Curious thought: Why isn’t it called an instability ball? After all, that’s what this rubbery sphere brings to workouts, forcing every muscle to work harder to keep the body righted. The traditional stability ball jackknife exercise is challenging enough for the abs and back, which must resist collapsing against the in and-out motion of the knees. Bringing the knees alternately to each elbow in this twisting version requires the additional engagement of the obliques, creating a waist-whittling core workout from every angle. Round out the total-body strength routine by ending with 3 sets of 10 reps to each side.

Brace Yourself
The secret to successful core exercises is what happens before moving. To work the abs most effectively, consciously engage them. Think about pulling the belly button to the spine. Another perspective: Brace the abs as if anticipating a sucker punch to the gut.

STEP 1
Before jackknifing, assume the plank position. Use the arm’s length to select a stability ball with a similar diameter. Lie atop the ball at hip height, place hands on the floor, and begin walking out until shins (easier) or toes (harder) are balanced. Brace through the core to keep hips level to shoulders (no sagging). If holding here for 10 seconds is challenging enough, save the leg section for a later date. Work up to a 30-second hold before advancing.

STEP 2
From that solid, well-braced plank, on shins or toes, pull the knees in toward one elbow, allowing the hips to come just high enough to accommodate the incoming legs. With control, extend legs back out to straight. Be sure to also press strongly through the chest and arms, keeping shoulder blades flat, not collapsed towards each other. Brace abs again, then pull knees to the opposite side to complete one rep.


fitness model ryan fatscher photographed at on the go fitness, st. james