Mitochondria: Unpacking Cellular Power

Mitochondria are found in every cell of the human body, except red blood cells. These energy-producing organelles play a central role in exercise performance, cellular homeostasis and the prevention of several diseases, including type 2 diabetes, obesity and peripheral artery disease (Groennebaek & Vissing 2017). They are also home to the aerobic ATP-producing cellular pathways known as the citric acid cycle and the electron transport chain.

And they are, indeed, mighty: During prolonged exercise, the energy that mitochondria produce in skeletal muscle can increase up to 100 times, due to efficient breakdown (i.e., oxidation) of food substrates (Drake & Yan 2019). Traditionally, mitochondrial adaptations to exercise have been viewed through the lens of aerobic exercise, but now some groundbreaking studies, reviewed in this column, suggest that resistance training (RT) may stimulate similar adaptations.

The Role of Mitochondrial Biogenesis

Mitochondrial function is often measured by an increased capacity for ATP synthesis.

Mitochondrial biogenesis is the synthesis of components within the mitochondria that lead to an increase in mitochondria volume density and/or function (Groennebaek & Vissing 2017). Groennebaek & Vissing summarize research showing that mitochondrial biogenesis slows down with aging, inactivity and chronic disease.

Several measurable factors indicate mitochondrial biogenesis—an increase in mitochondrial density (more mitochondria per unit of muscle tissue), a boost in cristae density (an increase in the folds of the mitochondrion’s inner membrane that provide surface area for chemical reactions), and an increase in citrate synthase activity (citrate synthase is an enzyme in the citric acid cycle) (Groennebaek & Vissing 2017).

Mitochondrial function is often measured by an increased capacity for ATP synthesis, a process Groenne-baek & Vissing call high-resolution respirometry. The researchers propose that maximal ATP-producing capacity can be improved independently of changes in mitochondrial content.

See also: The Mighty Mitochondria

Short-Duration, High-Intensity Resistance Exercise Increases Oxidative Potential in Muscle

Source: Tang, Hartman & Phillips 2006.

Study question: Can a muscle hypertrophy resistance training program also improve muscle oxidative metabolic potential?

Participants: Twelve untrained male volunteers (average age 22), with no formal RT experience, took part. First, they were familiarized with the testing and training procedures of the study.

Exercise program: Participants resistance-trained 5 days per week (Monday–Friday) for 12 weeks. They used a 3-day cycle that consisted of

day 1: leg exercises (prone hamstring curl, incline leg press, seated knee extension and seated calf raise);

day 2: pushing exercises (vertical bench press, horizontal bench press, seated chest fly and seated triceps extension); and

day 3: pulling exercises (seated wide row, lat pulldown, seated narrow row, seated rear fly and seated biceps curl).

Each workout session was supervised by one of the researchers and lasted 45–60 minutes, following a 12-week linear periodization program (see chart, right):

Results: There was a 46.5% increase in strength for all exercises. Participants significantly increased cross-sectional areas of type 1 (+5%), type IIa (+12%) and type IIx (+17%) muscle fiber types in the vastus lateralis, as measured by muscle biopsies. Additionally, the oxidative potential of skeletal muscle rose significantly, as shown by increases in two oxidative enzymes in the mitochondria: citrate synthase (+24%) and of ß-HAD (+22%).

Major take-away: This study shows that a short-duration, high-intensity RT program that emphasizes hypertrophy can also stimulate aerobic adaptations in mitochondria.

Lower-Load Versus Higher-Load Resistance Exercise: Which Is Better for Enhancing Mitochondria?

Source: Lim et al. 2019.

Study question: Is lower-load or higher-load resistance exercise more effective in eliciting increases in mitochondrial protein content and function?

Participants: Twenty-one untrained males (average age 23) were randomly assigned to three groups:

1. high-load, low-repetition group: exercises at 80% of their 1-RM (all exercises) to failure (the point where the neuromuscular system can no longer produce adequate force to overcome a specific workload)
2. low-load training at 30% of 1-RM (all exercises), completing the same total work as the high-load group (i.e., groups were work-matched)
3. low-load training at 30% of 1-RM to failure

Exercise program: All participants resistance-trained 3 days per week for 10 weeks. The program focused on the lower body; subjects performed 3 sets of leg press, knee extension and hamstring curl. To ensure that training load stayed on track throughout the study, 1-RM on the exercises was remeasured after weeks 4 and 8.

Results: Group 1 (80% of 1-RM to failure) and group 3 (30% of 1-RM to failure) both saw a significant increase in the cross-sectional area of type 1 (slow-twitch) muscle fibers, whereas group 2 (30% of 1-RM work-matched) saw no significant improvement in this area. All three groups showed a significant boost in type II (fast-twitch) muscle fiber CSA. Interestingly, a number of mitochondrial proteins related to metabolism increased with resistance exercise, but mitochondrial adaptations were greatest in the group performing the higher-volume, lower-load resistance exercise to failure (group 3).

Major take-away: Training at 30% of 1-RM to failure, which involves performing significantly more repetitions per set than the other protocols, is more effective at stimulating mitochondrial adaptations.

See also: Loaded Movement Training

Practical Applications for Personal Trainers

Any periodized training program can include a block of high-volume, low-intensity exercises to evoke metabolic stress.

The great news is that personal trainers have multiple options to help clients attain favorable musculoskeletal and mitochondrial outcomes. Clearly, the linear periodization hypertrophy emphasis study by Tang, Hartman & Phillips may provide a very popular training model, as fitness pros can easily customize the program for clients who are seeking to increase muscle size.

Similarly, any trainer who offers periodized training programs can include a block of high-volume, low-intensity exercises—as demonstrated in the Lim et al. study—to evoke the metabolic stress required for muscle cells to increase their oxidative capacity.

For example, you can select a load that clients can do for 20–30 consecutive repetitions and have them complete their sets very close to muscular failure (a 9 out of 10 on an RPE scale). Have them perform 2–3 sets, allowing for 2–3 minutes of rest between sets so clients can sustain a high level of effort. Use this for any multiple- or single-joint exercise for any muscle group.

Another high-volume, low-intensity training idea you can incorporate with clients is the use of  drop sets. Begin with a load your clients can lift for 8–12 consecutive repetitions. After they reach failure, drop the load by ~25% and have them perform another set to failure with no rest in between. Repeat for 3–4 drops or until they have completed 30–40 repetitions. For safety, it may be best to complete drop-set training on RT machines with weight stacks.

For additional variety, perhaps have clients perform some battle rope exercise (i.e., single- or double-arm waves) with maximal effort for 30 seconds. Allow them to rest for 60 seconds. Repeat 6–8 times.

Can resistance training enhance mitochondria? Yes! Go for it!

See also: Exercise Has Significant Impact on the Cellular Level