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Task-dependence of Activity/ Bite-force Relations and its Impact on Estimation of Chewing Force from EMG

Department of Prosthodontics, University Dental Clinic, University of Erlangen-Nürnberg, Glueckstrasse 11, D 91054 Erlangen, Germany;

View larger version (36K): [in a new window]   Figure 1. Examples of activity/bite-force relations in two selected patients with the activity used as the independent variable. Fest and Fme denote estimated and measured chewing forces. Patient A: Working-side masseter (mass. ws) showing a strong correlation in clenching on the bite-fork (r = 0.96) but a weak one in chewing (r = 0.6). The mean activity/bite-force ratio was higher in chewing (0.45 µV/N) than in clenching (0.23 µV/N), resulting in overestimation of chewing force (613 N) when the mean chewing activity (128 µV) was substituted into the activity/bite-force regression of clenching. Dashed arrows elucidate the estimation procedure. Balancing-side masseter (mass. bs) showed good correlations and almost equal activity/bite-force ratios (0.41 µV/N, 0.39 µV/N) in both biting tasks. Hence, the estimated chewing force deviated by only about 5% from the measured force. Patient B: Weak correlations in chewing because of missing small forces. However, in both muscles, activities per unit bite-force in the 2 biting tasks were similar, resulting in fairly correct force estimations.

View larger version (17K): [in a new window]   Figure 2. Group means and standard errors (N = 9) of activity/bite-force ratios for each muscle. Data next to the arrows indicate the factor by which the activity/bite-force ratio in chewing exceeded the activity/bite-force ratio in clenching. Since the bite-force cancels out, these factors also indicate the chewing/clenching ratios of the absolute activities. Abbreviations: mass = masseter, temp = temporalis, ws = working side, bs = balancing side.

View larger version (23K): [in a new window]   Figure 3. Actually measured masticatory forces with standard errors and forces estimated from the chewing activities of the 4 muscles in all subjects. Patients are arranged according to ascending values of measured forces (connected for the sake of clarity). The right-most compartment shows group means and standard errors (N = 9) of measured and estimated chewing forces. Patients #8 and #3 correspond to patient A and patient B of Fig. 1. Standard errors of measured forces are based on the number of chewing cycles performed during the recording period. The number of cycles varied interindividually between N = 12 and N = 32. Estimated forces have no error bars, because only one value could be obtained per patient and muscle. Abbreviations: est = estimated chewing force, mass = masseter, temp = temporalis, ws = working side, bs = balancing side.