Regular weightlifting may increase the risk of the eyesight-threatening condition glaucoma, research suggests.
Weightlifting can stress the body
Brazilian researchers found lifting heavy weights was linked to a temporary increase in pressure within the eye - especially when holding the breath.
They say that this could increase the risk of glaucoma, as the condition is more common in people subjected to frequent changes in eye pressure.
The study appears in the journal Archives of Ophthalmology.
Glaucoma is usually high pressure inside the eye that damages the optic nerve and can result in permanent vision loss
Normal-tension glaucoma leads to optic nerve damage and vision loss despite a normal pressure inside the eye
Thought to be caused by an unusually fragile optic nerve
Intraocular pressure - the pressure within the eyeball - is generally decreased after aerobic exercise, such as running and biking, and nonaerobic exercise, including weightlifting.
However, higher intraocular pressure has been reported during the Valsalva manoeuvre, in which air is forced against a closed windpipe and pressure increases in the chest.
This action occurs during coughing, vomiting, playing wind instruments - and sometimes weightlifting.
The latest research was led by Dr Geraldo Magela Vieira, of the Institute of Specialized Ophthalmology and UNIPLAC School of Medicine, Brasília.
They measured intraocular pressure during weightlifting in 30 men with no sign of glaucoma, and whose intracular pressure was in normal range at under 21mm of mercury.
The volunteers each performed four repetitions of a bench press exercise in two ways.
The first time, pressure was measured in the right eye and the weightlifters held their breath during the last repetition.
The second time, pressure was measured in the left eye and the participants breathed normally throughout the exercise. Eye pressure was measured during the fourth repetition.
During the first round of exercise, intraocular pressure increased in nine out of 10 of the participants, by an average of 4.3 mm of mercury.
During the second round, pressure increased in 62% of weightlifters by an average of 2.2 mm of mercury.
The researchers said the increased pressure could be due to the Valsalva manoeuvre or a similar motion performed during bench presses.
They believe the even higher pressure during the first repetitions may be due to greater chest pressure caused by the air retained in the lungs when the subjects held their breath.
Writing in the journal, the researchers said: "Prolonged weightlifting could be a potential risk factor for the development or progression of glaucoma.
"Intermittent intraocular pressure increases during weightlifting should be suspected in patients with normal-tension glaucoma who perform such exercises.
"Patients with normal-tension glaucoma should be questioned as to a history of regular weightlifting."
David Wright, chief executive of the International Glaucoma Association, said weightlifting was likely to pose a small risk for people who were vulnerable to the condition.
"Any fluctuation in pressure within the eye can cause problems," he said.
"The optic nerve tolerates steady pressure much better than it tolerates fluctuating pressure."
Mr Wright said glaucoma had also been linked to trumpet playing, and to wearing neckties too tightly.