EDGE Holistic Fitness

Metabolic vs. Mechanical Stress

November, 16 2012

For years, a litany of fitness professionals, journals, magazines, and blogs have touted on and on regarding the ‘superior methods’ of resistance training for achieving muscle size and strength.  Their argument?  To induce maximum muscle hypertrophy (size) and strength, one must lift heavy loads.  Excellent supposition!  No doubt based on years of scientific inquiry and data…

Indeed, according to many exercise physiologists and their meticulously designed experiments of yester-year, the ideal model for inducing muscle strength and hypertrophy consists of a simple recipe: utilize loads >80% of 1-RM, perform less than 10 reps per set, and take long rest intervals (IE:  2-5 minutes) to allow full recuperation between subsequent sets. Cumulatively, these variables can be categorized into a fundamental training modality that I refer to as Mechanical Stress.

Mechanical stress stimulates muscle adaptations via heavy loading.  In terms of strength improvement, studies have demonstrated that heavier loads (IE:  >80% of 1-RM) may exhibit superior motor unit recruitment patterns as compared to lighter loads.

This intuitively makes sense.  The heavier the load, the more muscle fibers must be simultaneously recruited (by associated muscle neurons) to produce greater force.  This phenomenon is also associated with high velocity contractions.

Regarding muscle hypertrophy, Mechanical Stress-type exercise programs induce surges in testosterone, which then signal protein synthesis (directly or through the action of IGF-1).  Thus, testosterone secretion is believed to be the primary mechanical stress response – a powerful anabolic stimulus for muscle hypertrophy (which likely also contributes to strength adaptations).  As a side, strength adaptations can also be achieved through an increase in neuromuscular functionality – AKA “rate coding” – consequential to heavy loads implemented.

Conversely, more recently, exercise physiologists began conducting research on the possible merits of high intensity, high volume, low-moderate load resistance training programs as an alternative model for muscle hypertrophy and strength adaptations.  Specifically, test subjects would use <75% of 1-RM, 12-25 reps per set, and extremely short rest intervals (IE:  30-60 seconds).  Cumulatively, this type of resistance training program induces what I refer to as Metabolic Stress.

2 fundamental approaches to resistance training

Metabolic Stress-type exercise programs are notoriously renowned for their effect on lactic acid accumulation and the illustrious burning sensations associated (anyone who has done this knows exactly what I am talking about).  Interestingly, a plethora of research demonstrates a strong correlation between lactic acid accumulation (primary cause of ‘metabolic stress’), catecholamines, growth hormone, and EPOC (excess post oxygen consumption – or simply an increase in metabolism post exercise).  Cumulatively, the downstream effects of metabolic stress (lactic acid accumulation) contribute to greater caloric consumption, lipolysis, fatty acid oxidation, and the anabolic/catabolic effects of growth hormone.

Deductively, here is how Metabolic Stress works…

As previously stated, high-volume, short-rest interval  training is a perfect recipe for lactic acid accumulation.  Lactic acid stimulates the metabolic stress response, further increasing metabolic rate, and increasing the secretion of catecholamines and growth hormone.  Within the workout, catecholamines stimulate the breakdown triglycerides into fatty acids (and glycerol) – the fatty acids can then undergo oxidation.  Growth hormone secretion significantly kicks in post exercise, which then assists in the uptake and use of aforementioned fatty acids, in addition to exerting an anabolic effect.  Specifically, growth hormone, perhaps similar to testosterone, also signals protein synthesis (either directly or through IGF-1).  Again, the cumulative effect is an increase in metabolic rate, fat burning, and muscle growth – the primary contribution to strength gains.  However, due to the lower loads implemented, motor unit recruitment and associated strength adaptations are believed to be inferior (low load = low motor unit requirement).

From a practical perspective, Mechanical Stress-type exercises require immense force production per rep, making each rep feel more difficult.  Conversely, Metabolic Stress-type exercises feel much easier per rep in the beginning of a set, but as one progresses to the end of the set, lactic acid accumulates, increasing perceived exertion.  The long rest intervals of  Mechanical Stress-type exercises attenuate this response – lactic acid simply is not a part of the metabolic pathway.

Thus, it could be said that Mechanical Stress workouts are systemically easier – HR remains lower, and overall fatigue may be low due to lack of metabolic stress.  But this ease – per se – may indeed sacrifice caloric expenditure (metabolic rate is lower during and post exercise). On the other hand, Metabolic Stress-type workouts significantly increase HR, rate of perceived exertion, and systemic fatigue – overall contributing to a much more difficult experience – per se; which is why most people avoid it.

Interestingly, Mechanical Stress exercises preferentially increase testosterone, but have demonstrated an insignificant effect on growth hormone.  And yes, as you may have already guessed, Metabolic stress exercises preferentially increase growth hormone, but not testosterone.  Suggesting fundamental causal mechanisms of each hormone:  heavy load = testosterone, moderate load + short rest = growth hormone.

Despite these differences, these two fundamental training modalities appear to be viable in eliciting strength and muscle hypertrophy responses.  Therefore, BOTH training modalities should be incorporated into a progressive resistance training model.  One is NOT absolutely better than the other, but rather, should be implemented according to the individuals’ goals (Metabolic Stress may contribute to greater fat utilization/loss, but perhaps less absolute strength), and physiology (some will adapt better to one vs. the other).

Listed below is a simple comparison of two resistance training programs, using the same exercises and order; yet they yield two drastically different workouts and responses (perform reps till failure):

Mechanical Stress Workout:

  1. Chest Press – 8 reps, 4 sets, 2 minutes rest.
  2. BB Squats – 8 reps, 4 sets, 2 minutes rest.
  3. Lat Pull Down – 8 reps, 4 sets, 2 minutes rest.
  4. Shoulder Press – 8 reps, 4 sets, 2 minutes rest.

Metabolic Stress Workout:

  1. Chest Press – 15 reps, 4 sets, 60 seconds rest.
  2. BB Squats – 15 reps, 4 sets, 60 seconds rest.
  3. Lat Pull Down – 15 reps, 4 sets, 60 seconds rest.
  4. Shoulder Press – 15 reps, 4 sets, 60 seconds rest.

As you can see, this is a very basic distinction between the two training modalities (Vanilla programs – as I call them).  Both programs can be performed in a circuit fashion to further increase systemic demand and increase training volume per unit of time. Moreover, the Metabolic Stress Workout may benefit from the incorporation of Drop sets (fatigue, decrase weight, continue, fatigue, decrease weight, continue…) or Giant sets (Chest Press followed by Chest Fly, etc.); while Mechanical Stress workout may benefit from the inclusion of Supersets (back-to-back exercises that utilize distinct muscle groups).

By the way, if Metabolic Stress sounds familiar, that is because it is the fundamental mechanisms of many of the emerging training fads – P90x, Insanity,etc. – in addition to traditional body building training.

My advice:  If you want to build muscle and burn fat – try Metabolic Stress.  If your primary goal is strength, perform mostly Mechanical Stress.  And always… keep switching it up to avoid training plateaus.


The above blog is a bite-sized, “in a nutshell” excerpt from a 23-page research paper I wrote for my Professional Writing for the Health Sciences class at USC, fall 2011.  Training Induced Lactate and its Effect on Muscle Metabolism and Development:  A Review, was – without exception – an exhilarating educational experience. The knowledge I acquired significantly changed the way I perceive resistance and high intensity interval training, and their subsequent effect on metabolism and muscle adaptations.  I hope you enjoyed reading this as much I did researching and writing it.


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