The interventions included biofeedback based on heart rate variability and neurofeedback based on electrical activity in the brain.

When effects for biofeedback and neurofeedback were combined, the authors reported a moderate improvement in pooled mental health outcomes (anxiety, stress, anger, fatigue, and depression) and large improvements in pooled athletic performance outcomes (golf, swimming, shooting, endurance, basketball, bowling, football, etc.) and pooled cognitive performance outcomes (attention, focus, and memory).

Separate subgroup analyses for each intervention and individual outcomes revealed fewer benefits: Biofeedback only improved anxiety and basketball performance, whereas neurofeedback only improved attention.

Do biofeedback and neurofeedback help athletes? Original paper