Thinking of Joining USA Triathlon?

Be a part of our 550,000 member community of multisport athletes. Membership benefits include a subscription to the quarterly USA Triathlon magazine, discounts from USA Triathlon partners, inclusion in the national rankings, excess accident insurance at events, and savings at races. To see why you should join or renew today, visit the membership benefits page. Already a member? Login below.

Forgot Your Password? | Login Help

The Impact of Resistance Training on Aerobic Performance

By Dr. Alexander Hutchison

Resistance training is a crucial element of any effective triathlon training program. With a quick internet search you can find numerous articles describing the general benefits of weight training and providing examples of some training plans and lists of exercises. However, to fully appreciate how much your aerobic performance can be enhanced by incorporating weight training one has to understand the physiological adaptations that occur after both aerobic and resistance training.

resistance trainingSo, let’s start by describing the basic anatomy and physiology of skeletal muscle. Muscle is composed of long, cylindrical cells that we will refer to as muscle fibers. These fibers contain two contractile proteins called actin and myosin. It is the complex interaction between these two proteins that produces contractions. 

There are three different forms of myosin that are expressed in muscle: types I, IIa, and IIx. Typically, each individual muscle fiber will contain only one form of myosin. But, within a whole muscle, there is a mosaic of all three fiber types. Generally speaking, endurance athletes, like triathletes, have more type I fibers in their muscles while sprinters, like those who run the 100-meter dash, have more type IIa and IIx fibers.

Type I fibers are smaller in diameter, produce less force when contracted, but are fatigue resistant, and can metabolize fat in addition to carbohydrate because they utilize oxygen to “burn” these fuels. Type IIx fibers are larger, produce more force, but almost exclusively utilize carbohydrates to produce energy, and fatigue rapidly. Per fiber, type IIx fibers produce the most lactic acid during high-intensity exercise. Type IIa fibers are intermediate fibers for all the aforementioned characteristics.

Although everyone’s muscles contain all three fiber types, the percentages of each are different between athletes. The proportion of fibers you have is largely determined by genetics and training does little to change this. So that means that no matter how much distance running an NFL linebacker does, he likely will never be a world class marathoner, and vice versa.

During exercise, our muscles contract to produce the force used for propulsion. For example, during cycling the quads contract to extend the leg while pushing the pedal down. During low-intensity exercise, much of this work is done by type I fibers. They can handle this effort because the pace is relatively slow and little force is required. As intensity increases, more force is needed and thus, more of the larger type IIa and IIx fibers are recruited.

The problem arises from the fact that IIa and IIx fibers fatigue relatively quickly. So, between two athletes in a race, the advantage goes to the one who can sustain the most force production while using the fewest IIa and IIx fibers. Since the type II fibers are the ones that produce the most lactic acid, another way to look at this scenario is to measure the lactate thresholds of the two athletes. The woman with the highest lactate threshold can sustain a higher intensity of exercise before lactic acid begins to accumulate in the blood faster than it can be removed. How weight training plays into this will be explained later.

Now that we understand how the muscles function during exercise, let’s examine how they change after training. Following several weeks of aerobic exercise, the muscles undergo some dramatic alterations that make them more efficient. The number and size of the mitochondria (the structures responsible for burning carbohydrate and fat to make usable forms of energy) each double, effectively increasing our capacity for energy production by fourfold. The size of type I fibers decreases while the number of blood vessels (capillaries) surrounding each fiber increases. These changes increase the amount of oxygen and nutrients that can be delivered to the working muscles. Finally, a small number of type IIx fibers transform into type IIa, further increasing the amount of more fatigue resistant muscle fibers.

The drawback of these adaptations is a loss in power production capacity. As the type I fibers decrease in size, there are fewer contractile proteins available to move the muscles. On the whole, the increase in aerobic efficiency far outweighs the reduction in maximal power output observed after long-term aerobic training. But, where the power reduction is most obvious is when an athlete needs a short, explosive burst of energy, e.g. when climbing, passing, or putting in a finishing kick on the run. This is where the triathlete who does resistance training will have an advantage. But, before we get into that too deeply, let’s review the physiological adaptations that take place in muscle after several weeks of resistance training.

Generally speaking, the effects of long-term resistance training on muscle physiology are opposite those observed following aerobic training. The size of the muscle fibers (IIa and IIx particularly) increases dramatically. Some of the type I fibers transform into IIa fibers, thus increasing the maximal force production capacity of these cells. These improvements occur at the expense of mitochondrial and capillary density, both of which decrease. These changes reduce the amount of oxygen and nutrients delivered to the working muscles, thus requiring them to shift to a form of metabolism that produces much more lactic acid. Overall, resistance training makes the muscles larger and more powerful while reducing their ability to resist fatigue.

So, after reading this list of weight training-induced adaptations you must be wondering why any coach would recommend adding resistance exercise to an athlete’s regimen. Here is the best part: when resistance training is added to an aerobic training program in the form of high repetitions (i.e. >30) and low weight, the muscle fibers get larger without sacrificing mitochondrial or capillary density. The estimated changes in fiber size are variable ranging from 3-10 percent. But even this small change can lead to significant performance improvements. Several studies have shown that elite endurance athletes who participated in 8-12 weeks of high-rep, low-weight resistance training (2 days/week) experienced significant improvements in lactate threshold and maximal sprint power output. On average, there was no change in VO2peak. This means that at the end of the 8-12 weeks, these athletes could perform at a higher absolute workload for longer before fatigue set in. Just as importantly, during supermaximal exercise (i.e. climbing, passing, sprinting) there were more, bigger muscle fibers available to produce more force, and thus more speed.

Although this article primarily dealt with the underlying science behind how resistance training works, there are other reasons to include weight training in your weekly schedule. Athletes who add resistance training have significantly fewer over-usage injuries. Aerobic training tends to overwork active muscle while underutilizing smaller, “stabilizing” muscles. This imbalance can be lessened with as little as two 30-minute sessions per week. There are numerous resources that you can use to find a good program to fit your own specific training needs. As with any exercise program, start by talking to your doctor, and then seek some help from a qualified strength coach.

Dr. Alexander Hutchison has a PhD in Kinesiology from University of Houston and is currently a research fellow at the University of Texas Health Science Center at the Houston Division of Infectious Diseases. He is a USA Triathlon Level I Certified Coach and the owner and operator of the Highlander Triathlon. You can email Alexander with your questions at