3 Tips to Strengthening an Athlete's Heart - Training & Conditioning (training-conditioning.com)

Intervals vs. Steady-State Cardio
SSC = steady-state cardio (i.e., low-effort continuous 4 mile or 45-minute run).

 MICT =  moderate-intensity continuous training (i.e., moderate-effort 2 mile or 20-minute run).

 HIIT = high-intensity interval training (i.e., shorter distance runs or timed drills at high-effort 

               followed by limited recovery time between each). Examples: 10 x 300 yards with 2:30 

               recovery or 20 x :40 stations/drills with :15 recovery).

“There is growing and robust evidence that high-intensity interval training (HIIT) shows similar or greater efficacy compared with MICT across a range of cardiovascular and metabolic measures, in both healthy populations and populations with a chronic illness,” according to 2018 research by Australian scientists at the University of New South Wales published in the Journal of the American Heart Association.

Further evidence of HIIT’s heart-strengthening ability was shown in a clinical trial of 93 patients with coronary artery disease published in JAMA Cardiology in 2020. After four weeks, cardiorespiratory fitness improved by 10% with HIIT compared to only 4% with MICT.