US researchers have found a person's body mass index is not a factor in predicting their level of disability after stroke.

Regular exercise may not always prevent a stroke but it could determine how well a person recovers from one.

A Harvard University study published in journal Neurology found those who exercised "vigorously" three times a week or more prior to a stroke were more likely to be independent after the medical emergency compared to those who were inactive.

"We also found that a person's body mass index (BMI) was not a factor in predicting their level of disability after stroke," said lead author Dr Pamela Rist.

Body mass index is a measure of a person's body fat based on their height and weight. Having too much body fat may be a risk to a person's health.

For the study, researchers followed more than 18,000 people who were initially stroke-free for an average of 12 years.

Participants were interviewed every other year about their ability to do basic activities and were also asked for their height, weight and whether they participated in vigorous physical activity or exercise.

Vigorous physical activity was defined as participating in sports, heavy housework or a job that required physical labour.

During the study, 1,374 of the participants had a stroke and survived and 479 people had a stroke and died before the next round of interviews.

Of those who did not have a stroke, 45 per cent were physically active, compared to 43 per cent of those who had a stroke and survived.

Among the stroke survivors, those who were physically inactive were 18 per cent less likely to be taking care of their basic activities such as bathing on their own three years after stroke than those who exercised regularly.

They were also 16 per cent less likely to be taking care of more complex activities such as managing money on their own.

"Our study was able to show that being physically inactive before stroke predicts a higher risk of being dependent both before and after stroke," Dr Rist said.

"Research is needed to look into whether more intense activity could improve stroke outcomes and whether people can change their activity patterns to improve stroke outcomes."