When asking the question of “what is better?” you need to always first consider the traits that make something better or worse. In other words, what are your goals? For cardiovascular fitness, running and cycling are both excellent. Cycling may take longer for some athletes to be able to maintain a good effort. Running is cheaper, but cycling will get you somewhere faster if you use it for transportation. Determining which of these traits are most desirable or undesirable is the first step to deciding which activity is going to be better for a trainee.
One often-overlooked aspect of exercise that is important for many trainees is the impact on bone health. Having a favorable mineral profile, density, flexibility, and strength in your bones is important to all athletes, especially aging athletes or those involved in high-impact sports. The way you exercise can make a substantial difference to your bone health, sending you into your older years better prepared and with fewer bone breaks.
According to a review published in BMC Medicine, when asking if cycling is better for bone health the answer is very clear. It is not. In fact, two thirds of all professional and masters level cyclists can be considered osteopenic, which means they have lower than average bone density. Osteopenia is considered a precursor to osteoporosis.