Body composition in the gym: how to use InBody and Biody to program training

By Ezequiel Hipperdinger — CEO of Full Fitness and co-founder of WizFit.

A body composition analysis gives a gym something the scale doesn't: how much of a member's weight is muscle, how much is fat, and what condition their cells are in. Used well, it does three things: better programming, showing members real progress, and adding a service. This article summarizes what the evidence says and how we apply it at Full Fitness with WizFit.

Summary

• Bioelectrical impedance (devices like InBody or Biody) is highly repeatable, but a single reading is an estimate: what matters most is comparing readings over time.
• Phase angle improves with strength training: about 0.5 degrees after 8 weeks or more, according to a meta-analysis.
• Eating less without training was the only strategy with significant muscle loss in a meta-analysis of 62 trials. Measuring body composition shows whether the plan is protecting muscle.

How reliable is bioelectrical impedance?

In a study that compared it with dual-energy X-ray absorptiometry (DXA) and air displacement plethysmography, bioimpedance showed very high agreement and excellent repeatability: repeated readings differed by less than 0.2%. But for an individual, body fat percentage can differ from the reference method by several points (von Hurst et al.).

For a gym, the practical takeaway is clear: always measure under the same conditions (same time of day, no training beforehand, similar food and hydration) and work with the trend, not a single day's number.

Phase angle: the metric most worth tracking

Phase angle reflects the distribution of water inside and outside cells, cell integrity and nutritional status. It isn't estimated with formulas: it's calculated directly from the resistance and reactance the device measures. A meta-analysis of 7 trials with 344 people aged 60 or older found that strength training for at least 8 weeks increased it by 0.52 degrees compared with those who didn't train, and proposes 0.5 degrees as the minimum change with a beneficial effect (Campa et al., 2023).

For coaches, that makes it a concrete indicator of whether training is improving muscle quality, not just quantity.

Why it matters more than weight

A meta-analysis of 62 trials with 4,429 participants found that calorie restriction alone was the only strategy with significant lean mass loss; adding strength training or low-to-moderate intensity aerobic exercise best optimized fat loss while preserving muscle (Xie et al., 2025).

A member who loses weight while losing muscle may see the scale go down and still be getting worse. Body composition shows it in time to adjust the plan.

How a gym uses it: the Full Fitness case

1. For programming. At Full Fitness, measurement results are loaded into WizFit. WizFit uses muscle mass, fat mass and phase angle to build and adjust each member's training plan.
2. To show progress. Repeating the measurement about every 8 weeks, the minimum period used in the phase angle studies, shows members changes the scale doesn't.
3. As a service. Full Fitness measures with InBody and Biody and offers the analysis to members and non-members, with a preferential rate for members. It's an entry point for people who don't train at the gym yet.

What to consider before adding it

• A fixed measurement protocol, so comparisons are meaningful.
• Someone on the team who explains the results: a number without context creates more questions than motivation.
• It's not a medical test. People with pacemakers or other implanted devices, or who are pregnant, should check before being measured.

For an explanation written for members, see the Full Fitness body composition guide (in Spanish).