Strengthening training Does it really work? Strengthening exercise may be unnecessary for many clients because the tongue and lips use only 10~30% of their maximum forces for speech, and the jaw only 2%, and that up to 1/3 of motor nerve fibers can be lost before functional impairments are encountered (by Duffy,1995) However, some experts still suggest strengthening training (Dworkin, 1991). Especially when the muscular weakness is the primary underlying impairment that contributes to the articulation deficits.