We’re not sure why you’d want to make a back squat harder than it is, but some researchers found a way to do it, and for a pretty good reason.
Scientists conducted an experiment, published in The Journal of Strength and Conditioning Research, in which they subjected 15 trained weightlifters to a parallel back squat, with both a stable load and unstable load (10 reps at 60 percent of their one-rep max).
When squatting with the unstable load, the lifters produced greater muscle activation in their rectus abdominis, external obliques, and soleus.
An easy way to replicate this in the gym is to hang kettlebells from each end of the barbell with a rubber exercise band. It might look a little funny, but when your abs are as hard as a diamond in an ice storm, nobody will be laughing. IM