Recently one of the athletes I coach (we’ll call him Scott) came to me with some concerns about the results of his latest DEXA scan and weigh-in. Although he had lost both overall weight and body fat, he had also lost some muscle mass, and the body-fat percentage in his arms had increased slightly. Scott wanted to know if he should add biceps curls and triceps dips to his strength workouts to correct these last two items. I told him absolutely not, and here’s why.

First off, it probably goes without saying that, although biceps curls and triceps dips are effective exercises for improving body composition in the arms, they achieve this effect by increasing muscle mass, and muscle mass—particularly in the arms— is dead weight for the long-distance runner. It can only slow you down.

I speak from experience. Last summer, when I trained with the Northern Arizona Elite team in Flagstaff, under the guidance of strength and conditioning coaches AJ and Wes Gregg I reduced my strength training frequency from three sessions per week to two and removed upper-body movements such as push-ups from my strength workouts. The resulting loss of upper body mass made a small but vital contribution, I believe, to my setting a marathon PR at age 46. (I promise to stop bringing this up after the one-year anniversary passes.)

Hardly shocking. But what may surprise you is that muscle mass in the legs is also dead weight for the long-distance runner. In 2004, exercise scientists at Ball State University examined the contractile properties of individual calf muscle fibers in college cross country runners over the course of a full cross country season. They found that the cross-sectional area of the runners’ muscle fibers decreased during this 12-week period, meaning their calf muscles shrank. That sounds bad, but the ratio of force-generating capacity to cross-sectional area of the muscle fibers increased during this same period, meaning that, pound for pound, their muscles got stronger.

The human body is not stupid. When you subject it to a specific type of training in preparation for a specific type of competition, it adapts in appropriate ways, even if certain adaptations seem negative at first glance.

Muscle tightness is another example. It is not uncommon for runners to go from being able to touch their toes when they start training to being unable to touch their toes after a few weeks or months of progressive running. An individual runner who experiences this change might think, “Oh, no! I’m losing flexibility!” But, like the decrease in muscle mass we just discussed, this tightening of particular muscles is also a beneficial adaptation to training.

It’s not tightness per se that you want as a runner but stiffness, which comes with tightness. The legs function as springs during running. Half of the energy that propels a runner forward comes from the ground as an equal and opposite reaction to the impact force delivered from the foot to the ground with each landing. A stiffer spring/leg is able to capture and reuse more of this free energy than a looser leg, improving running economy. A new study by researchers at the University of Calgary reported that greater Achilles tendon stiffness was associated with better running economy in a group of 46 elite runners, and prior research has shown that elite runners are generally less flexible than nonelite runners.

The bottom line is that if you are a runner seeking better race performance, you need to keep your eyes on the prize. Stay focused on the overarching goal—increasing the speed you are capable of sustaining over a given distance—and don’t get distracted by secondary goals such as maintaining muscle mass and flexibility. This is not to say that strength training and stretching should be avoided entirely, but these practices should be incorporated in targeted ways that contribute to the only thing that really does matter.