Axial hypertonicity in Parkinson's disease: Direct measurements of trunk and hip torque

A cardinal feature of Parkinson's disease (PD) is muscle hypertonicity, i.e. rigidity. Little is known about the axial tone in PD or the relation of hypertonia to functional impairment. We quantified axial rigidity to assess its relation to motor symptoms as measured by UPDRS and determine whether rigidity is affected by levodopa treatment. Axial rigidity was measured in 12 PD and 14 age-matched controls by directly measuring torsional resistance of the longitudinal axis to twisting (± 10°). Feet were rotated relative to fixed hips (Hip Tone) or feet and hips were rotated relative to fixed shoulders (Trunk Tone). To assess tonic activity only, low constant velocity rotation (1°/s) and low acceleration (< 12°/s²) were used to avoid eliciting phasic sensorimotor responses. Subjects stood during testing without changing body orientation relative to gravity. Body parts fixed against rotation could translate laterally within the boundaries of normal postural sway, but could not rotate. PD OFF-medication had higher axial rigidity (p < 0.05) in hips (5.07 N m) and trunk (5.30 N m) than controls (3.51 N m and 4.46 N m, respectively), which did not change with levodopa (p > 0.10). Hip-to-trunk torque ratio was greater in PD than controls (p < 0.05) and unchanged by levodopa (p = 0.28). UPDRS scores were significantly correlated with hip rigidity for PD OFF-medication (r values = 0.73, p < 0.05). Torsional resistance to clockwise versus counter-clockwise axial rotation was more asymmetrical in PD than controls (p < 0.05), however, there was no correspondence between direction of axial asymmetry and side of disease onset. In conclusion, these findings concerning hypertonicity may underlie functional impairments of posture and locomotion in PD. The absence of a levodopa effect on axial tone suggests that axial and appendicular tones are controlled by separate neural circuits.