When we talk about exercise recovery, we talk about muscle protein synthesis. Each time, we throw in a 1-2 sentence explanation of muscle protein synthesis (MPS), but it’s worth taking a closer look. MPS is important for all types of exercise, and this, of course, includes endurance exercise. So, while most of the times you may read about MPS on the good ol’ internet, MPS is described from the perspective of a weightlifting-based sport such as, powerlifting, bodybuilding, and obviously weightlifting, but this creates a misconception. MPS is not specific to the weight-based sports. When the goal is muscle development, there are many other factors that come into play. Those factors, not general MPS, is what makes the difference between muscle building and muscle recovery, as is the case with lifting versus endurance exercise, respectively. Therefore, endurance athletes should not be afraid to support a little MPS – you’re not going to gain (probably any) muscle. Let’s dive in!
Muscle Protein Synthesis Explained
Muscle protein synthesis is the primary driver of muscle repair, recovery, and development. MPS simply refers to the general process of repairing existing muscle proteins and/or adding new muscle proteins. Muscle proteins include the myofibrillar (contractile) proteins that we use to perform muscle actions (flexion, extension, running, jumping, all of the movements), sarcoplasmic (muscle intracellular contents – not muscle cell “size” per se, but can be thought of as such, independent from myofibrillar) proteins that include increasing oxygen-binding myoglobin, muscle glycogen capacity, and cellular fluids, and mitochondrial (energy-producing sub-cellular component [an organelle]) proteins that turn carbs, fats, and proteins into energy. One type of MPS typically does not occur wholly independent from another type, but the training stimulus is the deciding factor.
Clearly, athletes that do a lot of weight lifting and protein eating build large muscles, but even the type of weight lifting performed dictates what type of MPS is the primary type of MPS that occurs within the muscle. A powerlifter or Olympic weightlifter that squats with high loads (e.g., >85%) will mostly stimulate myofibrillar protein synthesis, with some sarcoplasmic protein synthesis, and very little mitochondrial protein synthesis. A bodybuilder that squats with moderate loads (e.g., 60-80%) will stimulate more sarcoplasmic and mitochondrial, but less myofibrillar, protein synthesis. Similarly, a runner that runs for 10 miles at a 6-minute mile pace will stimulate a lot of mitochondrial protein synthesis, some sarcoplasmic protein synthesis, and very little myofibrillar protein synthesis. In a nutshell, if you’re training for the thing you want – that’s what you’re going to get!
At the cellular level, muscle protein synthesis is the process of binding many free amino acids into proteins (myofibrillar, mitochondrial, etc.). All of the resultant protein products are formed specific to their function based on the DNA/RNA “blueprint” from which they are made. Basically, there is a bigwig construction company (nucleus) that has blueprints (DNA) for all of buildings (proteins including muscle, enzymes, etc.) in a major city (the cell), the company makes copies of the blueprints (RNA) and sends them out to the construction sites (ribosomes) to be built. Brick-by-brick (amino acids), the workers (transfer RNA) build the building (e.g., myofibrillar proteins actin and myosin) until it is complete and can then serve its function (in this case, muscle contraction). When the building is run into by a car (exercise), and needs to be repaired, it’s the same process, with greater specificity to only rebuild the damaged parts. Or more simply, muscle protein synthesis is taking one Lego (amino acid) and sticking it onto another Lego until you have a nice big tower (protein)! And when your older sibling kicks it to give you a hard time (exercise), the same process starts over! Fun fact – muscle cells are one of the only types of human cells to have more than one nucleus! Neato!
How is Muscle Protein Synthesis Regulated?
There are a number of ways that we can influence MPS. One of the most well-known is by eating tons of protein, but in reality, if you eat a lot of Calories and just a normal amount of protein, you can stimulate MPS as well. This is because MPS is also regulated by energy status. A very common example is with dieting – some people think they will only lose fat, but that’s not the case. Muscle is lost along the way (but it can be attenuated by doing other things that increase MPS, like eating protein). Carbohydrates may also stimulate some MPS by causing insulin secretion. Another fun fact – insulin is the most anabolic of human hormones. Even more than testosterone!
Exercise is another major way we can affect MPS. Resistance exercise is obvious, but did you know that endurance exercise can add muscle mass as well? This is true when first beginning an exercise regimen. A previously sedentary person that begins running or cycling or rowing or another type of endurance exercise will begin to add muscle. Moreover, even a trained athlete can stimulate MPS and add muscle by performing high-intensity interval training. And if you want to be any good at any endurance-based sport, you’re going to be doing some regular endurance work and HIIT.
MPS is also largely regulated by its opposite – muscle protein breakdown. There’s a saying in biology that goes, “every anabolic process is preceded by a catabolic process.” In other words, something must be broken down before something else can be built. Exercise also increases muscle protein breakdown. Depending on the type, volume, duration, and intensity of exercise, rates of muscle protein breakdown may even exceed that of stimulated MPS. This is common for endurance exercises. Conversely, resistance training can cause such a large increase in MPS that breakdown has a hard time keeping up. Fun fact #3 – walking at a pace >3 mph is sufficient to suppress myofibrillar MPS.
Muscle protein breakdown is increased by having low available amino acids (eating low quantities or qualities of protein), low energy (calorie restriction), extended bouts of exercise (>~1 hour [can also induce low energy status]), and high catecholamines (being stressed).
Why do Endurance Athletes Need MPS? And Why Won’t They Get SOOO JACKED?
All athletes … All humans need MPS. Endurance athletes need MPS, not to build muscle (technically, some, but if you’ve read this far, you probably already have all the muscle mass you will get from endurance exercise) but, to repair muscle damage and alleviate soreness caused by exercise. If you have fears of suddenly waking up with a mysterious set of huge new quads, that’s simply not going to happen. Endurance exercise is largely benefitted from having relatively small muscles, and endurance exercise causes adaptations to create that exact physique. Slightly larger muscles than an inactive person, and that’s all. An endurance athlete consuming more protein or even doing a little resistance training will not create any muscle growth except to perhaps increase the potential to produce ATP (mitochondrial protein synthesis) or increase the capacity to store glycogen (sarcoplasmic protein synthesis). All-in-all it would take a significant reduction in endurance training volume to start adding anything more than a little bit of weight (and even that little bit would need to be purposefully created with high volumes of weight training).
Because exercise also causes muscle damage, we can’t just do some resistance training after endurance exercise to boost MPS and heal tissue. That’s why the most efficacious interventions are nutritional. To recover from exercise (including endurance exercise) the athlete needs MPS. Even if you’re currently doing nothing on purpose to facilitate MPS and exercise recovery, gradually over time MPS fixes the muscle damage created by exercise. However, endurance athletes don’t have that luxury – we need our legs every day! This is why it is so important for the endurance athlete to foster MPS and the recovery process by consuming the adequate protein and calories necessary to get back to training faster.