- Email: [email protected]
bite-force function
Archs oral Bid. Vol. 39, No. 2, pp. I1 I-I 15, 1994 Copyright 0 1994 Elsevier ScienceLtd
Pergamon
Printed in Great Britain. All rights reserved 0003.9969/94$6.00+ 0.00
ESTIMATING HUMAN INCISAL BITE FORCES FROM THE ELECTROMYOGRAM/BITE-FORCE FUNCTION THOMASGAP,’ JILL RENDELL,’ AURELIE MAJOUREAU~and F. TIM MALONEY* ‘Department of BioStructure and Function, School of Dental Medicine, University of Connecticut Health Center, Farmington, CT 06030 and 2The Procter & Gamble Company, Cincinnati, OH 45241, U.S.A. (Accepted 6 October 1993) Some-This study used the electromyogram (EMG)~forc~fun~tion curve as a means of estimating functional bite forces. Surface EMG recordings were made for the masseter and anterior temporalis muscles bilaterally in ten adults while incisal bite forces were measured using a force transducer embedded in custom-made hydroplastic bite blocks. Measurements were made for mouth openings of 15 and 30 mm. Each subject was first instructed to develop a maximum level of bite force, and then to develop bite-force levels that corresponded to decreasing percentages of his or her maximum, from 90 to lo%, in 10% decrements. ARer the force measurements had been made, each subject was instructed to incise various food substances. The power-density spectrum and r.m.s. power level were calculated for each sample. Maximum bite-force levels varied considerably among the 10 subjects, ranging from a minimum of 2.5 kg to a maximum of 40 kg. For the subjects as a whole, estimated bite forces for the different types of food ranged from 3.0 to 5.5 kg. This range was independent of the subjects’ maximum bite-force levels. Thus, bite forces exerted to incise different foods probably depend more on the food than on the individual’s bite capabilities. There was variability within this range, although, as expected, bite forces were least for chocolate and greatest for apple and toffee. Key words: bite force, EMG.
INTRODUCTION Knowledge of the bite forces needed to incise different types of food is im~rtant for investigators interested in the biomechanics of the masticatory motor system and for orthodontists and prosthodontists who design and fabricate appliances that must withstand these forces. Although attempts have been made to measure these forces either directly while a subject or patient is biting down on different foods, or indirectly using simulated biting, results have not been entirely satisfactory (Adams and Zander, 1957; Ahlgren and Owall, 1970; Anderson, 1956; Helkimo and Ingervall, 1978). It is not possible to measure these forces directly during function. However, there is an approach that can be used to ‘estimate’ functional bite forces; this extrapolates bite-force levels from an EMG/bite-fork function curve. It is well known that a reliable and reproducible relation exists between changes in the amplitude of the EMG signal and changes in isometric muscle tension (Lippold, 1952). This relation appears as linear or quasilinear increments of muscle activity per unit of force. The range of forces produced by a given muscle system can be represented by a plot of EMG amplitude versus force; the slope of the curve, although variable from individual to individual and muscle to muscle, is consistent and reproducible for any given individual at the same extent of mouth
~~~~e~~~f~~~~ : EMG, el~tromyogram root mean square.
(-graphic); r.m.s.,
opening (Lippold, 1952; Lindauer, Gay and Rendell, 1991). Thus, by mapping the EMG/force-function curve for a muscle system during the application of known external forces, we can approximate the percentage of bite force generated during biting or chewing by determining where the EMG signal measured during those activities appears on the EMG/force-function curve. METHODS
AND MATERIALS
Subjects
Ten adults, six male and four female, served as subjects. All had normal dentition and no history of oral or temporomandibular joint pathology or disorders. Ages ranged from 24 to 52yr.
All recordings for both the superficial masseter and anterior temporalis muscles were made bilaterally using commercially available bipolar surface electrodes (Sensormedics) that were attached to the skin with double-sided adhesive tape. The site of electrode placement for both muscles was established partly by palpation. The superficial masseter site was determined as an area midway along a line connecting the inferior border of the zygomatic arch at the zygomaticotemporal suture to the gonial angle. The anterior temporalis site was determined by palpation during jaw clenching. Electrical impedance at sites of electrode contact was reduced by light abrasion and the application of a saline gel. The raw EMG signals were 111
led from the electrodes by paired wires to differential input preamplifiers (CWE, Model 831) where they were amplified at a gain of at least 1000, depending on the individual muscle and subject. The signals were band-pass filtered between 2-2000 Hz to remove any d.c. offset and high-frequency noise. The amplified data signals for each muscle were then stored on separate channels of a digital-data cassette recorder (Teat, RD- 1I 1T). Force me~s~rerne~t.~
Incisal bite-force nleasur~ments for each subject were obtained with a force transducer system that includes a miniature quartz transducer (6mm dia) and a charge amplifier (Kistler, Model 5004). In this system, the force applied to the transducer acts on a quartz element through two face plates. The longitudinal force effects on the two face plates induce a proportional electrostatic charge in the quartz element. The charge amplifier serves as both a power source and conditioning amplifier for the quartz element and outputs a force-proportional d.c. voltage that is recorded on a separate channel of the digital cassette recorder. The force transducer was imbedded in an acrylic bite block that was custom fabricated for each individual from cold-cure thermoplastic (Tak, Hydroplastic). Each bite block was fitted to span al1 four incisors and produce an interincisat distance of 15 or 30 mm. These two different mouth openings
were used to match the opening required for the different food types. This was necessary because EMG activity levels differ as a function of muscle length (Lindauer er al., 1991). Experimental protocol
After the EMG electrodes had been attached, the bite-force transducer was inserted between the teeth, the equipment was checked and calibrated, and the subject was instructed to develop and maintain a level of maximum bite force for a period of approx. 2 s. The signal from the bite-force transducer was branched off to the input of a storage oscilloscope and displayed as a moving dot where it was monitored and measured by one of the experimenters. Each subject’s maximum bite force was determined as the mean of three attempts. Each subject was then instructed to develop bite-force levels that corresponded to decreasing percentages of his or her maximum bite force, from 90 to lo%, in 10% decrements. These levels were achieved by the subject matching a force-related sweep line on an oscilloscope to the predetermined reference sweep line. After the force measurements had been made, each subject was required to bite and chew various food substances. These included 15 mm-thick pieces of carrot, pretzel, chocolate and toffee, a 30-mm piece of carrot, and an apple. All functional biting activities were repeated five times for each food type.
Bite
Pre-Bite
500 Time Fig. 1. Example
of pre-biting
(ms)
and biting segments of EMG signal. two components.
Note silent period
that separates
the
Estimating
functional
113
bite forces
EMG/Bite Force: Low Maximum Force Mouth
MouthOpening = 30 mm
opening= 15 mm
Maximum Bite Force = 5 kg
Maximum Bite Force = 8 Lrg
60
I
carrot
50
3 ;
Lo ; 8
30
Pretzel
% 2n
i,
10
Cheolate
10
20
30
1’ 0
:
40
10
20
30
40
Bite Force (kg)
Bite Force (kg)
Fig. 2. Linear regression curve of mean EMG/bite-force function for a subject (AM) who was able to generate maximum bite-force levels of 8 kg at 15-mm mouth opening and 5 kg at 30 mm. Estimated bite forces are superimposed on the composite curve. Circles represent EMG values obtained at each 10%
bite-force level.
Data acquisition and processing The EMG and force data were input and analysed on an IBM PS/2, Model 80 microcomputer, using a commercially available data acquisition and analysis
hardware and software system (Crisal PC, Version 2.5). The EMG signals and the d.c. force signal were played back from the digital-data cassette recorder through an analogue-to-digital converter at a sampling rate of 2 kHz. Two-second samples for each
EMG/Bite Force: Moderate Maximum Force MouthOpening = 15 mm Maximum Bite Force = 16 kg
Mouth Opening = 30 mm Maximum Bite. Force = 8 kg
60
”
z ;
50
a
Pretzel
Toffee
I2 20
Chocolate
Small Carrot
0
0
10
20
30
Bite Force (kg)
Fig. 3. Linear generate
regression maximum
4.0
50
10 L 0
10
20
30
40
Bite Force (kg)
curve of mean EMG/bite-force function for a subject (JR) who was able to bite-force levels of 16 kg at 15mm mouth opening and 8 kg at 30 mm.
114
THOMAS
GAY el al.
EMG/ESiteForce: High Maximum Force Month Opening = 15 mm MAUlUlllBi~PorCe=4Okg
YApple
20+ 10 0
LO
xl
30
40
Bite Force (kg)
LargeCarrot
1°0U
50
Bite Force (kg)
Fig. 4. Linear regression curve of mean EMG/bite-force function for a subject (PL) who was able to generate maximum bite-force levels of 40 kg at 15-mm mouth opening and 30 kg at 30 mm.
force level were input to the computer. Each sample was then demultiplexed and stored as a raw data file. Each raw data file was then digitally high-pass filtered at 2 Hz to remove any d.c. offset voltages. The power-density spectrum and r.m.s. power level for each sample was then calculated with the results stored in a separate analysis file. Both the mean and median frequencies of the power density spectrum were calculated by another subroutine. Figure I shows an example of a food-biting EMG sample used for analysis. The pre-bite isometric EMG is easily separated from the bite-related EMG signal because it is separated from the anisometric portion by a short silent period. The power-density spectrum and r.m.s. power levels were calculated for each of the pre-bite samples. For each subject, plots of EMG (r.m.s. power) and bite force (in kg) were constructed for each muscle. Because of trade-offs in the contributions of the
!hocolate RCt?.Cl Large C&t Small carrot Toffee Apple
Summary
of estimated bite forces required the various foods.
to i nc :ise
various muscles, a composite linear regression curve was plotted using the overall mean values of the four muscles. The r.m.s. power values (mean of five samples) obtained for the various biting activities were superimposed on the composite EMG/forcefunction curves to obtain numerical values corresponding to relative functional bite forces. RESULTS
As expected, maximum bite-force levels varied considerably among the 10 subjects, ranging from 4.0 to 40.0 kg at 15mm mouth opening and from 2.5 to 30.0 kg at 30 mm. All subjects were able to generate greater levels of bite force at 15-mm opening. The EMG/bite-force curves were, for the most part, linear for all four muscles across the 10 subjects. In some subjects non-linearity appeared between the 90 and 100% intervals. Figures 24 illustrate our main finding: functional bite forces are clustered in a range that is independent of an individual’s ability to generate bite force. Figure 2 shows the linear regression of the EMG/ force-function curves for a subject who was able to generate a maximum of 8 kg force at 15-mm mouth opening and 5 kg at 30 mm. The circles represent the mean value of the four muscles for each 10% interval in bite force. Estimated incisal bite forces range from 1.6 to 2.5 kg at 15-mm opening, and from 2.4 to 3.9 kg at 30 mm. These forces correspond to 20-32% maximum voluntary bite force at 15-mm opening and 48-78% at 30mm. Figure 3 shows the data for a subject who was able to generate 16 kg of maximum bite force at 15-mm mouth opening and 8 kg at 30 mm. Her estimated functional bite forces ranged from 1.6 to 3.0 kg at
Estimating functional bite forces 15-mm mouth opening and from 4.0 to 6.3 kg at 30mm. These forces correspond to l&19% maximum voluntary bite force at 15-mm opening and 6&78% at 30mm. Figure 4 shows the data for a third subject who was able to generate 40 kg of maximum bite force at 15-mm mouth opening and 30 kg at 30 mm. His estimated functional bite forces ranged from 3.1 to 4.3 kg at 15-mm opening and from 3.4 to 6.1 kg at 30 mm. These force levels correspond to 8-11% maximum voluntary bite force at 15 mm and 1l-20% at 30mm. Figure 5 summarizes the estimated forces required to bite the various foods; it shows the mean estimated incisal bite-force levels of all 10 subjects. Although the mean differences are relatively small, the distribution of forces is not unexpected, with chocolate requiring the least and toffee the greatest levels. Standard deviations were: chocolate, 0.76; small carrot, 0.35; pretzel, 0.29; apple, 0.65; large carrot, 0.59; toffee, 0.3 1. DISCUSSION
This study produced two main findings: (1) numerical estimates of incisal forces needed to penetrate different food types and (2) that these bite forces are clustered in a range which is independent of an individual’s ability to generate bite force. It should be emphasized first that the numerical values we extrapolated from the EMG/bite-force function are estimates only. They were derived from a composite of four muscles accessible to electromyography; other elevator muscles also contribute to the generation of incisal bite force. They also vary from individual to individual, although this variability is less than the individual variability that characterizes the ranges of incisal bite forces that we measured. Finally, these estimates are related only to the prebite component of incision. We did not estimate the bite forces that accompanied muscle shortening during the chewing phase of the incision. This is because the EMG/force function is valid only for isometric muscle contractions (Lippold, 1952). However, our results were fairly consistent with those of Neil1 et al. (1989), who estimated chewing forces from a relation established between EMG amplitude and bite force. One interesting extension of our results relates to a possible explanation of the difficulties that individ-
II5
uals who wear prosthetic denture appliances confront when attempting to incise the types of food we studied. It has been shown that these appliances (full upper dentures) tend to dislodge at forces of approx. 1.5-2 kg (B. J. MacKay et al., unpublished). Our data showed that the various hard foods we studied require force levels above that threshold, at least 3 kg for chocolate, and over 4 kg for a carrot or apple, for example. Our results also demonstrate that an individual’s functional incisal bite forces are determined by the type of food being incised rather than inherent biteforce capability. This is an interesting finding from a physiological point of view because it suggests economy of effort in function. This finding also might have clinical implications in helping to explain the possible origin of dysfunction of the mandibular elevator muscles in disorders of the temporomandibular joint. If an individual incisal bite-force capability is inherently low, then that individual would necessarily function at the upper range of his or her bite-force curve. This might result in the muscle ‘hyperactivity’ that is generally believed to characterize this patient population. Acknowledgement-This research was supported in part by a grant from The Procter & Gamble Company, Cincinnati, OH, U.S.A.
REFERENCES
Adams S. H. and Zander H. (1964) Functional tooth contacts in lateral and centric occlusion. J. Am. dent. Ass. 69, 465.
Ahlgren J. and Owall B. (1970) Muscular activity and chewing force: a polygraphic study of human mandibular movements. Archs oral Biol. 15, 271-275. Anderson D. J. (1956) Measurement of stress in mastication. J. dent. Res. 35, 664670. Helkimo E. and Ingervall B. (1978) Bite force and functional state of the masticatory system in young men. Swed. Dent. J. 2, 167-175. Lippold 0. C. J. (1952) The relation between integrated action potentials in the human muscle and its isometric tension. J. Physiol., Lond. 117, 492499. Lindauer S. J., Gay T. and Rendell J. K. (1991) EMG-force function characteristics in the assessment of oral function. J. dent. Res. 70, 1417-1421. Neil1 D. J., Kydd W. L., Nairn R. I. and Wilson J. (1989) Functional loading of the dentition during mastication. J. Prosth. Dem. 62, 218-228.
Pergamon
Printed in Great Britain. All rights reserved 0003.9969/94$6.00+ 0.00
ESTIMATING HUMAN INCISAL BITE FORCES FROM THE ELECTROMYOGRAM/BITE-FORCE FUNCTION THOMASGAP,’ JILL RENDELL,’ AURELIE MAJOUREAU~and F. TIM MALONEY* ‘Department of BioStructure and Function, School of Dental Medicine, University of Connecticut Health Center, Farmington, CT 06030 and 2The Procter & Gamble Company, Cincinnati, OH 45241, U.S.A. (Accepted 6 October 1993) Some-This study used the electromyogram (EMG)~forc~fun~tion curve as a means of estimating functional bite forces. Surface EMG recordings were made for the masseter and anterior temporalis muscles bilaterally in ten adults while incisal bite forces were measured using a force transducer embedded in custom-made hydroplastic bite blocks. Measurements were made for mouth openings of 15 and 30 mm. Each subject was first instructed to develop a maximum level of bite force, and then to develop bite-force levels that corresponded to decreasing percentages of his or her maximum, from 90 to lo%, in 10% decrements. ARer the force measurements had been made, each subject was instructed to incise various food substances. The power-density spectrum and r.m.s. power level were calculated for each sample. Maximum bite-force levels varied considerably among the 10 subjects, ranging from a minimum of 2.5 kg to a maximum of 40 kg. For the subjects as a whole, estimated bite forces for the different types of food ranged from 3.0 to 5.5 kg. This range was independent of the subjects’ maximum bite-force levels. Thus, bite forces exerted to incise different foods probably depend more on the food than on the individual’s bite capabilities. There was variability within this range, although, as expected, bite forces were least for chocolate and greatest for apple and toffee. Key words: bite force, EMG.
INTRODUCTION Knowledge of the bite forces needed to incise different types of food is im~rtant for investigators interested in the biomechanics of the masticatory motor system and for orthodontists and prosthodontists who design and fabricate appliances that must withstand these forces. Although attempts have been made to measure these forces either directly while a subject or patient is biting down on different foods, or indirectly using simulated biting, results have not been entirely satisfactory (Adams and Zander, 1957; Ahlgren and Owall, 1970; Anderson, 1956; Helkimo and Ingervall, 1978). It is not possible to measure these forces directly during function. However, there is an approach that can be used to ‘estimate’ functional bite forces; this extrapolates bite-force levels from an EMG/bite-fork function curve. It is well known that a reliable and reproducible relation exists between changes in the amplitude of the EMG signal and changes in isometric muscle tension (Lippold, 1952). This relation appears as linear or quasilinear increments of muscle activity per unit of force. The range of forces produced by a given muscle system can be represented by a plot of EMG amplitude versus force; the slope of the curve, although variable from individual to individual and muscle to muscle, is consistent and reproducible for any given individual at the same extent of mouth
~~~~e~~~f~~~~ : EMG, el~tromyogram root mean square.
(-graphic); r.m.s.,
opening (Lippold, 1952; Lindauer, Gay and Rendell, 1991). Thus, by mapping the EMG/force-function curve for a muscle system during the application of known external forces, we can approximate the percentage of bite force generated during biting or chewing by determining where the EMG signal measured during those activities appears on the EMG/force-function curve. METHODS
AND MATERIALS
Subjects
Ten adults, six male and four female, served as subjects. All had normal dentition and no history of oral or temporomandibular joint pathology or disorders. Ages ranged from 24 to 52yr.
All recordings for both the superficial masseter and anterior temporalis muscles were made bilaterally using commercially available bipolar surface electrodes (Sensormedics) that were attached to the skin with double-sided adhesive tape. The site of electrode placement for both muscles was established partly by palpation. The superficial masseter site was determined as an area midway along a line connecting the inferior border of the zygomatic arch at the zygomaticotemporal suture to the gonial angle. The anterior temporalis site was determined by palpation during jaw clenching. Electrical impedance at sites of electrode contact was reduced by light abrasion and the application of a saline gel. The raw EMG signals were 111
led from the electrodes by paired wires to differential input preamplifiers (CWE, Model 831) where they were amplified at a gain of at least 1000, depending on the individual muscle and subject. The signals were band-pass filtered between 2-2000 Hz to remove any d.c. offset and high-frequency noise. The amplified data signals for each muscle were then stored on separate channels of a digital-data cassette recorder (Teat, RD- 1I 1T). Force me~s~rerne~t.~
Incisal bite-force nleasur~ments for each subject were obtained with a force transducer system that includes a miniature quartz transducer (6mm dia) and a charge amplifier (Kistler, Model 5004). In this system, the force applied to the transducer acts on a quartz element through two face plates. The longitudinal force effects on the two face plates induce a proportional electrostatic charge in the quartz element. The charge amplifier serves as both a power source and conditioning amplifier for the quartz element and outputs a force-proportional d.c. voltage that is recorded on a separate channel of the digital cassette recorder. The force transducer was imbedded in an acrylic bite block that was custom fabricated for each individual from cold-cure thermoplastic (Tak, Hydroplastic). Each bite block was fitted to span al1 four incisors and produce an interincisat distance of 15 or 30 mm. These two different mouth openings
were used to match the opening required for the different food types. This was necessary because EMG activity levels differ as a function of muscle length (Lindauer er al., 1991). Experimental protocol
After the EMG electrodes had been attached, the bite-force transducer was inserted between the teeth, the equipment was checked and calibrated, and the subject was instructed to develop and maintain a level of maximum bite force for a period of approx. 2 s. The signal from the bite-force transducer was branched off to the input of a storage oscilloscope and displayed as a moving dot where it was monitored and measured by one of the experimenters. Each subject’s maximum bite force was determined as the mean of three attempts. Each subject was then instructed to develop bite-force levels that corresponded to decreasing percentages of his or her maximum bite force, from 90 to lo%, in 10% decrements. These levels were achieved by the subject matching a force-related sweep line on an oscilloscope to the predetermined reference sweep line. After the force measurements had been made, each subject was required to bite and chew various food substances. These included 15 mm-thick pieces of carrot, pretzel, chocolate and toffee, a 30-mm piece of carrot, and an apple. All functional biting activities were repeated five times for each food type.
Bite
Pre-Bite
500 Time Fig. 1. Example
of pre-biting
(ms)
and biting segments of EMG signal. two components.
Note silent period
that separates
the
Estimating
functional
113
bite forces
EMG/Bite Force: Low Maximum Force Mouth
MouthOpening = 30 mm
opening= 15 mm
Maximum Bite Force = 5 kg
Maximum Bite Force = 8 Lrg
60
I
carrot
50
3 ;
Lo ; 8
30
Pretzel
% 2n
i,
10
Cheolate
10
20
30
1’ 0
:
40
10
20
30
40
Bite Force (kg)
Bite Force (kg)
Fig. 2. Linear regression curve of mean EMG/bite-force function for a subject (AM) who was able to generate maximum bite-force levels of 8 kg at 15-mm mouth opening and 5 kg at 30 mm. Estimated bite forces are superimposed on the composite curve. Circles represent EMG values obtained at each 10%
bite-force level.
Data acquisition and processing The EMG and force data were input and analysed on an IBM PS/2, Model 80 microcomputer, using a commercially available data acquisition and analysis
hardware and software system (Crisal PC, Version 2.5). The EMG signals and the d.c. force signal were played back from the digital-data cassette recorder through an analogue-to-digital converter at a sampling rate of 2 kHz. Two-second samples for each
EMG/Bite Force: Moderate Maximum Force MouthOpening = 15 mm Maximum Bite Force = 16 kg
Mouth Opening = 30 mm Maximum Bite. Force = 8 kg
60
”
z ;
50
a
Pretzel
Toffee
I2 20
Chocolate
Small Carrot
0
0
10
20
30
Bite Force (kg)
Fig. 3. Linear generate
regression maximum
4.0
50
10 L 0
10
20
30
40
Bite Force (kg)
curve of mean EMG/bite-force function for a subject (JR) who was able to bite-force levels of 16 kg at 15mm mouth opening and 8 kg at 30 mm.
114
THOMAS
GAY el al.
EMG/ESiteForce: High Maximum Force Month Opening = 15 mm MAUlUlllBi~PorCe=4Okg
YApple
20+ 10 0
LO
xl
30
40
Bite Force (kg)
LargeCarrot
1°0U
50
Bite Force (kg)
Fig. 4. Linear regression curve of mean EMG/bite-force function for a subject (PL) who was able to generate maximum bite-force levels of 40 kg at 15-mm mouth opening and 30 kg at 30 mm.
force level were input to the computer. Each sample was then demultiplexed and stored as a raw data file. Each raw data file was then digitally high-pass filtered at 2 Hz to remove any d.c. offset voltages. The power-density spectrum and r.m.s. power level for each sample was then calculated with the results stored in a separate analysis file. Both the mean and median frequencies of the power density spectrum were calculated by another subroutine. Figure I shows an example of a food-biting EMG sample used for analysis. The pre-bite isometric EMG is easily separated from the bite-related EMG signal because it is separated from the anisometric portion by a short silent period. The power-density spectrum and r.m.s. power levels were calculated for each of the pre-bite samples. For each subject, plots of EMG (r.m.s. power) and bite force (in kg) were constructed for each muscle. Because of trade-offs in the contributions of the
!hocolate RCt?.Cl Large C&t Small carrot Toffee Apple
Summary
of estimated bite forces required the various foods.
to i nc :ise
various muscles, a composite linear regression curve was plotted using the overall mean values of the four muscles. The r.m.s. power values (mean of five samples) obtained for the various biting activities were superimposed on the composite EMG/forcefunction curves to obtain numerical values corresponding to relative functional bite forces. RESULTS
As expected, maximum bite-force levels varied considerably among the 10 subjects, ranging from 4.0 to 40.0 kg at 15mm mouth opening and from 2.5 to 30.0 kg at 30 mm. All subjects were able to generate greater levels of bite force at 15-mm opening. The EMG/bite-force curves were, for the most part, linear for all four muscles across the 10 subjects. In some subjects non-linearity appeared between the 90 and 100% intervals. Figures 24 illustrate our main finding: functional bite forces are clustered in a range that is independent of an individual’s ability to generate bite force. Figure 2 shows the linear regression of the EMG/ force-function curves for a subject who was able to generate a maximum of 8 kg force at 15-mm mouth opening and 5 kg at 30 mm. The circles represent the mean value of the four muscles for each 10% interval in bite force. Estimated incisal bite forces range from 1.6 to 2.5 kg at 15-mm opening, and from 2.4 to 3.9 kg at 30 mm. These forces correspond to 20-32% maximum voluntary bite force at 15-mm opening and 48-78% at 30mm. Figure 3 shows the data for a subject who was able to generate 16 kg of maximum bite force at 15-mm mouth opening and 8 kg at 30 mm. Her estimated functional bite forces ranged from 1.6 to 3.0 kg at
Estimating functional bite forces 15-mm mouth opening and from 4.0 to 6.3 kg at 30mm. These forces correspond to l&19% maximum voluntary bite force at 15-mm opening and 6&78% at 30mm. Figure 4 shows the data for a third subject who was able to generate 40 kg of maximum bite force at 15-mm mouth opening and 30 kg at 30 mm. His estimated functional bite forces ranged from 3.1 to 4.3 kg at 15-mm opening and from 3.4 to 6.1 kg at 30 mm. These force levels correspond to 8-11% maximum voluntary bite force at 15 mm and 1l-20% at 30mm. Figure 5 summarizes the estimated forces required to bite the various foods; it shows the mean estimated incisal bite-force levels of all 10 subjects. Although the mean differences are relatively small, the distribution of forces is not unexpected, with chocolate requiring the least and toffee the greatest levels. Standard deviations were: chocolate, 0.76; small carrot, 0.35; pretzel, 0.29; apple, 0.65; large carrot, 0.59; toffee, 0.3 1. DISCUSSION
This study produced two main findings: (1) numerical estimates of incisal forces needed to penetrate different food types and (2) that these bite forces are clustered in a range which is independent of an individual’s ability to generate bite force. It should be emphasized first that the numerical values we extrapolated from the EMG/bite-force function are estimates only. They were derived from a composite of four muscles accessible to electromyography; other elevator muscles also contribute to the generation of incisal bite force. They also vary from individual to individual, although this variability is less than the individual variability that characterizes the ranges of incisal bite forces that we measured. Finally, these estimates are related only to the prebite component of incision. We did not estimate the bite forces that accompanied muscle shortening during the chewing phase of the incision. This is because the EMG/force function is valid only for isometric muscle contractions (Lippold, 1952). However, our results were fairly consistent with those of Neil1 et al. (1989), who estimated chewing forces from a relation established between EMG amplitude and bite force. One interesting extension of our results relates to a possible explanation of the difficulties that individ-
II5
uals who wear prosthetic denture appliances confront when attempting to incise the types of food we studied. It has been shown that these appliances (full upper dentures) tend to dislodge at forces of approx. 1.5-2 kg (B. J. MacKay et al., unpublished). Our data showed that the various hard foods we studied require force levels above that threshold, at least 3 kg for chocolate, and over 4 kg for a carrot or apple, for example. Our results also demonstrate that an individual’s functional incisal bite forces are determined by the type of food being incised rather than inherent biteforce capability. This is an interesting finding from a physiological point of view because it suggests economy of effort in function. This finding also might have clinical implications in helping to explain the possible origin of dysfunction of the mandibular elevator muscles in disorders of the temporomandibular joint. If an individual incisal bite-force capability is inherently low, then that individual would necessarily function at the upper range of his or her bite-force curve. This might result in the muscle ‘hyperactivity’ that is generally believed to characterize this patient population. Acknowledgement-This research was supported in part by a grant from The Procter & Gamble Company, Cincinnati, OH, U.S.A.
REFERENCES
Adams S. H. and Zander H. (1964) Functional tooth contacts in lateral and centric occlusion. J. Am. dent. Ass. 69, 465.
Ahlgren J. and Owall B. (1970) Muscular activity and chewing force: a polygraphic study of human mandibular movements. Archs oral Biol. 15, 271-275. Anderson D. J. (1956) Measurement of stress in mastication. J. dent. Res. 35, 664670. Helkimo E. and Ingervall B. (1978) Bite force and functional state of the masticatory system in young men. Swed. Dent. J. 2, 167-175. Lippold 0. C. J. (1952) The relation between integrated action potentials in the human muscle and its isometric tension. J. Physiol., Lond. 117, 492499. Lindauer S. J., Gay T. and Rendell J. K. (1991) EMG-force function characteristics in the assessment of oral function. J. dent. Res. 70, 1417-1421. Neil1 D. J., Kydd W. L., Nairn R. I. and Wilson J. (1989) Functional loading of the dentition during mastication. J. Prosth. Dem. 62, 218-228.