As Noted In Previous Question My Wife Is Battling A
I would recommend as follows:
Detailed Answer:
Hello!
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I passed carefully through your question and would explain that light physical activity is recommendable in such cases.
She can start with light weights in both arms. Aeorobics is another possible option of light physical exercise. But, considering her tachycardia, it is advisable short periods of time (some minutes) and then make some breaks during training.
Yoga is another possible option (with some limitations due to her knee problem), which can help improve her physical performance, without causing tachycardia.
Hope you will find this answer helpful!
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Wishing good health,
Dr. Iliri