# Verification of Industrial Worker Walking Efficiency with Wearable Hip Exoskeleton
## 2 Materials and Methods
### Measurement  
A wireless surface electromyography measurement system (Ultium EMG, Noraxon Corp., Scottsdale, AZ, USA) consisting of a wireless transmitter, electromyography (EMG) sensor, and Ag/AgCl electrode was used to measure the activity and fatigue of the lower limb muscles.  
As representative muscles involved in flexion and extension of the lower-extremity joints, the rectus femoris (RF) and semitendinosus (ST), which are responsible for flexion and extension of the hip and knee joints, and the tibialis anterior (TA) and medial gastrocnemius (GCM), which are responsible for dorsiflexion and plantar flexion of the ankle joint, were selected [22]. The measured muscle activity (\(\upmu\)V) used the normalized value for the individual's maximum voluntary contraction (MVC), and the unit of measure is % MVC.  
Eight electromyography electrodes were attached to both lower extremities and the average muscle activity (% MVC) and median frequency (Hz) of the muscles were analyzed.  
To analyze muscle fatigue, the median frequency, the frequency at which the EMG power spectrum is halved into equal power regions, was analyzed.  
Because muscle fatigue causes the EMG frequency spectrum to shift downward over time, analysis of median frequency is one of the most effective methods for processing EMG signals during fatigue analysis [23].  
In this study, the slope of a second-order polynomial curve, which can predict the expected attenuation pattern of the central frequency spectrum, was analyzed. This means that the greater the slope in the negative direction, the more severe the muscle fatigue.  
To analyze the energy expenditure, we used a human spirometry system (K4b2, Cosmed Corp., Rome, Italy) consisting of a portable O\({}_{2}\)/CO\({}_{2}\) gas analyzer, flow meter, data transceiver, mask, and heart rate monitor. Oxygen uptake (VO\({}_{2}\)/kg), heart rate (beats per minute), energy expenditure per minute (kcal/min), total energy expenditure (kcal), and net energy expenditure per minute (kcal/min) were analyzed.  
\begin{table}
\begin{tabular}{l l} \hline Overall size & \(23.8\times 10.0\times 5\) cm\({}^{3}\) \\ \hline Weight & 1.4 kg (including battery, fastener) \\ \hline Operating time per charge & Approximately 2 h (assist mode) \\ \hline Battery & Lithium-ion battery, 14.4 Vd.c., 3.35 Ah \\ \hline \multirow{3}{*}{Applicable body size} & Main body—one size \\ \cline{2-3}  & Waist or thigh fastener—two sizes (waist 26\({}^{\prime}\)-36\({}^{\prime}\)) \\ \hline Adaptive thigh frame stroke & 160–350 mm \\ \hline \end{tabular}
\end{table}
Table 1: WIM size dimensions and hardware specifications.  
Oxygen uptake measures an individual's ability to absorb oxygen and deliver it to working tissues and the ability of working tissues to utilize oxygen. Cardiorespiratory capacity is defined as maximum oxygen uptake [24].  
The total energy expenditure (total EE) is calculated from the gas exchange of oxygen and carbon dioxide, measured through indirect calorimetry, and refers to the body's total energy expenditure for heat production and work output [25]. The energy expenditure per minute (EEM) is the total energy expenditure divided by minutes, and the net energy expenditure per min (net EEM) was the EEM while walking minus the EEM at rest [13].