Prior cycling measurably degrades running mechanics even when running economy looks unchanged, and a variable bike power file makes the problem worse
Interpretation
Stewart et al. found mechanical efficiency drops sharply after cycling (42.1% vs 48.1%) despite no significant change in oxygen cost — the brick effect is a force-production problem, not purely a metabolic one. Etxebarria et al. then showed that holding the same average bike power with high variability (vs. steady) made the following 9.3km run measurably slower, concentrated in the run's early stages.
Practical implication
Brick-run pacing advice should address neuromuscular/mechanical readjustment in the opening kilometres, not just assume a settled oxygen cost means a settled stride. Riders who race or train with highly variable power should expect a worse transition run than their average-power number alone predicts.
Sources
2 sources, independently verified.
Mechanical efficiency was significantly lower running after cycling than running alone (42.1±2.5% vs 48.1±2.5%, p=0.02), even though running economy itself (oxygen cost) was not significantly different — the bike leg changes how the body produces running force, not just how much oxygen it costs.
Two 1-hour cycle trials at identical average power (variable vs. constant), each followed by a 9.3km run time trial. The run was 42±37s slower after variable-power cycling, with the performance decrement concentrated in the early stages of the run.