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NatureInterface > No.01 > P056-057 [Japanese]


Masticatory Strength Affects Your Locomotion Capacity

A wearable sensor has been developed for measuring daily masticatory functions and has important implications for sports medicine.

It has been said since old times that you can get strength by clenching your teeth. There is a famous anecdote of Mr. Sadaharu Oh, formerly a baseball player for the Yomiuri Giants and now the manager of the Fukuoka Daiei Hawks. Whenever he came to bat, he clenched his teeth every time he took a swing. Finally, his teeth became scraped.

A boxer wears a mouthpiece in order to prevent his teeth from the shock of the punches, so that cerebral damage may be reduced.

As these examples indicate, teeth-clenching and mastication have important implications for athletes. This is why a wearable mastication sensor has been developed. What is it like?

From various methods of measuring mastication and clenching to the development of a wearable sensor

The head of a human body can move freely because it is not completely fixed to the trunk, the bones of the back and neck. This causes an unbalanced condition for athletes who need to coordinate their eyes and body movements. Therefore, they need to build up their neck and back muscles.

On the other hand, clenching the back teeth or chewing thoroughly provides another support for the head and trunk, and clearly correlates with muscle strength and locomotion capacity. Thus, a wearable sensor for measuring masticatory functions would play a significant role in measuring locomotion capacity. Fig. 1 shows the objective of the development: to estimate locomotion capacity by measuring clench strength.

There are two ways to measure mastication and clench strength: 1. Direct measurement by a sensor installed inside the cavity of the mouth. 2. Indirect measurement by muscle and jaw movement. Fig. 2 shows various methods for measurement.

Photo-occlusion and an oral sensor are examples of the direct method. In these examples, the sensor should be maintained inside the oral cavity, and masticatory measurement is difficult. It also prohibits respiration, so it is not suitable for field usage.

Examples of indirect measurement include measuring jaw movement and measuring the activity of the muscles involved in mastication. In measuring jaw movement, it is difficult to distinguish between mouth closure and clenching. So this method is not preferable for measuring mastication and clenching.

The development of this wearable sensor started from paying much attention to the physical change in masticatory muscles that occurs during mastication and clenching, because the conventional electromyogram measurement used to be disturbed by the impedance change resulting from sweating and the movement of mimetic muscles.

Fig. 3 shows the physical change in the anterior muscle bundle of the temporal muscle. This is where the developers focused much of their attention.

The anterior muscle bundle near the temple contracts a lot during mastication or clenching. Therefore, the anterior muscle bundle reflects masticatory and clenching movements. The temple, where the anterior muscle bundle is located, is a good point on which to install the sensor, since in that location it would not disturb the body movement and would not easily be removed unintentionally.

Determining the best measurement method

* Selection of the sensor

In measurement during exercises, the movement of the head itself has much influence on the result.

First of all, a small accelerometer is suitable for the oscillation component under high frequency such as clenching under a repose condition. But it is not suitable for the measurement of low frequency displacements, such as clenching with macro muscle movement, because it is difficult to distinguish the muscle movement from the noise, such as head movement, which is in the low frequency area even under the repose condition. In this method, the head position should be stabilized during mastication.

Gyroscopes can be suggested as a more efficient sensor for the low frequency oscillation component, but they would also be influenced by the head movement.

So we need to remove the influence of the head movement. The measurement of tension, by fixing a manometer to the temple by a headband and then taking the measurement by directly sticking a strain gauge onto the temple, may be proposed. However, these methods lack accuracy because they measure the pressure or the strain, and it is not clear how either of these correlate with displacement.

A laser displacement meter, to measure the relative displacement at the brow, can be also proposed, but at present such a device would be too heavy to serve as a wearable sensor. It may also have a bad influence on the eyes because the temple is near the eyeballs.

The best method found is to measure temple deformation by detecting the relative displacement as an angle using a potentiometer, which is highly accurate angle sensor. As Fig. 6 shows, the body of the potentiometer touches the brow by affixing the potentiometer to the right temple. The rotation angle under the mastication or clenching condition is detected and is transformed into voltage as the output. The relationship between the sensor output and the occlusal power is detected in this way (Fig. 7).

The results from the measurement with a potentiometer indicated several facts, as follows: 1. The displacement of the temple muscle is influenced by the movement of the lower jaw position such as during vocalization. 2. However, the movement of the lower jaw and clenching should be distinguished by the difference in muscle contraction levels. 3. There is a clear logarithmic correlation between the sensor output during clenching and the occlusal power.

Applying a potentiometer to estimate the locomotion capacity of athletes

Now you understand that using a potentiometer is the best way to measure mastication and clenching during exercises. One of the main features of this approach is that the clenching status is measured with time elapse; when and how much a person clenches during exercises is measured, so that the relation between clenching and various sports may be clarified. The maximum value of the output of the sensor has a clear correlation with the occlusal power, so that the occlusal power is also estimated.

The above-mentioned story about Mr. Oh is just anecdotal, but it has become possible to estimate scientifically the influence of clenching on locomotion capacity through the measurement with this sensor. It is expected that the locomotion capacity of various athletes will be estimated using this method.

Fig. Caption

Fig. 1 The objective of measuring mastication and clenching through wearable sensors.

"Chewing" Mastication Measurement of mastication time Increase in mastication time

Clenching Measurement of bruxism during sleep and of clenching during awakening

 → Detection and inhibition of clenching

Measurement of clenching during exercises

 → Estimation of clenching's influence on exercises

Fig. 2 Classification of the measurement method

Mastication and clench measurement Direct measurement Oral sensor, Photo-occlusion

Indirect measurement Measurement of the jaw movement Mandible position measurement sensor Chinstrap-type detection Earplug-type detection

Activity of masticatory muscles Electromyogram Headband-type detection

Fig. 3 Lower jaw descent (left) and elevation (right)

外側翼突筋 Lateral temporal muscle

舌骨上筋 Suprahyoid muscle

舌骨下筋 Infrahyoid muscle

側頭筋 Temporal muscle

内側翼突筋 Medial temporal muscle

咬筋 Masseter muscle

Fig. 4 Temporal muscle

前部筋束 Anterior muscle bundle

中部筋束 Middle muscle bundle

後部筋束 Posterior muscle bundle

側頭筋 Temporal muscle

頬骨弓の切断面 Cross-section of zygomatic arch

筋突起 Muscular process

Fig. 5 Comparison of measurement methods

Sensor Noise (*1) Weight Calculation of displacement Remarks

Accelerometer ×   ○   ×   Suitable for the measurement of MV and MS.

Gyroscope ×  ○   △(*3)   Influenced by head movement.

Strain gauge ○   ○   ×   The relationship between strain and displacement is not clear.

Manometer ○   ○   ×

Laser displacement sensor ○(*2)   ×   ○   Too heavy to be put on the brow.

Potentiometer ○   ○   ○   Fixed as an angle sensor.

*1 Those influenced by head movement are indicated by ×, while those not influenced are indicated by ○.

*2 When it is fixed on the brow.

*3 Displacement is calculated by one integration.

Fig. 6 Installation position of the sensor.

額(固定面) Brow (The fixation point)

ポテンシオメーター Potentiometer

接触棒の長さ:r Contact stick length: r

回転角:θ Rotation angle: ?

側頭筋変位:x Displacement of the temporal muscle: x

ポテンシオメーター Potentiometer

回転軸 Rotation axis

接触棒 Contact stick

Fig. 7 Relation with the occlusion sensor

咬合面積 Occlusal area

平均圧力 Average pressure

咬合力 Occlusal power

有効圧% Rate of pressure available (%)

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