**1. Introduction**

Walking behaviour can express information of human body-like pathological symptoms. For example, Parkinson's disease patients are characterized by special pace rhythm [1].

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People increase the respiratory ventilation when they are walking or exercising. However, those people who suffer from chronic obstructive airway disease (COAD) cannot increase their respiratory ventilation quick enough to maintain the exercising behaviour. Consequently, they change their behaviour such as walking slowly so that they can maintain their respiratory ventilation. We can perform gait analysis on COAD patients because their walking behaviours are different with normal people when they are exercising because of their respiratory function.

Chronic obstructive pulmonary disease (COPD) is one condition of COAD. Nowadays, there are many chronic diseases in our daily life. COPD is one of them. COPD is a chronic airway disease characterized by progressive going downhill of the breathing functions [2]. One characteristic of COPD disease is the decreasing of forced expiratory volume in 1 s (FEV1) because of the obstructive airway [2]. Depending on their disease severity, they have different walking behaviours. Therefore, gait analysis can be used to judge the COAD patients' airway condition by observing their walking behaviour. However, it is difficult to collect the data of COAD patients without medical staff. Without clinical data, we cannot verify the correctness of our gait analysis algorithm.

By cooperating with Shuang-Ho Hospital in New Taipei, Taiwan, we set up an experiment. We film the side view of the subjects when they are performing a 6-min brisk walking test. By gait analysis, we can extract the features from walking behaviour such as pace distance and walking speed variation. However, in order to obtain that physiological information, we may need to wear sensors or markers on the subjects. Our method does not need to wear any sensors on the subjects.

In gait analysis, it is common to wear markers or sensors to record walking behaviour. In the experiment [1], the subjects need to wear a recorder on the ankle so that it could record the stride interval. In another experiment [3], the subjects also need to place a designed insole with 12 sensors into their own shoes.

However, there are some drawbacks of using makers or sensors. Firstly, it is inconvenient and uncomfortable of attaching them on human body and might affect the normal walking. Secondly, some sensors are heavy or hard to use for the elderly. Thirdly, some sensors have the electromagnetic interference that might affect and harm human body. In addition to the sensor problem, it is hard to tell a subject suffering from COAD disease or not by a single experiment. Without a complete examination, it is hard to judge whether the subjects are COAD patients or not. Consequently, we decide to access the respiratory function. By the pulmonary spirometer, we can obtain the tested subjects' respiratory data. Nowadays, the parameters from pulmonary spirometer are the standard to access the respiratory function.
