Ashwagandha supplementation at doses of 300-600mg daily for 8 weeks increases VO2 max, a measure of cardiovascular endurance, in healthy human adults.

Study: Effects of Ashwagandha ( Withania somnifera) standardized root extract on physical endurance and VO 2max in healthy adults performing resistance training: An eight-week, prospective, randomized, double-blind, placebo-controlled study

This study gave people 600mg of Ashwagandha daily for 8 weeks and found they could run farther (higher VO2max) compared to people who took a placebo pill. The results were very clear with strong statistical significance.

Study: A double-blind, randomized, placebo-controlled trial on the effect of Ashwagandha (Withania somnifera Dunal.) root extract in improving cardiorespiratory endurance and recovery in healthy athletic adults.

This study gave people 600mg of Ashwagandha root extract daily for 8 weeks and found they significantly improved their VO2 max (how well their body uses oxygen during exercise) compared to people who took a placebo pill. This matches exactly what the claim says should happen.

Study: Effects of an 8-week high intensity interval training (HIIT) and ashwagandha supplementation on aerobic capacity, muscle oxygenation and haematological parameters in healthy men

This study gave people either ashwagandha or a placebo while they did high-intensity exercise training. Both groups got stronger, but the ashwagandha group didn't get any extra boost compared to the placebo group. This means ashwagandha didn't actually improve cardiovascular endurance beyond what exercise alone does.