Study participants and training
Hostrup and colleagues' 2025 crossover trial enrolled 11 healthy men, comparing two-week clenbuterol and placebo periods. Moesgaard and colleagues' 2026 parallel trial enrolled 32 adults, half women, during eight weeks of resistance training.
In a crossover trial, the same participants receive both conditions in separate periods. In a parallel trial, different groups receive the interventions. Placebo comparison helps separate the drug response from changes that occur during the experiment itself.
Muscle, strength and aerobic results
| Study | Body composition | Performance |
|---|---|---|
| Hostrup et al., 2025 | Lean mass was 0.91 kg higher; no effect on fat mass. | Maximal oxygen uptake fell 7% and exercise capacity fell 4%; sprint power was unchanged. |
| Moesgaard et al., 2026 | Lean-mass gain was 1.7 kg greater; fat-mass reduction did not differ. | Bench-press gain was 6 kg greater and leg-press gain 16 kg greater. |
Values describe group comparisons under the trial conditions. They are not expected changes for every person, and lean mass is not identical to new muscle protein.
Detraining and retraining
The 2026 trial included 16 weeks of detraining and eight weeks of retraining. After detraining, lean-mass and bench-press differences were no longer detectable, while a leg-press difference remained. Retraining produced no extra lean-mass gain from prior exposure, although bench-press gain was 3 kg greater.
This sequence combined drug cessation with reduced training. It cannot isolate what would happen after stopping the drug while continuing an unchanged training program.
What the tests measure
| Assessment | What it captures | Interpretation |
|---|---|---|
| DXA | Estimated fat, lean soft tissue and bone mineral. | Lean tissue includes water and other non-fat tissues. |
| One-repetition maximum | The greatest load completed in a specified lift. | Technique and coordination contribute alongside muscle force. |
| Maximal oxygen uptake | The highest measured rate of oxygen use. | Assesses aerobic capacity rather than maximum lifting strength. |
| Muscle biopsy | Local tissue and biochemical measurements. | Samples a particular muscle, not every tissue in the body. |
Training status and participant health
These were small studies of selected healthy volunteers. They cannot establish long-term cardiovascular safety or predict outcomes with uncertain products and combinations of performance-enhancing substances.
A statistically significant group difference still allows considerable individual variation. Confidence intervals, training background and the duration of follow-up matter when judging how broadly a result applies.
Published trials
Hostrup et al. (2025), The Journal of Physiology: lean mass, muscle protein and cardiorespiratory fitness.
Moesgaard et al. (2026), Sports Medicine: resistance training, detraining and retraining.