not all reps in a set taken to failure are equally effective for activating these high-threshold motor units. In fact, the last 5 reps of a set taken to failure are the most effective for recruiting these motor units. This is because as you approach failure, the lower-threshold motor units begin to fatigue, which means the higher-threshold motor units are recruited to maintain force output. By pushing yourself to complete the last 5 reps of a set, you ensure that you are activating these high-threshold motor units and creating maximal mechanical tension in your muscles.
The below picture demonstrates clearly how load does not determine the amount of hypertrophy and how mechanical tension is instead the driver of muscle growth. (Credit: @pheasyque on instagram)
To summarize, mechanical tension is a critical factor in muscle growth, and training to failure is an effective way to create significant amounts of mechanical tension through the activation of high-threshold motor units. By focusing on completing the last 5 reps of a set taken to failure, you can maximize the activation of these motor units and create the greatest possible amount of mechanical tension in your muscles. So if you want to maximize your muscle growth, focus on lifting heavy weights, pushing yourself to the point of failure, and prioritizing the last 5 reps of each set. And don't forget to give your muscles time to rest and recover between workouts, as this is when they will grow and adapt to the mechanical tension you've placed upon
Schoenfeld BJ. The mechanisms of muscle hypertrophy and their application to resistance training. J Strength Cond Res. 2010 Oct;24(10):2857-72. doi: 10.1519/JSC.0b013e3181e840f3. PMID: 20847704.
Burd NA, West DW, Staples AW, et al. Low-load high volume resistance exercise stimulates muscle protein synthesis more than high-load low volume resistance exercise in young men. J Physiol. 2010 Jan 1;588(Pt 1):33-43. doi: 10.1113/jphysiol.2009.179215. Epub 2009 Nov 2. PMID: 20711498.