Mapping the Body: How Muscle Distribution Shapes Athletic Performance

Two teenage swimmers stand side by side at the poolʹs edge. Both have body fat percentages hovering around 13%. Their total lean body mass is nearly identical. To the casual observer, they look remarkably similar. Yet when the splash settles and the stopwatch clicks at the 50-meter mark, nearly two seconds separate their times. The answer may lie in a detail thatʹs easy to overlook—where their muscles are located.

A study of adolescent competitive swimmers [1] revealed a notable finding: upper body strength dramatically influences 50-meter performance, accounting for 98% of performance variance in males and 82% in females, while lower body strength contributed far less. In other words, two athletes with identical total muscle mass will perform very differently if one carries more muscle in the arms and shoulders while the other carries more in the legs. For short-distance swimming, the former holds a clear advantage.

Think of two orchestras, each with fifty musicians. If one orchestra assigns thirty players to strings and the other concentrates them in brass, theyʹll produce entirely different sounds. The human body works the same way. Equal amounts of muscle, distributed differently, yield vastly different athletic performances.

Traditional body composition analysis tells us body fat percentage and lean body mass: numbers that reflect overall health status and athletic potential. But these figures are incomplete. They canʹt tell us how muscle is distributed across the body. For a swimmer who needs upper body explosive power, having 20kg of muscle in the arms and 15kg in the legs produces completely different results than the reverse distribution.

Research on twelve 400-meter runners [2] illuminates this principle further. While sprinters need powerful leg muscles for explosive starts, the middle-distance 400-meter event tells a different story. MRI measurements showed that overdeveloped calves actually become a liability, increasing rotational inertia and forcing each stride to consume more energy. Elite 400-meter runners display a precise muscular balance: well-developed thigh muscles provide power, while relatively streamlined calves minimize energy expenditure.

So how do we "see" these distribution differences?

Most people using bioelectrical impedance analysis (BIA) devices receive only whole-body data: body fat percentage, total muscle mass. These numbers canʹt tell us how muscle is distributed across arms, legs, and trunk. Itʹs like knowing a riverʹs total flow without understanding conditions upstream, midstream, and downstream. Where the current runs swift, where sediment accumulates, where deep pools form: these crucial details vanish.

Traditional BIA measures total electrical resistance from wrist to ankle based on a simple assumption. It treats the human body as a uniform cylinder. Muscle, rich in water, conducts electricity easily; fat, containing less water, resists current. By measuring resistance, we can estimate lean body mass. But this "whole-body average" approach canʹt capture compositional differences between body regions.

Segmental measurement technology [3] breaks through this limitation. It builds on the resistivity model: resistivity (ρ) = R × A / L, where R is measured resistance, A is the limbʹs cross-sectional area, and L is limb length. Rather than treating the body as a single homogeneous cylinder, this model measures each segment separately. Arms are long and thin with higher resistance; the trunk is short and thick with lower resistance; legs fall somewhere between. By measuring separately, we can see actual muscle and fat distribution across different body regions.

In practice, electrodes are placed on the right wrist, left wrist, right ankle, and left ankle. Weak electrical currents separately measure values for upper limbs, trunk, and lower limbs. Itʹs like viewing terrain from above. Where mountains rise (muscle-dense areas) and where plains stretch (higher-fat regions) becomes immediately clear.

When a coach reviews a swimmerʹs segmental measurements and discovers significantly less muscle mass in the left arm compared to the right, they can adjust training to strengthen the weaker side. Two basketball players might have similar overall measurements, but if one has sufficient leg muscle with weak upper body while the other shows the opposite pattern, their training needs diverge completely. The first needs upper body strength work for defensive stability; the second should focus on lower body explosive power to improve vertical leap.

For injury prevention, segmental measurement can identify risks early. Research [4] shows that martial arts involving predominantly unilateral movements (karate, fencing, taekwondo) create muscular imbalances between dominant and non-dominant sides over time.

A taekwondo study [5] found that left-right leg muscle asymmetry reduced kicking performance by 21%. More tellingly, when athletes entered fatigue states (later rounds when energy depletes), those with bone density asymmetries showed noticeably fewer kicks. This suggests training shouldnʹt merely increase muscle mass but should also address asymmetries by strengthening the weaker side.

The same principle applies to 400-meter runners. If measurements reveal excessive calf muscle mass in an athlete, coaches might reduce lower leg strength training while enhancing thigh and gluteal muscle groups. This refined adjustment becomes possible only by "knowing where the muscle is."

Segmental BIA technology isnʹt perfect, of course.

Compared to limbs (relatively simple, elongated structures), the trunk region presents challenges. Internal organs create complexity, tissue density varies, and electrical current follows more intricate paths. This makes trunk measurements less accurate than limb measurements. Thatʹs why electrode placement and current application require particular attention when developing measurement protocols.

Moreover, current research focuses primarily on young athletes with normal body composition. How this measurement approach performs for overweight individuals, older populations, or those with specific health conditions requires further investigation.

Just as geographers progressed from measuring Earthʹs circumference and sketching continental outlines to marking mountains and rivers, we need more than totals; we need a complete map. For athletes, this map indicates where strength resides, where weaknesses lie, where growth potential exists.

This is why, in developing BIA body composition analysis technology, we pursue not only whole-body accuracy but also more detailed physical information, including our proprietary bone density measurement capability. True health and athletic performance have never been captured by a single number. We must see every part of the body, understand how components work together, and recognize each oneʹs distinct role.

Muscle isnʹt just about having "enough." It must be in the "right place." And to know whatʹs "right," we first need to see the complete map. On this map, every marker carries meaning, every contour tells the bodyʹs story. This is each personʹs unique terrain, worth careful charting and thorough understanding.

References
[1] Almeida-Neto PF, Baxter-Jones A, de Medeiros JA, et al. Are there differences in anaerobic relative muscle power between upper and lower limbs in adolescent swimmers: A blinded study. Sports Med Health Sci. 2023;5(4):290-298.
[2] Muratomi K, Tarumi T, Furuhashi Y, et al. Effectiveness Index of Mechanical Energy Utilization in Male 400-m Sprinters and the Relation Between Muscle Cross-Sectional Area of the Trunk and the Lower Limb. Scand J Med Sci Sports. 2025;35(2):e70023.
[3] Organ LW, Bradham GB, Gore DT, et al. Segmental bioelectrical impedance analysis: theory and application of a new technique. J Appl Physiol (1985). 1994;77(1):98-112.
[4] Mala, L., Maly, T., Cabell, L., et al. Body Composition and Morphological Limbs Asymmetry in Competitors in Six Martial Arts. Int J Morphol 2019;37(2): 568-575.
[5] Ojeda-Aravena A, Warnier-Medina A, Brand C, et al. Relationship between Body Composition Asymmetry and Specific Performance in Taekwondo Athletes: A Cross-Sectional Study. Symmetry 2023;15(11):2087