Explain what core muscles do and why it's important to have strong set of core muscles

The "core" is where all movement starts, and it is our body’s center of gravity. It consists of 29 different muscles that attach at the lumbo-pelvic hip complex. Having a strong core is important for the transfer of strength and power from the lower extremities into the upper body. Having a weak core will increase your chances of hip, low back, neck, shoulder, and knee pain.

Pe.dometer would be the most useful to you for monitoring heart rate.