A maximum-force-dependent protocol for assessing labial force control

A protocol based on target levels, referenced to maximum voluntary closing force (MVCF), was used to assess static force control in the perioral musculature. Twenty normal-speaking subjects were instructed to produce rapid and accurate isometric force trajectories to end-point target levels and maintain force on-target for 5 sec while viewing a computer monitor. Each lip was assessed on a number of quantitative force control measures at 10, 20, 50, and 75% of MCVF. The 10% and 20% target levels were included so that static force control could be evaluated within the presumed physiological operating range for normal speech. Results of the target-level scaling procedure revealed that the upper lip was more stable in controlling force at all four targets. The use of MVCF-based target scaling is discussed in relation to absolute nonscaled force analysis methods applied to the perioral motor system.