A&s oral Bid. Vol. 36, No. I,pp. 54>541, 1991 Printed in Great Britain. All rights reserved
0003~9969/91 $3.00+ 0.00 Copyright 0 1991Pergsmon Press plc
SHORT COMMUNICATION THE PRECISION OF MOTOR CONTROL IN HUMAN JAW AND LIMB MUSCLES DURING ISOMETRIC CONTRACTION IN THE PRESENCE OF VISUAL FEEDBACK DANIEL VAN STEENBERGHE,BWNARD EGNTE, HILDE SCHOL.S,RJZINHILDEJACOBS and AN SCHOTTE Department of Periodontology, Laboratory of Oral Physiology, Faculty of Medicine, Catholic University of Leuven, Capucijnenvoer 7, B-3000 Leuven, Belgium (Received
6 July 1990; accepted 29 January
1991)
Summary-Experiments showed that the human capacity to maintain a particular isometric force in the presence of visual feedback during a force-level, pursuit-tracking experiment is less developed for the jaw-closing muscles than for the limb muscles. This finding may indicate that the projection on the trigeminal motoneurone pool from visual inputs is poor, or that the trigeminal effector system itself is less finely tuned. Key words: motor control, pursuit-tracking, jaw-muscles, mastication.
(b) Jaw clenching. Subjects bit isometrically on the force transducer with the incisor teeth. The temporal and masseter muscles contributed to this force production. (c) Pinching. The force transducer was pinched between the thumb and index finger of the preferred hand, according to the subject’s handedness. The hands lay in the lap. The flexor pollicis longis and brevis, the opponens pollicis and the flexor digitorum superhcialis contributed to this force. (d) Plantar extension. The foot was stretched on the force transducer as if in operation of a car accelerator. The soleus and gastrocnemicus produced the force. Although most individuals tend to be right-handed and -footed (porac and Coren 1982), in this experiment the left foot was always used because car driving or using the foot control of a dental micromotor might result in a better right- than left-footedness. Thus using the left foot should lead to less skilful responses while selection of preferred hand enhances the assessment of the skill.
Grasping or maintaining an object in a particular position implies the development of a precise isometric force. We have compared the capacity to maintain a precise isometric contraction force in jaw-closing muscles and limb muscle groups. Nine healthy adult subjects (6 men; maximum age, 40 yr, mean, 23 yr) with no known neurological symptoms (comfortably seated with their hands in the laps in a quiet room), were asked to concentrate on
an oscilloscope screen on which a target force level was presented. All subjects were students in the final year of dental training, which would imply that they had well-developed manual dexterity. They were instructed to react to the target-force trajectory as quickly and accurately as possible. This signal, produced by a function generator, was presented as a step wave with a variable duration of around 3 s. The chosen amplitude corresponded to a force level approximately between 15 and 20% of the individually determined maximal voluntary contraction. The subject had to generate this actual force on a strain gauge with a deep groove, which allowed force changes to be studied under near isometric conditions. The force produced was presented on the same oscilloscope screen to provide visual feedback. Four different muscle groups were tested, as illustrated in Fig. 1: (a) Cutting. The bent index and the middle finger were approximated on the force transducer. The subject used whichever hand he preferred according to his handedness, with the hands in the lap. Interosseus muscles contributed predominantly to this force production. AOB 36,7-E
The task was to mimic the generated square waves by the generation of a force ramp (the dynamic phase) followed by the maintenance of a stable force (the static phase). Two display modes were used for the target-force square waves. In the A-mode a slow time base was used so that the target force was displayed as a line that deflected (square wave) abruptly up and down for 3 s each (see Fig. 1). In the B-mode a fast display was used so that only a horizontal line crossed the entire screen, moving up and down every 3 s. 545
DANIEL VAN STEENBERGHEet
546
Fig. 1. Five trials (-) on the strain gauge are superimposed for all different tasks in one and the same subject (No. 9): cutting in trace A, jaw clenching in trace B, pinching in trace C and plantar extension in trace D. T’he force produced on the transducer is indicated in Newtons. The time scale is in seconds. At time 0 ms a square wave (----) was generated and presented to the subject on
an oscilloscope.
At the beginning of the experiment we allowed the subjects to become familiar with the oscilloscope and the force transducer. To avoid loss of concentration, the subjects could relax between trials and after the pre-experimental testing. The actual experiments involved four trials, each consisting of 10 square-wave presentations on the Aand B-mode. The testing sequence of the muscle groups was randomized to avoid a learning bias (Poulton, 1974). Data were collected on a magnetic tape of an Ampex PR 2230 recorder and A/D converted for analysis on an IBM-AT computer.
al.
The reaction time was defined as the delay between the initial appearance of the slope of the step-wave and the initiation of the force development. The r.m.s. values of the differences between the actual forces generated and the reference step-wave were also determined. The r.m.s. gives more weight to important deviations between the target and actual forces than absolute error calculation. A larger r.m.s. value implies a greater deviation from the target force and a less precise motor control. Statistical analysis was based on the Student’s paired t-test on the means of the four trials. Measured reaction times always exceeded the minimal delay of 200 ms described in humans (Poulton, 1981). The mean reaction times for different muscle groups were 335 ms (271 ms) for cutting in mode A (B), 336 ms (305 ms) for jaw clenching in mode A (B), 314 ms (273 ms) for pinching in mode A (B) and 343 ms (326 ms) for plantar extension in mode A (B). The reaction for plantar extension in the B-mode of tracking was significantly longer (p < 0.1) than for jaw clenching, pinching and cutting in 6 out of 9 subjects. In the A-mode of visual feedback the difference in reaction time, although still present, was often not significant. The increased delay of onset of force in the lower limb was logical because of the longer motor conduction time. While the delay was also longer for hand muscles than for jaw muscles, this seemed to be compensated for by either the existence of a more direct neuronal circuit between the visual imput and hand muscle motor nuclei or by a more efficient motor programme in the hand. The r.m.s. values (see Table 1) of jaw clenching were systematically larger than for pinching, cutting or plantar extension (p < 0.05) indicating a less precise motor-control skill in jaw clenching. In 2 out of 9 subjects the difference did not reach the level of significance for the A-mode of step-wave presentation. The same applied to subject 6 in the B-mode. In most subjects, especially for the B-mode presentation, the level of significance reached a p-value below 0.01. The findings are in accord with our hypothesis that the jaw movements cannot reach the same level of skill as limb movements when visual feedback is provided. The deliberate use of the left foot, which is less used for precise force development, still gave a smaller error than the jaw. The lower jaw only performed better, for the reaction time, than the left foot, which seems normal considering the very different neuronal conduction distances. The importance of visual feedback in the control of low isometric forces during prehensile grasping in normal humans has been documented (Mai er al., 1985). Cerebellar lesions impair the accurate control of force and of preprogrammed ballistic movements (Allen and Tsukahara, 1974). The same seems to apply to isometric force developments using the finger muscles in a grasping task (Mai et al., 1988). Mai et al. also documented the role of visual feedback in man. The role of visual input on jaw-muscle motor control has received little attention and remains unknown (Sessle, 1977). The role of auditory feedback on jaw coordination, such as during speech production, has been reviewed by Glencross (1977). The unique anatomical situation of the mandible,
Precision of motor control in human jaw and limb muscles
541
Table 1. p-Values of differences of r.m.s. values between different modes of force development Mode of target force presentation Subject No.
m-cl
cu-cl
tic1
ni-cu
ni-nl
CU-D1
r0.5
0003~9969/91 $3.00+ 0.00 Copyright 0 1991Pergsmon Press plc
SHORT COMMUNICATION THE PRECISION OF MOTOR CONTROL IN HUMAN JAW AND LIMB MUSCLES DURING ISOMETRIC CONTRACTION IN THE PRESENCE OF VISUAL FEEDBACK DANIEL VAN STEENBERGHE,BWNARD EGNTE, HILDE SCHOL.S,RJZINHILDEJACOBS and AN SCHOTTE Department of Periodontology, Laboratory of Oral Physiology, Faculty of Medicine, Catholic University of Leuven, Capucijnenvoer 7, B-3000 Leuven, Belgium (Received
6 July 1990; accepted 29 January
1991)
Summary-Experiments showed that the human capacity to maintain a particular isometric force in the presence of visual feedback during a force-level, pursuit-tracking experiment is less developed for the jaw-closing muscles than for the limb muscles. This finding may indicate that the projection on the trigeminal motoneurone pool from visual inputs is poor, or that the trigeminal effector system itself is less finely tuned. Key words: motor control, pursuit-tracking, jaw-muscles, mastication.
(b) Jaw clenching. Subjects bit isometrically on the force transducer with the incisor teeth. The temporal and masseter muscles contributed to this force production. (c) Pinching. The force transducer was pinched between the thumb and index finger of the preferred hand, according to the subject’s handedness. The hands lay in the lap. The flexor pollicis longis and brevis, the opponens pollicis and the flexor digitorum superhcialis contributed to this force. (d) Plantar extension. The foot was stretched on the force transducer as if in operation of a car accelerator. The soleus and gastrocnemicus produced the force. Although most individuals tend to be right-handed and -footed (porac and Coren 1982), in this experiment the left foot was always used because car driving or using the foot control of a dental micromotor might result in a better right- than left-footedness. Thus using the left foot should lead to less skilful responses while selection of preferred hand enhances the assessment of the skill.
Grasping or maintaining an object in a particular position implies the development of a precise isometric force. We have compared the capacity to maintain a precise isometric contraction force in jaw-closing muscles and limb muscle groups. Nine healthy adult subjects (6 men; maximum age, 40 yr, mean, 23 yr) with no known neurological symptoms (comfortably seated with their hands in the laps in a quiet room), were asked to concentrate on
an oscilloscope screen on which a target force level was presented. All subjects were students in the final year of dental training, which would imply that they had well-developed manual dexterity. They were instructed to react to the target-force trajectory as quickly and accurately as possible. This signal, produced by a function generator, was presented as a step wave with a variable duration of around 3 s. The chosen amplitude corresponded to a force level approximately between 15 and 20% of the individually determined maximal voluntary contraction. The subject had to generate this actual force on a strain gauge with a deep groove, which allowed force changes to be studied under near isometric conditions. The force produced was presented on the same oscilloscope screen to provide visual feedback. Four different muscle groups were tested, as illustrated in Fig. 1: (a) Cutting. The bent index and the middle finger were approximated on the force transducer. The subject used whichever hand he preferred according to his handedness, with the hands in the lap. Interosseus muscles contributed predominantly to this force production. AOB 36,7-E
The task was to mimic the generated square waves by the generation of a force ramp (the dynamic phase) followed by the maintenance of a stable force (the static phase). Two display modes were used for the target-force square waves. In the A-mode a slow time base was used so that the target force was displayed as a line that deflected (square wave) abruptly up and down for 3 s each (see Fig. 1). In the B-mode a fast display was used so that only a horizontal line crossed the entire screen, moving up and down every 3 s. 545
DANIEL VAN STEENBERGHEet
546
Fig. 1. Five trials (-) on the strain gauge are superimposed for all different tasks in one and the same subject (No. 9): cutting in trace A, jaw clenching in trace B, pinching in trace C and plantar extension in trace D. T’he force produced on the transducer is indicated in Newtons. The time scale is in seconds. At time 0 ms a square wave (----) was generated and presented to the subject on
an oscilloscope.
At the beginning of the experiment we allowed the subjects to become familiar with the oscilloscope and the force transducer. To avoid loss of concentration, the subjects could relax between trials and after the pre-experimental testing. The actual experiments involved four trials, each consisting of 10 square-wave presentations on the Aand B-mode. The testing sequence of the muscle groups was randomized to avoid a learning bias (Poulton, 1974). Data were collected on a magnetic tape of an Ampex PR 2230 recorder and A/D converted for analysis on an IBM-AT computer.
al.
The reaction time was defined as the delay between the initial appearance of the slope of the step-wave and the initiation of the force development. The r.m.s. values of the differences between the actual forces generated and the reference step-wave were also determined. The r.m.s. gives more weight to important deviations between the target and actual forces than absolute error calculation. A larger r.m.s. value implies a greater deviation from the target force and a less precise motor control. Statistical analysis was based on the Student’s paired t-test on the means of the four trials. Measured reaction times always exceeded the minimal delay of 200 ms described in humans (Poulton, 1981). The mean reaction times for different muscle groups were 335 ms (271 ms) for cutting in mode A (B), 336 ms (305 ms) for jaw clenching in mode A (B), 314 ms (273 ms) for pinching in mode A (B) and 343 ms (326 ms) for plantar extension in mode A (B). The reaction for plantar extension in the B-mode of tracking was significantly longer (p < 0.1) than for jaw clenching, pinching and cutting in 6 out of 9 subjects. In the A-mode of visual feedback the difference in reaction time, although still present, was often not significant. The increased delay of onset of force in the lower limb was logical because of the longer motor conduction time. While the delay was also longer for hand muscles than for jaw muscles, this seemed to be compensated for by either the existence of a more direct neuronal circuit between the visual imput and hand muscle motor nuclei or by a more efficient motor programme in the hand. The r.m.s. values (see Table 1) of jaw clenching were systematically larger than for pinching, cutting or plantar extension (p < 0.05) indicating a less precise motor-control skill in jaw clenching. In 2 out of 9 subjects the difference did not reach the level of significance for the A-mode of step-wave presentation. The same applied to subject 6 in the B-mode. In most subjects, especially for the B-mode presentation, the level of significance reached a p-value below 0.01. The findings are in accord with our hypothesis that the jaw movements cannot reach the same level of skill as limb movements when visual feedback is provided. The deliberate use of the left foot, which is less used for precise force development, still gave a smaller error than the jaw. The lower jaw only performed better, for the reaction time, than the left foot, which seems normal considering the very different neuronal conduction distances. The importance of visual feedback in the control of low isometric forces during prehensile grasping in normal humans has been documented (Mai er al., 1985). Cerebellar lesions impair the accurate control of force and of preprogrammed ballistic movements (Allen and Tsukahara, 1974). The same seems to apply to isometric force developments using the finger muscles in a grasping task (Mai et al., 1988). Mai et al. also documented the role of visual feedback in man. The role of visual input on jaw-muscle motor control has received little attention and remains unknown (Sessle, 1977). The role of auditory feedback on jaw coordination, such as during speech production, has been reviewed by Glencross (1977). The unique anatomical situation of the mandible,
Precision of motor control in human jaw and limb muscles
541
Table 1. p-Values of differences of r.m.s. values between different modes of force development Mode of target force presentation Subject No.
m-cl
cu-cl
tic1
ni-cu
ni-nl
CU-D1
r0.5
