Using EMA and EMG simultaneously to determine oral residence time

residence times

3.3.3.1. Introduction

So far, two methods have been used to determine the ORT. Both of them have strengths and weaknesses. EMG collects muscle activities data during oral processing, while EMA collects dynamic tongue and lower jaw movement data during oral processing of foods. All these data are required for this study. The two methods were therefore simultaneously used to collect data and ORT estimated from each method were compared.

3.3.3.2. Materials and methods

Subjects and food samples

Two male subjects (23 y and 24 y) took part in this experiment. They met the screening criteria outlined in Section 3.2.1.

The same five food samples were used in the previous work were also used in this experiment (Table 3-1.). All food samples were left in a temperature controlled room

(20ȗC) for at least 1.5 h to achieve room temperature before serving. Food temperature

was checked using an electronic thermometer.

Master and slave mice

The master and slave mice comprised a custom designed device to link up to three computer mice together in order to start and end different programmes in each computer at the same time. Two computer mice were connected in this experiment in order for EMA and EMG programmes to work simultaneously. Each mouse was connected to its own computer. The left click button of one mouse (master mouse) controlled all left click buttons of the other mouse (slave mouse). Each mouse was connected to its own computer and switch box. Both master and slave mice had full functionality on their own computer (can use the mouse as normal) when the switch box was in independent

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mode. In master/slave mode, the left click button of slave mouse was disabled, and the master mouse worked as normal, but it also controlled the left click button of the slave mouse.

Experiment procedure

All food samples were tested in one session in triplicate during one session. Samples were presented in random order. The subjects sat in the EMA cube, and 6 sensor coils were fixed at: a.) near tongue tip; b.) tongue back; c.) upper incisor; d.) lower incisor; e.) behind left ear; f.) behind right ear. Two pairs of EMG electrodes were fixed on the subject’s right masseter and submental muscles (Section 3.3.2.1). The experimental procedure was as described in Section 3.3.1.2. The subjects paused for at least 3s after food ingestion and swallowing. The master mouse was used to start and end two programmes on different computers simultaneously during the experiment.

3.3.3.3. Results and discussion

Time lag between two methods

The master / slave mouse was connected to EMA and EMG computers to check the time lag between the two methods. EMA collected data every 0.005 seconds (200Hz). EMG recorded 1000 points per second, but the collected data was rectified to RMS data every 0.02 seconds. The EMA method had a longer recording time than the EMG method while using the master / slave mouse. The time lag was between 0.14 to 0.35 s, with an average value was 0.26 s. The time lag was attributed to delays in both starting and ending of the programme.

The oral residence time from EMG and EMA trials

The ORT was easy to determine from displacement-time plots. The tongue and lower jaw movement were displayed in displacement-time plots and 2D displacement plots (Figure 3-21).

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Figures 3-21a, b and c. Three sensor coils movement on tongue and tooth on X axis (a), Z axis (b), and Y

axis (c) during consumption of plum jam from EMA for subject A.

In this case, the ORT starts at 7.14 s and ends at 14.69 s on the X axis; on Z axis it starts at 7.19 s and ends at 14.66 s. The earliest start time and the latest end time are used to calculate ORT – that is 7.55 s (the same as in the ORT on X axis). Plots show the tongue and lower jaw movement during food processing as well. As plum jam is a semi- solid food, it flows when certain a shear stress is added (e.g. the tongue is tilted, or

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pressed against the hard palate) under certain temperature. It explains why there is no typical rhythmic chewing activity. The sensor coil on the lower incisor does not move much, indicating that the subject does not use their teeth to chew. Only the tongue back is active; it moves slowly and irregularly and the displacement is within 10 mm both on the X and Z axis. The amplitude of movement reduces in turn from the Z axis to the X axis, to the Y axis. The movement pattern on X and Z axes are similar; the start and end time of ORT on both axes are similar. This indicates that the subject processes plum jam without teeth, and the tongue rubs and squeezes the food on the hard palate mainly in the anterior-posterior and vertical directions.

In EMG RMS plots (Figure 3-22), the ORT of plum jam starts at 7.78 s (masseter

muscle), and ends at 14.48 s. The ORT is 6.7 s. EMA recorded ORT (ORTEMA) is 7.55 s,

0.85 s longer than the EMG recorded ORT (ORTEMG). It is likely that the tongue

movement is earlier than where the masseter and submental muscles are activated; because after being activated the first movement during food processing after ingestion is to transport food to the molars on one side (soft-solid and hard-solid) or to the back of the oral cavity (liquid and soft-solid) by the tongue.

The EMG data provides not only the ORT, but also muscle activity information, such as muscle voltage which relates to the muscle force, the area under the curve which relates to the muscle work, chewing pattern or oral processing style and swallowing. For example, in the case of consumption of plum jam, the right masseter muscle does not

use much voltage to process food (Voltage max is 0.04 mV) and there are no regular

chewing cycles. The submental muscle data shows the subject swallows twice and uses

more voltage than with the masseter muscle (Voltage max was 0.15 mV). The masseter

muscle does 0.02 mV.s work and the submental muscle does 0.15 mV.s work. These data indicates that plum jam is easy to process, but the ORT is not short, the reasons are considered to relate to food properties and individual preferences.

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Figures 3-22 a and b. The rectified EMG RMS plots from right masseter muscle and submental muscles

during consumption of plum jam. The black arrow indicates the start time of oral residence time; the white arrow indicates the end of oral residence time.

Strengths and weaknesses of both methods

Both EMA and EMG methods have strengths and weaknesses. Based on the findings presented above, the strengths of EMA are: 1) accuracy of ORT recording; 2) tracking of tongue, tooth and jaw movement, even when the oral cavity is sealed; 3) dynamic recording of sensor coils’ data in three dimensions during oral processing. However, its weaknesses are: 1) taking more time to fix sensor coils on the tongue surface; 2) restricting oral movements due to sensor coils and wires in the oral cavity, especially when chewing activity is required; 3) difficult to guarantee the sensor coil is fixed to the exact same position across different sessions.

The EMG has been widely used in many fields for a few decades, the strengths are: 1) the instrument is easy to operate, and the electrodes are easy to fix on subject; 2) it is

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accurate on ORT measurement and muscle activity (muscle voltage, work, and oral processing style) recording; 3) the recorded data is easy to interpret. It also has some weaknesses, such as 1) the researcher cannot tell whether the movement of the muscle is caused by the tongue, tooth or lower jaw; 2) the muscle voltage and the area under the curve is highly related to the positions of the electrodes; 3) for liquid and some semi- solid food, the muscle activity is weak, and the data is not as accurate as for soft-solid and hard solid foods.

When using both instruments simultaneously, the restriction feature of EMA (due to wires restricting movement) makes EMG data collected simultaneously with EMA different from using EMG separately. The ORT time is longer and the maximum voltage (especially masseter muscle) is smaller when using EMG simultaneously with EMA than EMG on its own in ORT and food ingestion time (e.g. Figure 3-23 and 3-24). Therefore, using EMG on its own was chosen to investigate muscle activities during food oral processing and for recording the oral residence time. The trajectories of EMA sensor coils are not significantly different when using EMA separately and using EMA simultaneously with EMG.

Figure 3-23. Plot a shows the masseter (channel 1) and submental muscle (channel 3) EMG traces during

oral processing of plum jam using EMG on its own from subject B. Plot b shows EMG traces during consumption of Nutella when using EMG simultaneously with EMA from subject B.

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Figure 3-24. Plot a shows the masseter (channel 1) and submental muscle (channel 3) EMG traces during

oral processing of Nutella using EMG on its own from subject B. Plot b shows EMG traces during consumption of Nutella when using EMG simultaneously with EMA from subject B.

3.3.3.4. Conclusions

a) To measure ORT, EMG alone is the best method, because it is simple, accurate and

non-invasive. Alongside this, it does not interfere with natural oral processing behaviours. This is described further in Chapter 5.

b) To detect chewing appearance, EMA alone is the best method because of its

accuracy and dynamic multipoint tracking feature in the oral cavity. This is also described further in Chapter 5, even though the wires may affect oral movements.

c) To characterize tongue movement, EMA is the only approach (Chapter 7).

Generally, both methods are accurate to measure ORT, but the EMA method has an earlier detection of the tongue movement during oral processing. However, considering the difficulty of operation and the purpose of the research, using the EMG only method is preferred for further studies determining ORT.

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3.4.

Conclusions

The sensors position was determined to be the near tongue tip, tongue back and lower incisor in the oral cavity; reference sensors were located at the upper incisor and behind both ears. The start point of ORT was defined as the time when the subject starts oral processing after the food sample is loaded onto the tongue surface and the subject has paused for a few seconds. The end point was defined as the time when clearance and the terminal swallow have finished.

EMA and EMG methods are both accurate to measure the ORT: the former tracks the tongue and lower jaw movement dynamically; the latter records muscle activity. Further research will use EMA and EMG separately for different purposes and with different food types.

These experiments also indicate that oral processing behaviour is affected by various factors, the food physico-chemical and material properties being the most important. Therefore, food properties must be characterised before further investigation and this is the purpose of Chapter 4.

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