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Discipline of Physiology, School of Molecular and Biomedical Science, The University of Adelaide, Adelaide, SA 5005, Australia
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Abstract |
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(Received 16 July 2003;
accepted after revision 27 October 2003;
first published online 31 October 2003)
Corresponding author T. S. Miles: Discipline of Physiology, School of Molecular and Biomedical Science, The University of Adelaide, Adelaide, SA 5005, Australia. Email: timothy.miles{at}adelaide.edu.au
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Introduction |
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This raises the question of whether the mandible is supported actively during brisk vertical head movements. It has recently been shown that the vertical position of the mandible relative to the maxilla moves up and down within a small range during locomotion (Shiller et al. 2001; Flavel et al. 2003). Common experience indicates that the teeth do not come into contact with each step even during vigorous running, which suggests that there is some active (i.e. reflex) control of mandibular vertical position when the head moves up and down in locomotion.
The jaw-closing muscles are known to have sensitive short- and long-latency stretch reflexes (Poliakov & Miles, 1994); hence we hypothesized that stretch of the jaw-closing muscles during locomotion induces reflex responses in these muscles which tend to restore the mandible towards its rest position when it is displaced during head movements.
In the present study, the vertical position of the mandible was monitored at rest and during various forms of locomotion, and was correlated with the activity in its main antigravity muscle, the masseter. The kinematic data obtained during a subset of the experiments in this study are reported in detail elsewhere (Flavel et al. 2003).
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Methods |
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Movements of the head and the mandible were measured using a method developed in this laboratory (Flavel et al. 2002). Briefly, small accelerometers were glued to the upper and lower incisor teeth with glass ionomer dental cement (GC Fuji IX GP, GC Corporation, Tokyo, Japan). The outputs of these matched accelerometers were recorded separately, band-pass filtered (DC to 500 Hz), and summed electronically in a differential amplifier. The difference between their outputs gave acceleration of the mandible relative to the head. A small Hall-effect device was also incorporated into the system to give a record of the separation between the upper and lower teeth. Mandibular velocity relative to the head was obtained by integrating the differential acceleration signal.
A separate accelerometer taped to the right ankle indicated foot-strike during locomotion.
The EMG of the right masseter muscle was recorded with surface electrodes placed along the long axis of the muscle. EMG signals were filtered (sixth-order Butterworth, bandwidth 50–500 Hz), and all signals were recorded continuously on digital tape during the various manoeuvres described below.
The latency of the monosynaptic reflex response to stretch of each subject's jaw-closing muscles was determined by tapping briskly with a tendon hammer on a finger placed on the chin. An accelerometer in the tendon hammer indicated the time of impact.
Hopping
In the initial experiments on hopping, head acceleration was measured with an accelerometer attached to tightly fitting welding goggles whose frame was moulded to the face and which was held in place by a head strap, rather than with the accelerometer mounted on an upper incisor tooth. Eight healthy male subjects hopped barefoot on one leg on a force plate. Each run consisted of 100 hops at one hop per 2 s. After every 25 hops, the subjects rested for about 2 min, then changed to hopping on the other leg to reduce fatigue. The hopping height was approximately 100 mm, and subjects held their head erect and maintained a forward gaze. In different runs, subjects deliberately landed on either the toe or the heel of one foot.
In a separate series of trials, subjects kept their teeth firmly clenched together to prevent jaw movement while they hopped a further 100 times and landed on their heels. This clench gave a masseter EMG level approximately 10% of that produced in a maximal voluntary contraction.
Locomotion
In the second series of experiments on a different group of eight male and four female subjects, the EMG and kinematic variables were recorded while subjects stood, walked and ran barefoot on a motorized treadmill (Tetley Heartmaster Mk 2, Tetley Electronics, Sydney). After some practice, each subject walked or ran forwards for 4 min while the treadmill was activated at speeds of 0.7, 1.4, 2.1 and 2.8 m s-1 (2.5, 5. 7.5 and 10 km h-1). Subjects rested for at least 2 min between every run. The subjects ran again at the same speeds when the inclination of the treadmill was changed to a 5 deg negative slope (‘downhill’) and then again with it set to a 5 deg positive inclination (‘uphill’). Subjects were instructed to keep their head erect and to gaze forward at a target placed at eye level at 2 m distance, and to maintain a comfortable, relaxed facial and jaw posture. They were constantly monitored and given verbal feedback to assist with this.
The data were analysed off-line with a computer-based laboratory interface (1401plus, CED Ltd, Cambridge, UK) which sampled all signals at 5 kHz. Whenever possible, the downward acceleration of the mandible relative to the maxilla was used to trigger an ensemble average of the rectified and unrectified EMG signals from the jaw-closing muscles, and the various kinematic variables during locomotion. However, in the paradigms that caused only very small mandibular movements (walking, toe-landing when hopping, and heel-landing with the teeth clenched), the averager was triggered with the foot-strike signal from the force-plate.
ANOVA was used to assess the effect of treadmill speed on the size of masseter EMG reflex responses and peak downwards displacement of the mandible following landing. Scheffe's test was used for post hoc comparisons, where indicated. For all comparisons, significance was reported for P < 0.05.
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Results |
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Hopping
Hopping and landing on the toes of one leg was a familiar and comfortable task for all subjects. However, landing on the heels was unfamiliar, and uncomfortable because it resulted in a strong jolting of the head. The ground reaction forces and the transmission of these to the skull in the three hopping tasks are shown for a representative subject in Fig. 1. During toe-landing (left column), the footplate record showed an initial brisk deceleration upon initial toe contact, followed by a slow phase of deceleration as the impact of landing was absorbed by the muscles of the leg. Hence, much of the shock of landing was absorbed before it reached the skull. About 10 ms after the initial heel impact, the mandible also began to decelerate. This small, slow, vertical displacement did not evoke a reflex response in the masseter in any subject.
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As the subject landed, the mandible initially moved slightly upwards relative to the head as the head tilted forward. However, its inertia then caused it to move about 0.3 mm downwards relative to the maxilla, increasing the separation between the teeth and stretching the jaw-closing muscles. This stimulus evoked a brisk reflex excitation of the masseter muscle at a latency of 7.5 ms after the first detectable relative acceleration of the mandible with respect to the maxilla. The masseter activity was followed by a jaw-closing movement which began about 12 ms after the onset of the EMG response, peaking at 50 ms.
When the same subject now repeated the heel-landing hops with his teeth firmly clenched together (Fig. 1, right column), the maxilla acceleration was similar to the situation when the jaw muscles were relaxed, but the movement of the mandible relative to the maxilla was negligible after landing. The reflex burst of activity in the masseter was abolished, indicating that it was evoked by movement of the mandible and not the head. The reflex is therefore not due to activation of vestibular afferents.
There was no evidence for ‘anticipatory’ muscle activity before landing in any subject, and the pattern of responses seen was very similar in all subjects.
The mean latency of the reflex measured from the first detectable relative difference in mandibular acceleration with respect to the maxilla during heel-landing was 7.7 ± 0.8 ms (mean ±S.E.M.; n= 8), which was slightly but significantly longer than the 7.4 ± 0.5 ms latency of the ‘jaw-jerk’ reflex evoked in these subjects by tapping on the chin (paired t test; P < 0.02).
Locomotion
In subjects standing quietly, the mandible remained in a stable vertical position with respect to the maxilla, with the incisor teeth separated by 2–6 mm, and no tonic activity was detected by visual inspection of the raw EMG in the masseter muscle in any subject.
While walking, all subjects touched down first on their heel, after which the sole of the foot came down to contact the treadmill before the take-off into the swing phase of the next step. The raw data recorded in one representative subject while walking at two speeds and running at two speeds on a horizontal treadmill is shown in Fig. 2. The uppermost trace shows the record from the accelerometer taped to the lateral malleolus of one ankle. Below this are the records of the mandibular acceleration (derived from the difference in acceleration between the maxilla and the mandible), mandibular position relative to the maxilla, and the unrectified EMG signal from one masseter muscle.
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During running at both speeds, both the ankle acceleration and the mandibular acceleration following each heel-strike were much larger, as was the vertical displacement of the mandible. The mandible accelerated rapidly downwards about 10 ms after heel-strike (only the alternate heel strikes were registered) and then upwards, and these jaw movements were synchronized with bursts of masseter EMG activity.
The events that occurred following foot-strike are more clearly seen in the averaged records in Fig. 3, which show the various kinematic parameters of the mandibular movement relative to the maxilla, together with the rectified masseter EMG in another subject during walking and running on surfaces with different slopes. Note first that there was little if any tonic activity in the masseter that would support the mandible against gravity under any of the conditions tested.
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The pattern of responses during running was more complex, and depended on the speed of running and whether the treadmill was inclined upwards or downwards. As in walking, all subjects landed first on their heels when running on a horizontal or downwards-inclined surface, but on their toes when running on the upwardly inclined surface.
When this subject ran at a moderate pace on a level treadmill, the downward acceleration of the mandible after landing was markedly higher, and the mandible moved about 0.4 mm downwards from its original position. The downward movement then slowed and was followed by a brisk upward movement in which the mandible slightly overshot its original vertical position. This pattern of movement was greater when the subject ran at the same speed on a downwards-sloping treadmill and less when it was inclined upwards. The brisker downward movements during running evoked a burst of activity in the masseter muscles at a latency of about 8 ms (the precise time at which the acceleration began could be estimated only to the nearest millisecond, and the averaging procedure tended to smear the onset time). The amplitude of the EMG response varied with the amplitudes of the mandibular acceleration and position. There was no evidence for inhibitory or longer-latency excitatory reflex responses.
The excitation of the masseter was followed by an upward movement of the mandible to overshoot its prelanding position. The mean delay ±S.D. between the onset of the EMG response and the onset of the upward movement of the mandible was 32 ± 7 ms, and the mean onset latency of the reflex measured from the initial downward acceleration of the mandible relative to the skull was 7.3 ± 0.4 ms for all subjects.
Running at different speeds and on different inclinations of the treadmill led to different patterns of acceleration, velocity and displacement (equivalent to stretch of the jaw-closing muscles) of the head and the mandible. These were clearly the result of the different patterns of gait and forces of landing under these different conditions. For example, subjects running downhill landed on their heels, and therefore landed more forcefully (they also fell a little further). This resulted in increased acceleration of the head and hence relative acceleration of the mandible, compared with running on a level surface. On the other hand, when subjects ran ‘uphill’, they were compelled to land on their toes, and also landed less forcefully (having fallen a smaller distance): this reduced the forces acting on the head and jaw.
The masseter EMG and jaw displacement data from all 12 subjects are summarized in Fig. 4. Walking at the two lowest treadmill speeds produced small amounts of jaw opening and no significant reflex response in the masseter. Running at the two highest treadmill speeds resulted in larger displacements of the mandible, and these were greater for level and downhill slopes than for the uphill slope (Scheffe's test, P < 0.01). The peak jaw displacement was similar for the two fastest treadmill speeds. Reflex activation of the masseter was seen after landing when running at 2.1 m s-1 and 2.8 m s-1, and this was larger at the higher speed (Scheffe's test, P < 0.0001). The reflex was smaller when running uphill compared with running downhill (Scheffe's test, P < 0.05), or on a level surface (Scheffe's test, P= 0.05).
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Discussion |
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While the function of stretch reflexes in control of the jaw-closing muscles during chewing is well accepted, it has generally been considered that the jaw-closing muscles would not be subject to stretches in the course of normal everyday activities. However, the mandible moves up and down relative to the maxilla during hopping, walking and running (Flavel, Nordstrom & Miles, 2003), indicating that the jaw-closing muscles are subject to stretches during normal daily activities.
Hopping
Normally when hopping on one leg, people land on their toes which is quite comfortable because the antigravity muscles in the ankle, knee and hip absorb much of the shock of the landing. The present study shows that there is minimal vertical movement of the jaw relative to the head after toe-landing and that this does not evoke reflex responses in the jaw-closing muscles. The vertical position of the jaw is presumably maintained by visco-elastic forces in soft landings (Peck et al. 2002).
However, in order to hop and land on one heel, the knee must be fully extended and the ankle dorsiflexed. Consequently, much less of the shock of landing is absorbed by the antigravity muscles at the knee and ankle, and the resulting jolt is transmitted to the skull. This is uncomfortable: hence heel-landing is normally avoided during activities such as walking down stairs. Figure 1 shows that heel-landing was followed by a brisk downward movement of the mandible relative to the maxilla: this stretched the masseter, evoking a brisk reflex response in this muscle. The latency of this response is 0.4 ms longer than the monosynaptic reflex elicited by a tap on the chin. The slightly greater latency can be explained by the reduced temporal resolution that occurs when many trials are averaged.
After its downward movement relative to the maxilla, the mandible not only moved upwards but overshot its original vertical position (Fig. 1): however, the teeth did not come into contact in any subject. The overshoot indicates that the upward movement was not solely the result of elastic forces, but must be the result of reflex muscle activation.
The upward movement began about 32 ms after the onset of the masseter EMG. Nordstrom & Miles (1989) have shown that, in low-threshold masseter motor units, i.e. those that are likely to be activated in a stretch reflex (Miles et al. 1995), an action potential induces a twitch that peaks at about 38 ms (They could not calculate the electromechanical delay precisely because of the unknown time taken for the motor unit action potential to propagate along the muscle fibres to the intramuscular electrode.) These data strongly suggest that the reflex contraction of the jaw-closing muscles acts in concert with visco-elastic forces to arrest the downward-moving mandible and restore it to its normal postural position.
To exclude the possibility that this reflex is the result of vestibular afferent activation, subjects kept their teeth clenched while heel-landing in one set of trials, thus preventing the jaw from moving and the masseter from being stretched. Figure 1 shows that this abolished the reflex response, indicating that it is not the result of the short-latency vestibulo-masseteric reflex (cf. Deriu et al. 2003).
Locomotion
The foregoing indicates that stretch reflexes contribute to the control of jaw posture during an extreme gravitational stimulus, but the more interesting question is whether or not they control the jaw during more conventional activities such as walking and running.
The first suggestion that reflexes contribute to active maintenance of jaw posture was the observation of bursts of jaw-closing muscle activity occurring in each step of cats walking on a treadmill (Lund et al. 1984). Shiller et al. (2001) recently reported that vertical jaw movements occur during human locomotion, although the mechanisms that maintain jaw posture were not investigated. The amplitude of mandibular displacement during locomotion in humans depends on the speed of locomotion and on the nature of the footfall, i.e. whether landing is heel-first or toe-first (Flavel et al. 2003).
In the present study, the jaw-opening that occurred during each step in walking was small and slow, and evoked an irregular reflex response in only three of the subjects tested, and then only at their fastest walking speeds. In pooled data from 12 subjects (Fig. 4) there was no significant reflex activation of the masseter at either walking speed. That is, stretch reflexes do not contribute to mandibular posture during walking. Instead, the small-amplitude, damped oscillations that are induced by the downward displacement of the jaw after landing suggest that mandibular displacement is minimized by passive visco-elastic mechanisms. This visco-elastic support derives from the soft tissues in the peri-oral area, including the non-active jaw-closing muscles (Peck et al. 2002).
However, in a subject who is running, the larger forces transmitted to the skull and mandible by the shock of landing cause the mandible to move more quickly and further downwards relative to the maxilla. This stretches the jaw-closing muscles at higher velocities and higher amplitudes, and evokes clear reflex responses in them (Fig. 3).
The reflex consists of a brief, synchronous discharge in the masseter at a latency of 7.0–9.5 ms (Fig. 3). The latency could not always be measured accurately because of the difficulty in identifying precisely when the downward movement began, particularly in averaged records which reduce temporal resolution. Nevertheless, the latency indicates that the reflex response to the downward jaw movement traverses a segmental, probably monosynaptic, pathway. No evidence for longer-latency stretch reflexes was seen (cf. Poliakov & Miles, 1994; Miles et al. 1995).
This reflex excitation in the masseter was followed by an upward movement of the mandible (e.g. Fig. 3). As in hopping, the overshooting of the original jaw position indicates that the upward movement is at least partly the result of active muscle activity rather than passive elastic mechanisms alone, and its timing strongly suggests that the movement was caused by the reflex muscle contraction.
Running at different speeds and on different inclines led to different patterns of forces acting on the mandible and consequently different patterns of stretch reflex responses. In particular, the mandible moved less relative to the maxilla when subjects landed on their toes during uphill running, compared with landing on their heels during downhill and horizontal running (Figs 3 and 4). However, even toe-landing elicited a reflex response during running.
There is no reflex in the masseter during walking at up to 1.4 m s-1, but at faster speeds the reflex increases fairly linearly with running speed (Fig. 4). The relationships between the reflex and the various kinematic parameters of mandibular movement are less clear-cut, as shown in the representative data from three subjects in Fig. 5. In particular, there do not appear to be clear length or velocity thresholds for evoking the stretch reflex in a given subject. That is, it is not possible to draw a vertical line in the jaw-opening (stretch) or velocity panels that indicates a value for mandibular displacement above which the reflex EMG begins.
The amplitude of the reflex response did not always continue to increase in parallel with the amplitude or velocity of the downward movement of the mandible: Figs 4 and 5 show that the maximum amplitude and/or velocity of downward jaw movement often decreased or remained constant when the amplitude of the EMG was larger. The most likely explanation is that these records do not show a stimulus–response relationship of the kind that occurs in the normal laboratory situation where a servo-controlled stretch is imposed on the muscles. Rather, in the freely moving subject, the downward movement of the mandible that stretches the jaw-closing muscles evokes a reflex muscle excitation which then restrains the jaw movement. That is, when the stretch is sufficient to evoke reflex EMG activity, the muscle activation then prevents further downward jaw movement. Stronger stretches evoke more muscle excitation which results in smaller maximal downward jaw movement.
Because there was no clear relationship between either the amplitude or the velocity of stretch and the amplitude of the reflex response, we have plotted the kinematic parameters and EMG data for three representative subjects on three-dimensional axes in Fig. 6. This shows that maximal displacement increased fairly linearly with maximal velocity (shown on the horizontal plane). However, while there was not a clear threshold for the onset of the reflex response, the EMG amplitude increased sharply beyond a certain combination of stretch and velocity, particularly in the subjects whose data are shown in the right and left panels. This pattern is less clear in the central panel, however.
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Finally, the observation that the posture of the mandible is not regulated by stretch reflexes in the masseter even during walking in all subjects contributes to the long-standing debate over whether or not such reflexes are responsible for mandibular posture at rest (Woda et al. 2001). Together with the evidence from our earlier study (Jaberzadeh et al. 2003), this observation strongly supports the conclusion that the mandibular position at rest is not controlled by reflex activation of the masseter: however, we did not record the activity of the other jaw-closing muscles (temporalis and medial pterygoid) so it is possible (although not likely) that activity in these muscles may play a role.
It is concluded that, in a subject who is stationary or walking at a moderate pace, the mandible is supported primarily by passive mechanisms arising from visco-elasticity of the soft tissues in the peri-oral area. However, in a subject who is running, the increased inertial forces acting on the mandible result in larger, faster displacements that trigger short-latency stretch reflexes in the jaw-closing muscles. The size of these reflexes depends on the size and speed of the initial stretch induced in the jaw-closing muscles: hence, the resulting muscle contraction tends to restore the mandible to its prestretch position. Thus, one of the functions of the stretch reflex in the jaw-closing muscles is to maintain and restore the postural position of the mandible when it is perturbed during rapid head movements. The effectiveness of this response is demonstrated by the observation that the maximal downward movement of the mandible was usually less than 1 mm during running. We believe this to be the only situation in which the function of a stretch reflex has been studied under conditions of entirely natural activation.
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References |
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TopAbstractIntroductionMethodsResultsDiscussionReferences |
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: downhill locomotion and 
: uphill locomotion.