Mechanical vs. Metabolic Failure: How Do They Impact Strength Training Success?

Mechanical vs. Metabolic Failure: How Do They Impact Strength Training Success?

(Last Updated On: June 21, 2020)

Mechanical vs. Metabolic Failure

When your goal is to build strength or muscle size, how you train, and the consistency of that training matters. The most basic principle of strength training is progressive overload, the practice of increasing the demands on your muscles over time. You start out using a light weight or no resistance at all, but after a time, moving that weight is easier and you can perform more reps. So, you increase the weight. In response to the stimulus you place on your muscles, your muscles adapt and become larger and stronger. It’s a tried-and-true formula for muscle and strength gains.

Most people think progressive overload means increasing the resistance by using a heavier weight, but there are other ways to supply overload, such as increasing the number of repetitions, total number of sets, tempo of the exercise, frequency of training, training volume, and more. You can even use advanced training techniques such as supersets to stress your muscles in a different way to jumpstart growth.

Another topic you hear about that relates to strength training is the concept of “failure.” No, this isn’t failing to train, but pushing your muscles to where they can’t complete another repetition using good form. Some fitness experts believe people should train to failure to maximize growth and strength gains, although research shows that working a muscle to failure isn’t necessary to build strength and muscle size. In fact, there are two types of failure: mechanical and metabolic failure. What’s the difference between the two?

Mechanical vs. Metabolic Failure

Let’s look at mechanical failure first. When your muscles reach mechanical failure, it fatigues the muscle to the point that the weight feels too heavy to complete a full repetition. The problem is mechanical fatigue, your muscles, and nervous system are so exhausted they can’t generate enough force to contract against the current resistance. You could only move the weight if you have someone assist you or switch to a lighter weight.

In contrast, metabolic failure is where you experience burning in your muscles because of acid build up in the muscles you’re working. Where does the acid come from? Hydrogen ions accumulate when your muscles can’t get enough oxygen to supply energy to the muscle via aerobic pathways. So, fatigued muscles resort to using anaerobic pathways that don’t require oxygen to produce ATP, the energy source your muscles use to contract.

When your muscles use anaerobic pathways, lactic acid builds and this decreases the pH inside muscle cells. You may have heard that the burn comes from lactic acid or lactate, but more recent studies suggest this isn’t the case. Lactic acid is a by-product of muscles using anaerobic energy pathways, but it’s the hydrogen ions that are responsible for the burning you feel when a muscle is exhausted. Nevertheless, when a muscle exhausts because of the build-up of hydrogen ions, it’s reached metabolic failure.

Training to Muscle Failure

Should you train to mechanical or metabolic failure? Some studies suggest you should lift to mechanical or metabolic failure to maximize training and that doing so is more effective than just fatiguing the muscles. However, some of these studies didn’t control for the total volume of training.

A theory as to why mechanical failure is beneficial goes like this. When you work a muscle to failure, you maximize muscle unit recruitment, including those high threshold units. Recruiting more muscle units is, theoretically, a way to boost strength and hypertrophy gains. However, there’s no powerful evidence that you must do this to maximize strength and hypertrophy, but you do need to fatigue the muscles.

What about metabolic failure? The idea behind training to metabolic failure is that the build-up of lactic acid when muscles use anaerobic pathways boosts the release of growth hormone and testosterone, hormones that help with fat burning and muscle gains. However, Spanish researchers also found that training to failure with every set boosts the level of catabolic hormones, such as the stress hormone cortisol, and that counters some potential muscle gains. Research also shows that failure training with every set reduced IGF-1, a powerful anabolic growth factor.

Plus, training to mechanical or metabolic failure during most sets is taxing on your nervous system. This is detrimental since an interplay between brain and muscle drives adaptations to strength training. Plus, the extra stress and fatigue that goes along with training to failure can lead to exhaustion and force you to reduce your total training volume and that can negatively affect hypertrophy gains.

The Bottom Line

There are benefits to training to mechanical or metabolic failure but do it in moderation. Taking every set to failure will cause excessive fatigue and work against your goals by stimulating greater cortisol release. Instead, research muscle failure for the last set of an exercise, as a “finisher.” There’s no established evidence that you need to train to failure to build muscle or gain strength. A better approach is to lift to failure on your last set for some exercises rather than every set. Even then, do it only a few times per week. Training to failure also isn’t a smart idea when you first start out and haven’t built up a baseline level of strength. Remember the basics too. If you’re training hard, optimize your nutrition and recovery strategies. Both are important for optimizing muscle growth.



  • World J Methodol. 2017 Jun 26; 7(2): 46–54. Published online 2017 Jun 26. doi: 10.5662/wjm.v7.i2.46
  • com. “Ask The Muscle Prof: Is Training To Failure Helping Or Hurting Me?”
  • Izquierdo M, Ibañez J, González-Badillo JJ, Hakkinen K, Ratamess NA, Kraemer WJ, French DN, Eslava J, Altadill A, Asiain X, Gorostiaga EM. Differential effects of strength training leading to failure versus not to failure on hormonal responses, strength, and muscle power gains. J Appl Physiol. 2006 May; 100(5): 1647-56.
  • Schoenfeld BJ. Potential mechanisms for a role of metabolic stress in hypertrophic adaptations to resistance training. Sports Med. 2013 Mar;43(3):179-94.
  • Nóbrega SR, Libardi CA. Is Resistance Training to Muscular Failure Necessary? Front Physiol. 2016;7:10. doi:10.3389/fphys.2016.00010.
  • Willardson, J.M. (2007). The application of training to failure in periodized multiple-set resistance exercise programs. Journal of Strength and Conditioning Research. 21(2), 628-631.


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Why Training to Failure When You Strength Train May Be Counterproductive

Muscle Fatigue vs Muscle Failure: What’s the Difference?

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Weight Training: Is It Better to Do More Sets?


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