Can your boss make you lift heavy things with a medical certificate?

I have sprained my rotator cuff in my right shoulder.

I have a medical certificate from my doctor to not lift anything more than 5kgs in a 30 minute time frame.

Today my boss got me to put 10 glasses, 10 plates, two platters of food on the cart in the photo. Then push it in the elevator and through two doors on the next level.

She knows about my arm injury and said "You can do that"

By the time I put everything on there and pushed the cart to the next level, my upper arm couldn't stop shaking.

Do I have a right to say no next time she asks?

Can your boss make you lift heavy things with a medical certificate?
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HR and my boss don't like me, as I've spoken about things before which they didn't want to hear. If they fire me for speaking up, I don't care.
Can your boss make you lift heavy things with a medical certificate?
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