Strength training and aerobic exercise – they both have health benefits. Strength training helps to build and preserve muscle strength and size, especially as you age. Working with weights, barbells, or resistance bands also help you build metabolically active muscle tissue to keep your metabolism from slowing over the years. Plus, having more muscle is metabolically healthy since muscle acts as a “glucose sink” by taking up glucose and removing it from the bloodstream. Yet, we also need to be concerned about the health of a small but important muscle – your heart. Aerobic exercise makes your heart a more efficient pump and lowers your risk of cardiovascular disease, the most common cause of death in Western countries.
Now you know why you need BOTH types of training – but how do you schedule them? Depending upon time availability, you might find it most convenient to do strength training and cardio on the same day. This approach is called concurrent training and it’s a hotly debated topic among fitness professionals. Some fitness trainers believe the two forms of training should be separated by at least 24 hours due to a possible interference effect, the concept that aerobic training and strength training adaptations are different and that doing them too close together interferes with gains from both types of exercise when you do concurrent training. The belief is mainly that aerobic training interferes with strength gains.
Adaptations to Strength and Endurance Exercise
The way your body adapts to strength and endurance exercise differ. In response to endurance exercise, mitochondria, the tiny organelles inside cells that produce ATP, increase in number. When there are more mitochondria, there’s a greater capacity to produce ATP since there are more ATP-generating machines to do the job. The enzymes that they use to make ATP also increase. The mitochondria essentially become more efficient producers of ATP, a cell’s energy source, and can provide muscles with more ATP to fuel contraction. As a result, aerobic capacity goes up.
Endurance exercise also activates AMPK, known as a cell’s energy sensor. Activation of AMPK increases fat oxidation during a workout and also turns on the building of new mitochondria. So, AMPK is critical to the positive adaptations you get from aerobic exercise as it turns on the production of new mitochondria, a key way cells adapt to aerobic exercise. More mitochondria mean more ATP, which leads to the greater capacity to support sub-maximal exercise.
Your body adapts to strength training in a different way. When you work your muscles harder than they’re accustomed to, it creates microscopic tears in the muscle fibers that must be repaired. During the repair process, additional myofilaments, the contractile units of muscle fibers, are laid down and the muscle can contract with greater force and is now stronger. Muscle protein synthesis is turned on by a pathway called the mTOR pathway, one of the most important anabolic pathways for muscle growth.
How might interference occur? When AMPK is activated, in response to endurance exercise, it turns off the mTOR pathway. Makes sense, doesn’t it? Remember, AMPK is a cell’s energy sensor. When energy is low, it’s activated and AMPK’s goal is to preserve what energy is remaining. That means turning off anabolic pathways, like the mTOR pathway that drives muscle protein synthesis. So, at a physiological level, you can explain how endurance exercise interferes with muscle growth, by activating AMPK and turning off mTOR and muscle protein synthesis.
So, if you want your muscles to grow, you don’t exhaust them with aerobic exercise and deplete them of ATP. Plus, endurance exercise that leads to fatigue activates cortisol, a hormone that interferes with muscle protein synthesis and can actually lead to muscle breakdown. You send your muscles mixed messages when you train them too close together and ask them to adapt via pathways that conflict. Plus, there’s the issue of fatigue. If you run five miles before doing a lower body workout, how hard will you be able to train with weights?
What Does Research Show about Concurrent Training & the Interference Effect?
As you can see, it makes sense physiologically that you don’t do endurance exercise and strength training too close together in time – but does research support this idea? It’s difficult to draw firm conclusions because there are so many variables. For example, a low to moderate-intensity aerobic workout that’s short might not have an interference effect whereas a high-intensity training session or a long duration endurance session may. However, a 2012 study, as well as several other studies, failed to show an interference effect. Yet with so many variables, it’s hard to completely rule it out in concurrent training.
One variable is which muscles you work after a strength workout. If you run prior to strength training, your lower body muscles will be fatigued but not your upper body. In fact, a well-conducted study showed that reductions in strength-training performance after aerobic exercise was limited to the muscles worked. So, if you run before strength training, you probably won’t perform as well on lower body exercises but your performance on upper body exercises may not suffer.
The take-home message? If you’re concerned about strength gains, it’s probably not a good idea to take an exhaustive run before strength training, regardless of what muscles you’re working. If you exhaust your energy stores, it turns on AMPK and that interferes with the mTOR pathway and can impede muscle protein synthesis. One solution is to strength train before aerobic exercise to ensure you’re not fatigued when you lift and do a short, cardio session afterward. Again, you don’t want to exhaust yourself as this can impede muscle protein synthesis. Of course, you can always do aerobic exercise and strength training on alternate days and, if you have the time, that’s likely the best solution.
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