Your strength is your independence.
So often people will do some form of aerobic exercise, walking, biking, jogging etc., but will neglect muscle strengthening. Getting a couple of sets of dumbbells at home, some theraband, or doing exercises such as wall slides, push ups against a wall, step ups, yoga are all great ways to work on improving strength. When people become frail it is often due to severe muscle atrophy or wasting. You can build and improve strength at any age. The key is to begin light and continue to push yourself to do a little more. Start with soup cans if needed, to make your own leg weights three rolls of pennies equals one pound, put them in a sock and tie it around the ankle, do leg extensions, leg lifts to build leg strength.
A key for heart patients with muscle building exercises is to EXHALE ON EXERTION.
Many will find they hold their breath or inhale when lifting a weight. It is the opposite, when lifting the weight blow out, this helps to keep the blood pressure from elevating dangerously high, and placing increased forces on the artery walls. You will find if you focus on the breathing technique you are able to lift more weights. It is crucial to maintain muscle mass to maintain independence, to be able to do activities of daily living such as carrying in groceries, taking out trash, moving furniture, lifting boxes, getting up off the floor etc.