Physiology Friday #177: Prior Strength Training Impairs the Mitochondrial Response to Endurance Exercise
Physiology Friday #177: Prior Strength Training Impairs the Mitochondrial Response to Endurance Exercise
Is there new evidence in support of the “interference effect”?
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Endurance athletes are typically advised to incorporate strength training into their routine. After all, having more muscle, less fat, and more strength and power will allow someone to train harder, avoid injury, and be a stronger, more well-rounded athlete.
However, resistance training also has some underappreciated benefits: it can also enhance mitochondrial content and function, a benefit often only ascribed to endurance exercise training.
As such, performing strength and endurance training at the same time — known as concurrent training — should be synergistic.
However, it was once thought that concurrent strength and endurance training would somehow limit the benefits of either modality, a concept known as the “interference effect.” Molecular responses to strength training were posited to somehow be incompatible with those of endurance training, and vice-versa. However, recent studies have mostly debunked the interference effect.
What is less known is whether performing a block of resistance training before a period of endurance training — let’s call it “strength preconditioning” — enhances, diminishes, or impairs adaptations. Interestingly, a recent study found that performing endurance training prior to resistance training enhanced strength adaptations. Is the opposite true?
It can be hypothesized that resistance-training-mediated increases in myonuclei and ribosome content (along with strength and other neuromuscular benefits) would allow for a greater muscle fiber capacity for transcription/translation of mitochondrial protein and thus, enhanced mitochondrial adaptations to subsequent endurance training.
This hypothesis was investigated in a study published in the Journal of Physiology.1
Twenty five healthy young men were randomized to one of two groups: endurance training only (ET-only) or resistance training + endurance training (RT+ET).
In the ET-only group, participants performed 7 weeks of high-intensity interval training (HIIT): exercise was performed 2–3 times per week and comprised a 3-minute warmup followed by a series of 1-minute sprints at 80–100% of participants’ VO2 max. Throughout the 7-week intervention, the number of sets increased from 5–10, the effort increased, and the recovery duration between sets decreased from 3 to 1.5 minutes.
In the RT+ET group, participants first performed 7 weeks of progressive resistance training: exercise was performed twice per week and included leg press, bench press, leg extension, cable pulldown, and leg curls.
After resistance training, participants waited 1 week and then performed 7 weeks of endurance training as described above.
Performance outcomes (VO2 max, speed at onset of blood lactate, and strength), body composition, and muscle-fiber-related variables including cross-sectional area, myonuclear content, myonuclear domain size, satellite cell number, and mitochondrial content were assessed before training and after each 7-week training period.
Results
The resistance training program elicited several common adaptations, including improved body composition, increased muscle strength and muscle thickness, greater mixed and type II muscle fiber cross-sectional area, and increased myonuclear number, ribosome content, and satellite cell number.
The resistance training program failed to induce mitochondrial adaptations: there were no changes in mitochondrial protein complex levels or markers of mitochondrial content.
The HIIT program also elicited several well-known benefits of aerobic exercise training, including an increase in relative VO2 max (13.4% in the RT+ET group and 10.6% in the ET-only group) and an improvement in speed at the onset of blood lactate (7% in the RT+ET group and 12% in the ET-only group).
Body fat % also decreased in both groups after 7 weeks of HIIT.
The endurance-performance-related improvements were not enhanced, by prior resistance training, despite the various beneficial adaptations observed in the RT group.
Furthermore, many of the mitochondrial adaptations to endurance exercise were impaired following the period of resistance training when compared to ET only.
Increases in the mitochondrial protein complexes I–IV were significantly lower (1–11%) in the RT+ET group compared to the ET-only group (32–66%). Relative mitochondrial content in mixed muscle fibers decreased by 13% in the RT+ET group but increased by 15% in the ET-only group.
There was no increase in type I muscle fiber mitochondrial content in either group.
What could explain the distinct responses in the RT+ET group?
The authors speculate that: “the transition from RT to ET in the RT+ET group promoted a sustained elevation in muscle protein breakdown mechanisms while diminishing the protein synthetic response. An ultimate consequence of this shift may have included myofibre atrophy accompanied by a decrease in cellular mitochondrial and ribosome content.”
In other words, the catabolic nature of endurance training may have actually led to the impaired adaptations.
Let’s be clear — the RT+ET group still experienced adaptations in mitochondrial content after training, they were just lower than those experienced by the ET-only group. On the other hand, performance adaptations were similar.
We must consider the fact that the participants in this study were healthy, albeit untrained young men without much prior resistance or aerobic exercise experience. Thus, whether similar adaptations/interference would occur in older participants, women, or trained participants/athletes is unclear and will have to be investigated in future studies.
Is this study a cautionary tale for endurance athletes to avoid resistance training? Will lifting weights increase strength and size at the expense of valuable mitochondria? Not quite.
Of course, choosing one type of exercise over another will always involve tradeoffs of some sort, and there is some evidence that strength and endurance exercise may interfere with one another.
However, if you’re consistently training strength and endurance concurrently (during the same training period), there’s probably little to worry about. I think this is illustrated by the fact that, despite the “maladaptive” mitochondrial responses to endurance training, actual performance didn’t decrease. The molecular outcomes didn’t manifest as worse function.
So, feel free to prioritize whichever exercise mode you enjoy, but know that strength and endurance can coexist.
Thanks for reading. See you next Friday.
~Brady~
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