Zac Efron's former personal trainer says 2 common ab exercises are a waste of time. Here's what he recommends instead.

• Patrick Murphy never does hanging knee raises because they work the hip flexors more than the abs.
• The celebrity trainer said Russian twists are ineffective too, and can lead to back pain.

Popular core exercises like Russian twists and hanging knee-raises aren't effective for training the abs, according to celerity personal trainer Patrick Murphy.

Murphy, whose clients include Cameron Diaz, Jennifer Lawrence, and Zac Efron (for his famously shredded body in "Baywatch"), said he never programs the common movements.

Murphy told Insider there's no need to overcomplicate ab training because the muscles work in a simple way.

Instead of hanging knee-raises and Russian twists, try crunches with resistance and anti-rotation resistance band holds, he said.

Do crunches instead of hanging knee-raises

Murphy never does hanging knee-raises, which involve hanging from a bar and bending at the hip to raise the legs with knees bent.

"The hanging knee-up is not necessarily an ab exercise," Murphy said. In a hanging knee or leg-raise, for many people it's the hip-flexors rather than the abs that are working, he said.

These exercises may put strain on the lower abdominal and hip region, but they're not an optimal way to train the abs, he said.

"If I want someone to have their abs pop, spinal flexion is the one, end all be all," Murphy said. Spinal flexion means bending forwards and contracting, with the ribs moving towards the hips. Example movements are crunches, such as on a resistance ball or with a cable machine.

Murphy says that when you do crunches, you should be able to feel the lower abs "firing." This means it's working, he said.

Murphy is a fan of supersetting spinal flexion exercises (performing two back-to-back), one with added resistance and one without.

However, it's important not to do too much flexion without also performing spinal extension, he said.

"If you only do too much flexion, you create frontal tightness and you create a weaker lower back," Murphy said. "Doing too much abs, too many spinal flexes, will weaken your lower back, so you have to turn and reciprocate your movements."

Back extension exercises can be performed on a hyperextension bench or the floor.

Do cable twists instead of Russian twists

Murphy said he would never program a Russian twist, and instead recommends controlled rotational movements for the abs, using bands and cables.

When people sit on the ground and move a heavy medicine ball or weight over their body from side to side, the hips stay straight and it forces the spine to rotate more than it should, he said.

"If you have a fixed pelvis and you're rotating a heavy ball from side to side, you're just wearing tear on your lumbar spine," Murphy said.

Physiotherapist Bryce Hastings agrees with Murphy: "Each vertebra in the lumbar spine has only about three degrees of rotation, so when you try to produce a lot of rotation in your lumbar spine you are actually taking the joint to end-range rotation," he wrote for Les Mills. This leads to added strain on the lower back.

Instead, try rotational movements that involve moving the whole body, including the pelvis, Murphy said.

Anti-rotation exercises are also effective, Murphy said, such as holding a resistance cable out in front of you and resisting it pulling you to one side. A similar exercise is a pallof press, which Noam Tamir, the founder and CEO of TS Fitness in New York City, recommended to Insider's Gabby Landsverk.

"Anti-rotation exercises are great for the core," Murphy said. "Rotational exercises are great for the core, as is spinal flexion."