New balance devices that improve stability have made shifting the new lifting of resistance training, fitness experts say, adding the challenge of instability to back-to-basic workouts.

Exercise balls, sandbags and load-shifting body bars are among the tools popping up in bodyweight training, the minimal-equipment exercise routine that the American College of Sports Medicine (ACSM) tagged as the top trend for 2015.

“The idea of bringing the body into an environment that challenges stability and balance is on the up rise,” said Michigan-based trainer Derek Mikulski. “Shifting resistance constantly challenges the body’s center of mass so the core has to work harder.”

The core refers to the muscles of the abdominals and back that support the spine and keep the body stable and balanced.

Mikulski is the creator of a new balance device called ActivMotion Bar. It looks like a body bar but is hollow and filled with steel balls that shift back and forth when moved.

It is designed so that gripping, steadying and moving the bars, which come in weights from four and one-half to 18 pounds, (two to eight kilograms), in basic moves from curls to lunges will boost calorie burn, core strength and balance.

Several U.S. fitness chains, including Life Time Fitness and Powerhouse Gym, have introduced the bar, which was rolled out earlier this year.

Stabilizing tools, from exercise balls to Bosu balls, which look like half-balls, have been used in rehabilitation for decades, said Dr. Audrey Lynn Millar, a physical therapist with ACSM.

But there haven’t been many studies about their impact on the healthy, she said.

“Yes, it is activating the core and improving stability and balance but there’s limited evidence on whether it makes the healthy person stronger or faster,” Millar said.

But she added that ACSM recommends balance training at least twice a week.

“Once people pass the 40s we start to see some minor balance issues, then more in 50s and 60s,” she said. “It’s not just an ageing thing, it’s a lack of use thing. If we stop using those muscles our body loses that fine tuning.”

Shawn Perine, editor in chief of the magazine Muscle & Fitness, said bodyweight training can improve a person's physique.

He said the ActivMotion Bar brings core muscles into play as you always try to balance.

“There are many paths to fitness. Most important is to find one that fits your personality,” he said.