Nearly a quarter of over-65s don't do any strength exercises, the Chartered Society of Physiotherapy (CSP) says.
And it warns that the rise of internet shopping means fewer people carry home their groceries.
Physiotherapists say not enough people realise the need to maintain strength as we age.
NHS guidelines suggest people do two strengthening sessions a week, such as exercising with weights, or lifting and carrying heavy loads such as groceries.
But a survey of more than 2,000 older people in the UK, carried out on behalf of the CSP, found nearly a quarter do no strengthening exercises at all.
Nearly one in five people said they didn't know how to do them, while a similar number said they just didn't want to.
Falls among the elderly cause the vast majority of hip fractures and cost the NHS around £1bn each year.
Physiotherapists say maintaining your strength into old age can help avoid falls and other forms of ill health.
They argue that encouraging people to keep their strength up - by doing simple things such as gardening or vacuuming - will pay dividends, helping them to live independent, healthier lives for longer.
Exercises to strengthen muscles
- lift weights
- work with resistance bands
- do exercises that uses your own body weight, such as push-ups and sit-ups
- heavy gardening, such as digging and shovelling
- carrying shopping
- yoga
Prof Karen Middleton, chief executive of the CSP, says it's not inevitable that we become weaker and frailer as we get older.
"Research shows getting stronger brings a whole host of health benefits so it is incredibly important that people don't overlook strengthening when being active."
She said that could mean digging in the garden or standing up out of a chair 10 times.
Prof Middleton also said online shopping had removed a potential source of muscle exercise.
"We're carrying fewer bags home from the supermarket because it arrives at our door.
"We're also waiting at home for other goods to be delivered when in the past we would have gone out to buy them."
She said we should look for ways to build strength exercises into our everyday lives.
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