By Jim Donovan

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PHILADELPHIA (CBS) — Fitness trackers have become popular among many people who want to improve their overall health but some users are reporting an unwanted side effect getting shocked.

Chris Nelson felt healthier with his new fitness tracker, counting his steps and tracking his heart rate.

But after a month, while putting his three-year old daughter Emily to bed, the Fitbit Charge 2 shocked him, causing him to drop his daughter.

“It was clearly an electric shock,” Nelson said.

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Emily was not hurt but Chris says the shock left a painful lump on his wrist and his hand temporarily numb.

“The best as I can figure, I was just shocked by the connector that was on my wrist,” he said.

Lauren Reiss of Yardley swears two different Fitbit models shocked her.

“It shot up my arm. It was intense,” she said. “As soon as I looked down at my arm, I noticed immediately that it still hurt.”

Reiss says one jolt left this lasting mark.

If you search federal complaints, you’ll find other Fitbit complaints.

“This is a danger that needs to be addressed,” Reiss said.

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Fitbit says it takes all reports of potential issues seriously and its products are designed to prevent electrical contact with the user.

In each reported case Fitbit investigated, it says the devices and batteries were found to be fully intact and functional, with no signs of overheating, voltage irregularities or malfunction of any kind.

They did suggest it is more likely that the shock was caused by static electricity built up on the person.

“I just don’t think it’s static; I think there is something else going on,” Kelly Robinson said.

She says static can build up on a body but static is often a scapegoat when companies can’t figure out what’s really wrong.

“There’s something seriously going on here and we need to keep at it and we need to keep looking into it; we need to fix it,” Robinson said.

The reports of shocks concern consumer attorney Stuart Talley.

“If you had a pacemaker, if you were driving a car, something like that could cause a really serious injury,” Talley explained.

Reiss does have a pacemaker and she now wears her Fitbit farther from her heart, on her ankle, instead of her arm.

“Since I’ve taken it off, I’ve actually felt a lot better,” Reiss said.

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Fitbit sent Chris Nelson a brand new advanced model and he has not felt any other jolts.

“People shouldn’t be getting shocked by their watch,” he said.

While Reiss now wears it around her ankle, Fitbit says they’re designed for the wrist and they encourage people to wear them properly to get the most accurate picture of their health.