CHICAGO — If you’re being treated for breast cancer, chances are you feel like something stuck to the bottom of a shoe.

"Going through cancer treatment, people become unwell; they lose muscle mass, cardiovascular fitness and their activity levels go down," says Margie McNeely, an assistant professor in the department of physical therapy and the department of oncology at the University of Alberta.

"Exercise won’t prevent the losses," McNeely says, but it can lessen their magnitude. "Those who exercise often have less of a loss of function."

But physical activity is a hard prescription to follow.

"Afterwards I just wanted to go to sleep, and that’s not who I am," said Sara Pomish, a 52-year-old advertising strategic planner in Farmington Hills, Mich., who had surgery for breast cancer last fall. "I was just exhausted."

"It’s the kind of fatigue like a bad flu," says Dr. Patricia Ganz, director of cancer prevention and control at the University of California at Los Angeles. For most people, the fatigue recedes on its own in the year after treatment, but 25 percent to 30 percent of survivors will have persistent exhaustion. For those people, Ganz says, research shows exercise improves the condition. As for fighting fatigue in that first year after treatment, "It is probable that exercise will (help with) that," Ganz says. "My gut feeling is that exercise will reduce inflammation and enhance the body’s ability to get back into balance."

In 2007, a study in the British Medical Journal looked at 203 women recovering from breast cancer, split into two groups: one got usual care; the other got the usual care with 12 weeks of supervised exercise. The latter group not only had improved physical functioning and shoulder mobility, but it also saw a "significant effect" in terms of psychological benefits, which were largely retained at the six-month follow-up. What’s more, the exercisers were half as likely to spend a night in the hospital during the six-month follow-up period, and they needed to visit their general practitioner less frequently. The study found exercise produced no adverse effects.

Women who have undergone treatments for breast cancer are often concerned about when it’s safe to start exercising. Pomish opted for a lumpectomy because she didn’t want a major surgery to derail her fitness regimen. Coupled with radiation and a five-year prescription for the cancer-treatment drug tamoxifen, she was feeling wiped out.

Pomish’s surgeon had her hold off on exercising for three weeks post-surgery, then she was encouraged to do as much as possible within reason. "I was told to listen to my body," she said, which is something professor McNeely echoed. "Exercise was absolutely critical in combating fatigue and loss of strength and endurance," Pomish said. "It’s also a wonderful antidepressant."

For 35-year-old office manager Jennifer McCrea from Calgary, Alberta, it was breast cancer that turned her into a runner.

While less invasive procedures have a lower incidence of problems, McNeely says, that didn’t stop McCrea, who started running just two weeks after a double mastectomy last summer.

McCrea has "a strong and devastating family history of breast and ovarian cancer," and has two very young children. "Exercise has been instrumental in my recovery. My doctors highly recommended that I stay active to thrive as a breast cancer survivor.

"At the start of my recovery, I could barely move my arms from my armpits, let alone lift my children or a jug of milk," recounts McCrea, who is also on the five-year plan for tamoxifen, which causes her fatigue. But two months after surgery, she ran her first 5K. The next spring she ran a half-marathon, and she’s still running. "I will not let this cancer take one more day from me again," she said.

McNeely strongly supports exercise as part of recovery, but also promotes caution. "If there are arm or shoulder problems, those need to be addressed before doing any type of upper-body exercises. There is no reason why they can’t start walking though," she said. Anything more vigorous needs doctor approval.

"We follow the ACSM guidelines," McNeely said, referring to a 2010 round table of the American College of Sports Medicine on exercise guidelines for cancer survivors. Published in Medicine & Science in Sports & Exercise, it states, "Exercise prescriptions should be individualized according to a cancer survivor’s pretreatment aerobic fitness, medical comorbidities, response to treatment, and the immediate or persistent negative effects of treatment that are experienced at any given time."

"The activities we promote are those that the woman enjoys doing," Ganz said. The average person with breast cancer is 61 and sedentary, so doctors focus on being realistic. "I usually recommend walking and using a step counter to gradually incorporate exercise. Just being a little more physically active every day will help them in many ways."

Pomish, who also had two lymph nodes removed, waited five weeks to return to running, but she was teaching fitness classes with an upper-body component just four weeks after surgery with the blessing of her surgeon.

Overall, it seems the medical advice in regard to exercise as part of breast cancer recovery is to err on the side of "go for it."