There's a book by William Parry that postulates that the core is involved in stuttering by the inadvertent triggering of the Valsalva maneuver. I read his book a few years ago, but was left largely disappointed. Sure there are theories as to how the Valsalva maneuverer is linked to stuttering, but no treatment plan related to this hypothesis is presented. Instead Parry diverts treatment recommendation to generic relaxation and fluency shaping principles. I don't see there being a meaningful link between core strength and fluency rate in the general population. Core strength often comes up when PWS feel tension in their abdomen. Weak muscle structure isn't the cause of this tension, rather the stutter's attempts to force out words is the root of the problem that manifests itself in tension of the core, lungs, neck, and jaw.