Molecular hydrogen might influence several aspects of exercise capacity to a small degree, but whether this is actually meaningful to an exercising person requires more research.
Molecular hydrogen may reduce fatigue, blood lactate levels, and the perception of exertion during high-intensity exercise. However, the reduction in fatigue and perceived exertion may only be significant in untrained individuals, possibly due to the body being less adapted to the stress of exercise. Furthermore, most studies have reported that hydrogen did not affect VO2max (a metric of aerobic exercise performance) or endurance performance in trained or untrained individuals.[1][2]
When benefits have been observed, they appear to be greater in intermittent, high-intensity exercise as opposed to continuous exercise and with molecular hydrogen provided as a single dose immediately before exercise rather than multiple-day dosing or after exercise.[1][3]