Electromyographic Heterogeneity in the Human Temporalis Muscle N.G. BLANKSMA and T.M.G.J. VAN EIJDEN Academic Center for Dentistry Amsterdam (ACTA), Department of Anatomy and Embryology, University of Amsterdam, Meibergdreef 15, 1105 AZ Amsterdam, The Netherlands
This study examined the possible existence of regional differences in activation of the temporalis muscle. Since the muscle is fan-shaped, its fibers pull in different directions. This suggests regional specialization for different motor tasks. EMG activity was registered by six bipolar fine-wire electrodes inserted anteroposteriorly across the muscle belly. Muscle signals were recorded during different static bite tasks for which both the direction and magnitude of bite force were specified. The results showed that the ratio of activities of the six muscle regions changed with the direction of bite force. This indicates a partitioning of the excitatory command to the muscle's motoneuron pool. Alteration in activity with changing bite-force direction was generally the smallest in the anterior-most region of the muscle, the largest in the posterior-most region, and the intermediate in the interjacent regions. Generally, all muscle regions exhibited the highest EMG activity when the bite force was in an approximately posterolateral direction. The muscle was activated uniformly only for bites in this preferential direction. Activity in the regions appeared to be scaled up or down in a linear way according to the desired bite-force level. The results indicate that the direction of pull of the muscle and the maximal force it can produce are not fixed, but depend on the direction of bite force. J Dent Res 69(10):1686-1690, October, 1990
Introduction. To obtain insight into the role a muscle plays in executing a particular motor task, knowledge is required about the way it is activated. There is increasing evidence that muscles are not always activated homogeneously, i.e., all motoneurons in a muscle's motoneuron pool receive the same input, but that selective activation of motoneurons is possible (Ter Haar Romeny et al., 1982; Hoffer et al., 1987; Van Zuylen et al., 1988; Jongen et al., 1989). These studies indicate that a motoneuron pool of a particular muscle may consist of subpopulations of motoneurons that can be activated more or less independently to perform different tasks. The levator muscles of the human jaw have a complex architecture with relatively large attachment areas. For this reason, each muscle would have a large variety of possible action lines in case of a selective activation of muscle parts. Consequently, the range of bite-force directions in which a jaw muscle might participate to produce bite force would increase (Van Eijden et al., 1988a). Traditionally, the muscles have been divided very crudely. For example, the temporalis muscle is usually partitioned into an anterior and a posterior part. Since their action lines differ in orientation, these two parts have the potential to behave differentially in performing various motor tasks. EMG studies have indicated that this is indeed true (Vitti and Basmajian, 1977; Van Eijden et al., 1990). The possible existence of smaller parts that differ in activation has been Received for publication February 22, 1990 Accepted for publication June 11, 1990
1686
suggested by Eriksson and Thornell (1983), who demonstrated regional differences in histochemical fiber-type composition. This points to differences in motor unit behavior. Since the temporalis muscle is fan-shaped, different fibers pull in different directions. This suggests that with, for example, a change in bite-force direction, particular regions may be activated preferentially. The aim of the present study was to examine the possible existence of regional differences in activation of subpopulations of fibers in the temporalis muscle.
Materials and methods. Subjects. -The study was performed on five male subjects, ranging in age from 27 to 39 years. They had normal Class I occlusions and showed no symptoms or signs of craniomandibular dysfunction. One of the subjects was missing a lower left premolar. The experiment was approved by the medical ethical committee of the Academic Hospital. Generation of bite force of a specific direction and magnitude. -A detailed description of the procedure is given by Van Eijden et al. (1988b). Briefly, both the direction and magnitude of bite force were registered by a three-component force transducer (dimensions, 24 x 24 x 10 mm, length x width x height), positioned between a pair of occlusal clutches; the inter-incisal distance was 18 ± 5 mm (mean ± SD of the five subjects). For feedback purposes, both the magnitude and direction of the resultant force applied to the transducer and the desired bite-force magnitude and direction were visualized on the screen of a computer. The magnitude of the exerted force was represented by a vertical bar of varying height, and the desired force level by a horizontal bar. The actual force direction was shown by a cross situated in a two-dimensional polar coordinate system. The desired direction, with a range of 50, was represented by a circle. Subjects could be instructed to produce a bite force of a particular direction and magnitude by matching the cross with the circle and the top of the vertical bar with the horizontal bar. In the present study, the center of the transducer coincided with the center of the second right upper premolar. Two experiments were carried out. In the first experiment, bite-force magnitude was kept constant at a level of 150 N. Forces were exerted in nine different directions, defined relative to the vertical z-axis (i.e., perpendicular to the occlusal plane of the upper teeth) and the anteroposterior x-axis, which was parallel to both the occlusal and mid-sagittal planes. The following directions were examined: vertical, anterior (z-angle, 20°; x-angle, 00), anterolateral (200, 450), lateral (20°, 90°), posterolateral (200, 1350), posterior (20°, 1800), posteromedial (200, 225°), medial (200, 2700), and anteromedial (200, 3150). The sequence of the nine bites was chosen randomly; between the trials, a rest period of at least two min was inserted so that fatigue effects would be avoided. The experiment was repeated five times, each time with another randomized order of bite-force directions. Hence, the total number of bites in each subject was 54 (nine directions x six trials). In the second experiment, in which three of the five subjects
Downloaded from jdr.sagepub.com at UCSF LIBRARY & CKM on April 9, 2015 For personal use only. No other uses without permission.
Vol. 69 No. 10
HETEROGENEITY IN THE TEMPORALIS MUSCLE
participated, only five bite-force directions were examined, viz., vertical, anterior, lateral, posterior, and medial. In each direction, bites of 50, 150, 250, 350 N, and maximal voluntary force were produced. EMG registration. -Six bipolar fine-wire electrodes were used for registration of the EMG activity in different anteroposterior regions of the right temporalis muscle. Each electrode pair consisted of two Teflon-insulated stainless-steel wires (diameter, 0.12 mm) with their tips bared (1 mm) and bent into a hook (Basmajian and Stecko, 1962); the separation between the bare ends of the two wires was approximately 2 mm. They were inserted with the aid of a 0.6-x-25-mm disposable needle. The electrodes were on a line parallel to the zygomatic arch at a level 1.5 cm above the lateral corner of the eye. The anterior- and posterior-most electrode pairs were at a distance of 1.5 cm from, respectively, the anterior and posterior borders of the muscle, identified by palpation; the electrode pairs were at an equal distance (1.7 + 0.2 cm, mean ± SD of the five subjects) from each other. A ground electrode was attached to the skin over the seventh cervical vertebra. The EMG apparatus consisted of differential amplifiers. Frequency bandwidth was from 200 to 1000 Hz; amplification ranged from 1000 to
10,000.
Data processing.-For each bite, the outputs of the force transducer (three signals, one for each force component) and of the EMG apparatus (six muscle signals) were simultaneously recorded on FM analogue tape. The analogue data were digitized (2400 samples/s/channel) with a DEC 11/73 minicomputer system. From each trial, a computer program automatically selected a continuous period of 0.5 s. Out of all possible 0.5-second periods, the one was selected where the resultant force was within 50 of the specified bite-force direction, and the mean-squared difference between the exerted and desired force level was the smallest (this difference was usually smaller than 5 N); in trials where maximal bite forces were exerted, the 0.5-second period with the largest mean bite force was chosen. The EMG activity of the six muscle regions in this period was quantified as the mean value of the rectified
1687
350 N). The dots represent EMG activity for the maximal bite force in the four directions. The following points deserve special attention.
First, all muscle regions were always active throughout the whole range of bite-force directions. This was a consistent finding in all subjects. It implies that the activity of a muscle part could not be fully comprehended from the direction of its action line. For example, the posterior-most region of the muscle (region 6) was expected to contribute to the production of bite force in the lateral and posterior directions. However, the muscle region was also active in antagonistic, medial and anterior, directions. We do not have an explanation or hypothesis to explain why regions 1 and 5 were similar with respect to greater activation on medial-force production. Second, in each bite-force direction, an increase in biteforce magnitude went along with an increase in EMG activity. For a particular muscle region, the curves obtained for various bite-force levels had basically the same shape. This implies that the ratio of muscle activity in various bite-force directions was constant, regardless of bite-force magnitude. The biteforce/EMG relationship was, broadly speaking, linear, although this was not always true for all muscle regions in all directions. As an example, Fig. 2 shows (for various bite-force directions) the bite-force/EMG relationship of the anterior-most (region 1) and posterior-most (region 6) regions of the muscle. For the anterior-most region, the relationship was not ob-
, LAT.
signal.
In the experiment where several bite-force levels were examined, EMG activity for each muscle region was expressed as a fraction of the maximal value found in that subject during the complete experiment. In the experiment where bite-force magnitude was kept constant (150 N), for each subject, muscle region, and bite-force direction, the mean + SD EMG was calculated over the repeated measurements (n = 6). The SD values were used as a measure of intra-individual variability for a particular electrode pair; the nine (i.e., number of biteforce directions) mean values were scaled to the maximum
value found. These scaled values were used in an analysis of variance to test influence of bite-force direction and muscle region on EMG activity. Furthermore, for each muscle region and bite-force direction, mean and SD over the five subjects were calculated.
-
subject E
POST.
PAST.
rection. -For visualization of the influence of bite-force direction on EMG activity, polar plots have been constructed, where the bite-force direction is depicted in the angular direction, and the scaled EMG activity along the radial direction. For each of the six anteroposterior regions of the temporalis muscle and for four directions (except the vertical one), such a plot is shown in Fig. 1. Points of the same bite force have been connected (inner contour line, 50 N; outer contour line,
PO
Fig. 1-EMG polar plots for the six anteroposterior regions of the temporalis muscle (region 1: anterior-most. . . . region 6: posterior-most). Scaled EMG activity is in the radial direction (ring represents 100% activity), and the direction of the bite force is in the angular direction. Each contour represents a single bite-force magnitude (four contour lines-50 N, 150 N, 250 N, 350 N; inner contour line-50 N . .. ; outer contour line-350 N). The dots represent EMG activity for maximal bite force in each direction. Ant., anterior bite-force direction; lat., lateral; post., posterior; med., medial. anterior-most region
EMG (%)
,l medial
100
-
ventical Von"
tera
so
Results. Bite-forcelEMG relationship as a function of bite-force di-
LAT.
posterior-most region
EMG (%)
100
. 80
60 40
20
L 50
150
250
350
450
550 650 Bite Force (N)
50
150
250
350
450 550 1650 Be Force (N)
Fig. 2-EMG activity of the anterior-most and posterior-most muscle regions as a function of bite-force magnitude for five bite-force directions (Subject: E).
Downloaded from jdr.sagepub.com at UCSF LIBRARY & CKM on April 9, 2015 For personal use only. No other uses without permission.
1688
J Dent Res October 1990
BLANKSMA & VAN EIJDEN
viously affected by bite-force direction, whereas this was clearly true for the posterior-most region. Hence, the influence of biteforce direction on muscle activity differed between these two muscle regions. Bite-force direction/EMG relationship.-When the relative EMG activity between the six muscle regions in bites of constant magnitude (150 N) in nine different directions were compared in the five subjects, the analysis of variance demonstrated that the EMG activity depended significantly on the muscle region (p
This study examined the possible existence of regional differences in activation of the temporalis muscle. Since the muscle is fan-shaped, its fibers pull in different directions. This suggests regional specialization for different motor tasks. EMG activity was registered by six bipolar fine-wire electrodes inserted anteroposteriorly across the muscle belly. Muscle signals were recorded during different static bite tasks for which both the direction and magnitude of bite force were specified. The results showed that the ratio of activities of the six muscle regions changed with the direction of bite force. This indicates a partitioning of the excitatory command to the muscle's motoneuron pool. Alteration in activity with changing bite-force direction was generally the smallest in the anterior-most region of the muscle, the largest in the posterior-most region, and the intermediate in the interjacent regions. Generally, all muscle regions exhibited the highest EMG activity when the bite force was in an approximately posterolateral direction. The muscle was activated uniformly only for bites in this preferential direction. Activity in the regions appeared to be scaled up or down in a linear way according to the desired bite-force level. The results indicate that the direction of pull of the muscle and the maximal force it can produce are not fixed, but depend on the direction of bite force. J Dent Res 69(10):1686-1690, October, 1990
Introduction. To obtain insight into the role a muscle plays in executing a particular motor task, knowledge is required about the way it is activated. There is increasing evidence that muscles are not always activated homogeneously, i.e., all motoneurons in a muscle's motoneuron pool receive the same input, but that selective activation of motoneurons is possible (Ter Haar Romeny et al., 1982; Hoffer et al., 1987; Van Zuylen et al., 1988; Jongen et al., 1989). These studies indicate that a motoneuron pool of a particular muscle may consist of subpopulations of motoneurons that can be activated more or less independently to perform different tasks. The levator muscles of the human jaw have a complex architecture with relatively large attachment areas. For this reason, each muscle would have a large variety of possible action lines in case of a selective activation of muscle parts. Consequently, the range of bite-force directions in which a jaw muscle might participate to produce bite force would increase (Van Eijden et al., 1988a). Traditionally, the muscles have been divided very crudely. For example, the temporalis muscle is usually partitioned into an anterior and a posterior part. Since their action lines differ in orientation, these two parts have the potential to behave differentially in performing various motor tasks. EMG studies have indicated that this is indeed true (Vitti and Basmajian, 1977; Van Eijden et al., 1990). The possible existence of smaller parts that differ in activation has been Received for publication February 22, 1990 Accepted for publication June 11, 1990
1686
suggested by Eriksson and Thornell (1983), who demonstrated regional differences in histochemical fiber-type composition. This points to differences in motor unit behavior. Since the temporalis muscle is fan-shaped, different fibers pull in different directions. This suggests that with, for example, a change in bite-force direction, particular regions may be activated preferentially. The aim of the present study was to examine the possible existence of regional differences in activation of subpopulations of fibers in the temporalis muscle.
Materials and methods. Subjects. -The study was performed on five male subjects, ranging in age from 27 to 39 years. They had normal Class I occlusions and showed no symptoms or signs of craniomandibular dysfunction. One of the subjects was missing a lower left premolar. The experiment was approved by the medical ethical committee of the Academic Hospital. Generation of bite force of a specific direction and magnitude. -A detailed description of the procedure is given by Van Eijden et al. (1988b). Briefly, both the direction and magnitude of bite force were registered by a three-component force transducer (dimensions, 24 x 24 x 10 mm, length x width x height), positioned between a pair of occlusal clutches; the inter-incisal distance was 18 ± 5 mm (mean ± SD of the five subjects). For feedback purposes, both the magnitude and direction of the resultant force applied to the transducer and the desired bite-force magnitude and direction were visualized on the screen of a computer. The magnitude of the exerted force was represented by a vertical bar of varying height, and the desired force level by a horizontal bar. The actual force direction was shown by a cross situated in a two-dimensional polar coordinate system. The desired direction, with a range of 50, was represented by a circle. Subjects could be instructed to produce a bite force of a particular direction and magnitude by matching the cross with the circle and the top of the vertical bar with the horizontal bar. In the present study, the center of the transducer coincided with the center of the second right upper premolar. Two experiments were carried out. In the first experiment, bite-force magnitude was kept constant at a level of 150 N. Forces were exerted in nine different directions, defined relative to the vertical z-axis (i.e., perpendicular to the occlusal plane of the upper teeth) and the anteroposterior x-axis, which was parallel to both the occlusal and mid-sagittal planes. The following directions were examined: vertical, anterior (z-angle, 20°; x-angle, 00), anterolateral (200, 450), lateral (20°, 90°), posterolateral (200, 1350), posterior (20°, 1800), posteromedial (200, 225°), medial (200, 2700), and anteromedial (200, 3150). The sequence of the nine bites was chosen randomly; between the trials, a rest period of at least two min was inserted so that fatigue effects would be avoided. The experiment was repeated five times, each time with another randomized order of bite-force directions. Hence, the total number of bites in each subject was 54 (nine directions x six trials). In the second experiment, in which three of the five subjects
Downloaded from jdr.sagepub.com at UCSF LIBRARY & CKM on April 9, 2015 For personal use only. No other uses without permission.
Vol. 69 No. 10
HETEROGENEITY IN THE TEMPORALIS MUSCLE
participated, only five bite-force directions were examined, viz., vertical, anterior, lateral, posterior, and medial. In each direction, bites of 50, 150, 250, 350 N, and maximal voluntary force were produced. EMG registration. -Six bipolar fine-wire electrodes were used for registration of the EMG activity in different anteroposterior regions of the right temporalis muscle. Each electrode pair consisted of two Teflon-insulated stainless-steel wires (diameter, 0.12 mm) with their tips bared (1 mm) and bent into a hook (Basmajian and Stecko, 1962); the separation between the bare ends of the two wires was approximately 2 mm. They were inserted with the aid of a 0.6-x-25-mm disposable needle. The electrodes were on a line parallel to the zygomatic arch at a level 1.5 cm above the lateral corner of the eye. The anterior- and posterior-most electrode pairs were at a distance of 1.5 cm from, respectively, the anterior and posterior borders of the muscle, identified by palpation; the electrode pairs were at an equal distance (1.7 + 0.2 cm, mean ± SD of the five subjects) from each other. A ground electrode was attached to the skin over the seventh cervical vertebra. The EMG apparatus consisted of differential amplifiers. Frequency bandwidth was from 200 to 1000 Hz; amplification ranged from 1000 to
10,000.
Data processing.-For each bite, the outputs of the force transducer (three signals, one for each force component) and of the EMG apparatus (six muscle signals) were simultaneously recorded on FM analogue tape. The analogue data were digitized (2400 samples/s/channel) with a DEC 11/73 minicomputer system. From each trial, a computer program automatically selected a continuous period of 0.5 s. Out of all possible 0.5-second periods, the one was selected where the resultant force was within 50 of the specified bite-force direction, and the mean-squared difference between the exerted and desired force level was the smallest (this difference was usually smaller than 5 N); in trials where maximal bite forces were exerted, the 0.5-second period with the largest mean bite force was chosen. The EMG activity of the six muscle regions in this period was quantified as the mean value of the rectified
1687
350 N). The dots represent EMG activity for the maximal bite force in the four directions. The following points deserve special attention.
First, all muscle regions were always active throughout the whole range of bite-force directions. This was a consistent finding in all subjects. It implies that the activity of a muscle part could not be fully comprehended from the direction of its action line. For example, the posterior-most region of the muscle (region 6) was expected to contribute to the production of bite force in the lateral and posterior directions. However, the muscle region was also active in antagonistic, medial and anterior, directions. We do not have an explanation or hypothesis to explain why regions 1 and 5 were similar with respect to greater activation on medial-force production. Second, in each bite-force direction, an increase in biteforce magnitude went along with an increase in EMG activity. For a particular muscle region, the curves obtained for various bite-force levels had basically the same shape. This implies that the ratio of muscle activity in various bite-force directions was constant, regardless of bite-force magnitude. The biteforce/EMG relationship was, broadly speaking, linear, although this was not always true for all muscle regions in all directions. As an example, Fig. 2 shows (for various bite-force directions) the bite-force/EMG relationship of the anterior-most (region 1) and posterior-most (region 6) regions of the muscle. For the anterior-most region, the relationship was not ob-
, LAT.
signal.
In the experiment where several bite-force levels were examined, EMG activity for each muscle region was expressed as a fraction of the maximal value found in that subject during the complete experiment. In the experiment where bite-force magnitude was kept constant (150 N), for each subject, muscle region, and bite-force direction, the mean + SD EMG was calculated over the repeated measurements (n = 6). The SD values were used as a measure of intra-individual variability for a particular electrode pair; the nine (i.e., number of biteforce directions) mean values were scaled to the maximum
value found. These scaled values were used in an analysis of variance to test influence of bite-force direction and muscle region on EMG activity. Furthermore, for each muscle region and bite-force direction, mean and SD over the five subjects were calculated.
-
subject E
POST.
PAST.
rection. -For visualization of the influence of bite-force direction on EMG activity, polar plots have been constructed, where the bite-force direction is depicted in the angular direction, and the scaled EMG activity along the radial direction. For each of the six anteroposterior regions of the temporalis muscle and for four directions (except the vertical one), such a plot is shown in Fig. 1. Points of the same bite force have been connected (inner contour line, 50 N; outer contour line,
PO
Fig. 1-EMG polar plots for the six anteroposterior regions of the temporalis muscle (region 1: anterior-most. . . . region 6: posterior-most). Scaled EMG activity is in the radial direction (ring represents 100% activity), and the direction of the bite force is in the angular direction. Each contour represents a single bite-force magnitude (four contour lines-50 N, 150 N, 250 N, 350 N; inner contour line-50 N . .. ; outer contour line-350 N). The dots represent EMG activity for maximal bite force in each direction. Ant., anterior bite-force direction; lat., lateral; post., posterior; med., medial. anterior-most region
EMG (%)
,l medial
100
-
ventical Von"
tera
so
Results. Bite-forcelEMG relationship as a function of bite-force di-
LAT.
posterior-most region
EMG (%)
100
. 80
60 40
20
L 50
150
250
350
450
550 650 Bite Force (N)
50
150
250
350
450 550 1650 Be Force (N)
Fig. 2-EMG activity of the anterior-most and posterior-most muscle regions as a function of bite-force magnitude for five bite-force directions (Subject: E).
Downloaded from jdr.sagepub.com at UCSF LIBRARY & CKM on April 9, 2015 For personal use only. No other uses without permission.
1688
J Dent Res October 1990
BLANKSMA & VAN EIJDEN
viously affected by bite-force direction, whereas this was clearly true for the posterior-most region. Hence, the influence of biteforce direction on muscle activity differed between these two muscle regions. Bite-force direction/EMG relationship.-When the relative EMG activity between the six muscle regions in bites of constant magnitude (150 N) in nine different directions were compared in the five subjects, the analysis of variance demonstrated that the EMG activity depended significantly on the muscle region (p
