There is a saying that "a stronger athlete is a better athlete." When applied to sports such as football, basketball, and baseball, most people would agree that strength training is beneficial. However, in the context of endurance activities such as swimming, cycling, and running there is less consensus of opinion. In fact, some would argue that aerobic athletes need to be as lean as possible, and strength training would make them bigger rather than better.
While it is true that you rarely see a successful triathlete who weighs over 200 pounds, this is essentially an unfounded fear. To begin, top triathletes typically have an ectomorphic body type. That is, they have a linear physique with firm but trim features. On a comparative basis they are relatively low on fat and relatively low on muscle. Due to fewer fat cells they do not add fat easily, and due to fewer muscle cells they do not build muscle easily. In other words they do not have the genetic potential to develop large, muscular physiques.
But they certainly can become leaner and stronger through a sensible program of strength exercise. For example, let's say that Tom weighs 140 pounds and is 12 percent fat. He therefore has 17 pounds of fat weight and 123 pounds of lean (muscle) weight. After 12 weeks of regular strength training Tom weighs 142 pounds. He has added 3 pounds of muscle for 126 pounds of lean weight, and lost 1 pound of fat for 16 pounds of fat weight. His percent fat is now 11 percent, and he is leaner than before even though he has a slightly higher bodyweight.
As an analogy, Tom has changed from a 6-cylinder engine to an 8-cylinder engine. Although a little heavier, he has increased his horsepower and should experience a higher level of athletic performance. Power is essential for endurance events as well as for sprint events. Because power equals work divided by time, the person who completes the triathlon (work) in the least time is the most powerful athlete, and the winner.
Our muscles are the engines of our bodies. But, unlike automobile engines, our muscles use energy when we are moving and when we are resting. Because each pound of muscle increases our resting metabolism by about 35 calories per day, Tom's 3 pounds of additional muscle requires over 100 calories a day just for tissue maintenance. That amounts to almost a pound of fat loss per month just to sustain normal metabolic function even when Tom is not exercising.
It should be clear that Tom has nothing to lose and much to gain by developing more functional muscles. Although not as obvious, the greatest benefit of a well-designed strength training program may be the experiences that he avoids. I am referring to the various overuse injuries that frequently plague triathletes. Because swimming, cycling and running all emphasize some muscles more than others, joint structures in the feet, knees, hips, back, shoulders and neck may become vulnerable to musculoskeletal problems. By performing a comprehensive program of strength exercise many potential injuries may be prevented.
For example, swimming emphasizes shoulder extension and adduction movements (pulling the arms through the water) more than shoulder flexion and abduction movements (recovering the arms through the air). Consequently, the stronger pectoralis major and latissimus dorsi muscles tend to overpower the weaker deltoid muscles which frequently leads to shoulder injuries, especially in the vulnerable rotator cuff area.