Well, the answer begins with, “It depends…..” This is a n=1 case study of a 69 year old man (me) that I think will generalize fairly well. I once talked with a trainer who was also a body builder and he argued that one could not have too much muscle. More muscle was better in any sport. I said, “What about those who run marathons? Wouldn’t that mean that Arnold Schwarzenegger could have run a sub 2 hour marathon”? He had no response.
I am an example of someone who has had muscle, lost muscle and gained some back for different reasons. When I graduated from High School, I weighed 160 pounds. In my early twenties, I was primarily a “poser”. The current meaning was not available then. I think we were called “players” then. I have an ectomorph frame and am 6’ tall. At that time I was 230 pounds, could military press 210 pounds and could do three sets of ten pull ups. I did no real aerobic exercise in addition to the weight lifting. Aerobic exercise wasn't really fashionable until Kenneth Cooper came along in the 1970’s. I was very strong but would wake up at night with a numb arm. I believe I had increased body mass without increasing my circulatory system.
I was drafted into the army and quickly decided that basic training was not geared to the heavy folks. There are mesomorphs who are exceptions to this. I was in basic training with Ken Bowman a center for the Green Bay Packers. He was huge, fast and fit but even he would not set any marathon records unless he was in the “Clydesdale” category. We were tested in the mile run and needed to finish in less than seven minutes to pass. We ran everywhere we went. Since running is a gravity dependent sport (as opposed to swimming), I began to lose weight. By the time I finished advanced individual training (AIT), I weighed 173 pounds. Because of the other combat proficiency tests like the horizontal ladder, I maintained proportionate strength but was not nearly as strong as before. It was a worthwhile tradeoff.
After the Army, I gradually gained weight and was up to 220 pounds again at mid-life. I had begun to have health problems including high Cholesterol and high blood pressure. I decided to start exercising again and began walking two miles a day. Since my life motto is “Everything worth doing is worth overdoing” I eventually worked up to a marathon. I finished my first marathon (the American Odyssey) is Wisconsin in last place in 6:20. I was about 200 pounds by then. It was obvious that my weight was the primary limitation. More and more I thought about my high school weight as proper for my frame.
After that I got into trail running and ultra-marathons. Eventually, my weight dropped to 185 pounds and I could finish a marathon in less than four hours. I also realized that at age 50 I would have to be at 160 pounds to meet the 3:30 qualifying time for the Boston Marathon. I qualified and ran the 100th anniversary Boston Marathon.
I wanted to finish the Western States 100 mile endurance run and finally finished after three tries at age 57 (20001) at 160 pounds. I was about seven percent body fat. 160 pounds is my optimal running weight but I began having trouble opening my friction fit windows in my house. I took on the challenge of the Hawaii Ironman (I was a lottery winner in 2004). In my training for Ironman, my weight increased to 165 pounds because of the upper body muscle developed by weight training and needed for swimming. I was still about 7 percent body fat.
As I age, it is also becoming evident that my heart and lungs are not as robust as they once were. This is another reason to keep my weight down. I still lift for upper body strength but my lower body is developed by running, biking, swimming and cross country skiing. I find that backpacking using trekking poles also is a great endurance workout. This summer, my weight is just below 160 pounds but my body fat is down to 5 percent. Climbing at altitude with a pack is quite a workout.
The point I am making is that each person has a particular frame size, V02 Max, sport and optimal weight. For the ordinary person like me to optimize my performance in endurance events, I need to be at my ideal weight. It only varies by about five pounds. It is exercise (and diet) that primarily shapes my body. I am no longer a poser. My training is directed to my real life activities. I can still do three sets of six pull ups. What I do now is directed less to competition and more to being in the Sierras.
If you play contact sports and are younger then that is another story for someone else to tell. Even in football there is room for the lean individual but that depends on the position he may play.