Study Design and Participants
This study was performed in a randomized, controlled, crossover design to the metabolic, cardiovascular, and muscular responses during NW and W (Fig. 1A). The experiment was conducted in a laboratory setting within 1 day, with no follow-up. Healthy adults over the age of 19 who had no Nordic pole experience were recruited as study subjects. Those with a history of arrhythmia, heart failure, or myocardial infarction or those who received treatment or medication for cardiovascular disease within the last 3 months were excluded from the study. We assumed the oxygen consumption of 18.3 mL·kg-1·min-1 for W and 20.5 mL·kg-1·min-1 for NW (standard deviation during W, 2.5 mL·kg-1·min-1) [7]. The estimated sample size was 11, with an alpha level of 0.05 and a beta level of 0.20. We expected a 30% dropout rate, and the final sample size was calculated as 16. The first participant was recruited on March 24, 2018. The experiment was completed by May 31, 2018, with a total of 16 people. During the experiment, 1 subject was not tested for surface electromyography; hence, the final analysis included the results of 15 subjects.
The experimental group constituted 15 healthy men. Table 1 shows the means and standard deviations for age, weight, body mass index, waist circumference, umbilicus height, resting systolic and diastolic blood pressure (SBP and DBP, respectively), resting heart rate (HR), room temperature, and room humidity. All participants had no experience with NW before this study and were fully informed about the study’s procedures and their participation’s possible risks. All voluntarily participated in the study after they provided written consent. This study was approved by the Institutional Review Board. It was conducted together with a study that analyzed the accuracy of HR measurement using a wearable band during NW [12].
NW Technique
Participants visited the exercise testing laboratory on the day of the graded walking tests. Participants were sent a homepage link (http://www.nordicwalking.or.kr) about the Nordic walking method in advance to the experimental day. On experiment day, they underwent a 1-h training on holding and releasing the Nordic pole and walked with Nordic pole on surface and on the treadmill with a NW instructor from the Korean Nordic Walking Federation/Korean Walking Association (Y. Ha) before the walking test. The NW technique emerged from a training modality that is typical of cross-country skiing, requiring a specific technique: moving the extended arms similar to the range of movement of natural walking, maintaining the upper body upright, maintaining a backward pole position during the loading phase, using the poles actively and dynamically, and controlling the poles by hands gripping with grasp/release patterns [13]. Each participant’s pole length (Nordic Friend, Gabel, Italy) was set at the umbilicus height. The weight of the walking pole was 196 g. After familiarization with NW on a treadmill, they were asked to rest for at least 30 min before the start of the walking test.
Graded Walking Test Protocol
The graded walking test protocol on the treadmill constituted 20-min stages of rest, adaptation, walking, and recovery. The participants warmed up and familiarized for 2 min at 2 km·h-1, the same speed with the first walking test stage. The walking tests constituted seven stages, with a 2-min duration for each stage: walking on a treadmill (STEX 8100TD; Taeha Mechatronics, Anyang-Si, Korea) at 3 km·h-1, 4 km·h-1, and 5 km·h-1 at 0% inclination and 1%, 3%, 5%, and 7% inclination at 5 km·h-1 (Fig. 1B).
The participants walked on the treadmill at a stepwise incremental speed and grade. Two walking conditions (NW and W) were used with a randomized sequence using a random number. The blood pressure cuff was worn on the left upper arm, and SBP/DBP were measured at the end of each stage. HR was assessed in 30-s intervals using 12-lead electrocardiography (Philips StressVue, Philips, the Netherlands), and the highest HR value was selected for each walking stage. Exertion was also rated at each walking stage; they were rated using the 6–20-point Borg rating of perceived exertion (RPE) scale [14].
Ventilatory Gas Analysis
The volume of oxygen consumed per minute (V̇O2), volume of carbon dioxide per minute (V̇CO2), expired ventilation per minute (V̇E), respiratory exchange ratio (RER), and respiratory rate (RR) were measured using a ventilatory gas analysis system (Ultima PFX®, MGC Diagnostics Corporation, St Paul, MN, USA). The measured values during the 2 min of each walking stage were averaged.
Surface Electromyography
The surface electromyographic signals of the mid deltoid (DEL, the midpoint between the acromion and the deltoid tubercle), biceps brachii (BB, the thickest muscle belly of BB), triceps brachii (TB, the midpoint between the acromion and the olecranon), vastus lateralis (VL, five finger’s breadth upward and lateral from the patella), medial gastrocnemius (GCM, the medial belly of the calf muscle), and tibialis anterior (TA, four finger’s breadth downward from the tibial tuberosity) in their right body were recorded using Model 586 Desktop DTS Receiver and Model 542 Desktop DTS EMG Sensor (Noraxon USA, Inc., Scottsdale, AZ).
Before applying the surface electrodes, the skin was cleaned with an alcohol swab to reduce impedance. A skilled physiotherapist attached all electrodes to the skin on the midpoint of the contracted muscle belly parallel to the muscle fibers with an adhesive tape. The sampling frequency was 1500 Hz. The data were filtered with a bandpass filter in the acquisition software. The root of the mean square value was acquired for each walking stage.
Statistical Analysis
The acquired data were analyzed using the SPSS ver. 23 (IBM, Armonk, NY). Hemodynamic responses, ventilatory gas analysis, and surface electromyographic results were compared between NW and W using the paired t-test. A P-value <0.05 was considered significant.
Simple and multiple linear regression analyses were conducted to estimate E for NW. The V̇O2 (mL·kg-1·min-1) with different walking speeds at flat walking (walking stage 1 to 3; grade = 0%; speed 3 to 5 km·h-1 or 50 to 83 m·min-1) was analyzed using a simple linear regression analysis. The V̇O2 in different grades with constant speed (walking stage 3 to 7; speed = 5 km·h-1 or 83 m·min-1) was estimated using a simple linear regression analysis. Multiple linear regression analysis was used for walking stage 1 to 7 for treadmill speed and the interaction of treadmill speed and grade.
E for W also estimated with simple and multiple linear regression analyses in the same way as NW.