Effects of bolus size and hardness on within-subject variability of chewing cycle kinematics.

This study analysed how bolus hardness and size affect within-subject variability of chewing cycle kinematics. Two independent prospective studies were performed; both tracked chin movements using an optoelectronic recording system. Computer programs identified each subject's ten most representative cycles, and multilevel modelling procedures were used to estimate variances. One study evaluated 38 subjects who chewed 1, 2, 4 or 8 g of gum presented in random order. The second study evaluated 26 subjects who chewed approximately 2.5 g of harder (670 g) or softer (440 g) gum, also presented in random order. In terms of bolus size, the 2g and 1g boluses produced the least and greatest relative within-subject variability, respectively; the largest differences were found for cycle duration and excursions. Within-subject variability when chewing the harder gum was consistently greater than when chewing the softer gum, except for lateral movement towards the balancing side. Because bolus hardness and bolus size influence within-subject variability differently, they must be taken into consideration when designing experiments to study masticatory kinematics. We conclude that a 2g bolus of soft gum should be used in studies of chewing cycle kinematics in order to reduce within-subject variability and increase statistical power.