Firefighters are more likely to be injured while exercising than while putting out fires, suggests research published online in Injury Prevention.
But carrying patients is the task most likely to require time off work, the study shows.
Combined firefighting and emergency medical services have one of the highest workplace injury and death rates in the US.
The authors looked at data for injuries sustained while at work for 21 fire stations serving the metropolitan area of Tucson, Arizona between 2004 and 2009.
The 650 employees included firefighters, paramedics, engineers, inspectors and battalion chiefs. Average age was 41, and all but 5% were men.
During the study period, the average annual incidence of new injuries was 17.7 per 100 employees, most of whom were in their 30s and 40s.
Injuries sustained while exercising accounted for a third of the total, despite the fact that exercising is designed to keep employees in good physical condition, in a bid to stave off the risk of injury while doing their job.
A further one in six injuries (17%) were caused while transporting patients, and just over one in 10 were sustained during simulated training drills.
Sprains and strains were the most common type of injury, accounting for between 40 and 85%, followed by cuts and bruising. Most (95%) of the injuries were minor in nature.
Only one in 10 injuries occurred during firefighting, but a greater proportion of these were more serious.
But almost half of time off work for injuries was caused by strains and sprains sustained while transporting patients.
The number of structural fires which need to be put out has been steadily falling since the 1970s, say the authors, but firefighters have taken on other responsibilities, and are now considered first responders for all types of medical emergency, including natural disasters and acts of terrorism.
In 2009, most of the call-outs (84%) required either basic or advanced life support, and just one in 10 involved the need to put out a fire. The rest entailed technical rescue activities and other daily responsibilities.