Is it possible to print a report of heart rate activity?

Not that I am aware of.

I'm not sure if you mean the ECG app on your phone or ,If you mean the heart rate monitor on the watch,if it is the heart rate monitor on the watch then I agree with Julieda if it's the ECG app then this might help;

How to share ECG results with your doctor

Take an ECG from your Apple Watch.

After taking at least one ECG, go into the Health app on your iPhone.

Tap on Health Data near the bottom, and then tap on Heart and then Electrocardiogram (ECG).

Tap on the ECG result you’d like to send to your doctor.

Tap on Export a PDF for Your Doctor.

Tap the share button on the top right.

Now choose how you want to send the PDF(Messages or Mail, for example).

You could also email the PDF to yourself if it's for personal use,the above assumes you want to send the ECG to your doctor.