Ten healthy adults (four men, six women) were included in the present study. All subjects regularly engaged in physical exercise. All subjects were familiarized with the experimental procedures, and it was ensured that the attachment of the experimental device to the body did not have any effects on the physiological responses of the subjects during NW and LW. Prior to participation in this study, all subjects were fully informed of the study objectives, procedures, and possible risks of participation in the study, and all subjects gave written informed consent to participate. This study was approved by Human Ethics Review Committee of Juntendo University (approval No.24-66).
The subjects were asked to ensure that they received sufficient sleep and refrained from engaging in severe exercise and consuming alcohol 24 hours before the experiment. On the day of the experiment, each subject ingested a meal at least 2 hours before the walking tests, and arrived at the laboratory 1 hour before the test was scheduled to take place. To ensure that the subjects were in good physical condition, they underwent a medical examination, including completion of questionnaires and measurement of blood pressure, and their height, body weight, and percentage body fat were measured. The mean age, height, body weight, and percentage body fat were 21.8 ± 1.0 years, 1.75.8 ± 2.1 cm, 66.8 ± 4.4 kg and 14.4 ± 2.3%, respectively, in men, and 21.7 ± 1.5 years, 166.2 ± 4.0 cm, 61.0 ± 7.6 kg, and 25.3 ± 5.2%, respectively, in women (Table 1).
Experimental conditions and length of Nordic pole
Each subject was taught the NW methods by an International Nordic Walking Federation (INWA)-qualified instructor (the master instructor), and was familiarized with NW.
The pole length used for NW was selected and adjusted to permit a smooth arm motion, based on the INWA formula (0.68 × body height (in cm), and to induce a near right-angle elbow flexion upon pole landing.
During the experiments, two submaximal walking tests consisting of NW and LW were performed by each subject. Thereafter, subjects were fitted with a respiratory mask for measurement, and with electrodes for electrocardiography (ECG) and electromyography (EMG) recordings. Resting and HR were determined over 5 minutes while the subject was sitting on a stool. After resting measurements were obtained, the subjects performed two submaximal walking tests, NW or LW, at random. Both walking tests were interspaced by a rest period, which permitted the physiological parameters and rating of perceived exertion, measured using the OMNI scale, to return to the resting values that were measured before the tests. Both tests began at an initial speed of 60 m/min for 3 minutes, followed by increases of 10 m/min every 2 minutes up to 120 m/min. During the tests, some physiological parameters were continuously measured over a 1-minute interval. Room temperature and relative humidity were 20.0 ± 1.8°C and 46.7 ± 12.9%, respectively.
Oxygen uptake, ventilation, heart rate and OMNI scale
Throughout the tests, expired respiratory gas was analyzed using an automated metabolic analyzer (MG-360; Minato, Tokyo, Japan), which had been calibrated with known concentrations of O2 and CO2 for fractional concentrations of both gases. In addition, expired gas volume was measured on a flow meter (RM-300; Minato)[25, 26] every 30 seconds. and were also calculated every 30 seconds during both tests. Perceived exertion was evaluated using the OMNI scale during the final 15 seconds of NW and LW. As described by Robertson et al., ratings of felt at the level of the whole body and the upper and lower extremities were evaluated, and all subjects were aware that these measurements were being evaluated to determine the stresses on the cardiorespiratory system and on the muscle groups of the upper and lower extremities, respectively. Perceived exertion was evaluated at rest and during each stage of the walking tests.
Electromyogram recording and analysis
EMG activities of the upper and lower extremities were recorded from the thickest part of each muscle (the ‘belly’ of the muscle) of each muscle using pre-amplifier mounted electrodes (EMG Isolator SX230; Biometrics Ltd, Newport, UK). The skin over each muscle belly was wiped with pure ethanol, followed by skin preparation gel (Skin Pure; Nihon Kohden, Tokyo, Japan), to lower the resistance of the skin between the electrodes to less than 10 kΩ.
Based on the results of previous studies[23, 29, 30], EMG activities were recorded from four muscles in the left lower extremity and one muscle in the right upper extremity. Specifically, the muscles of lower extremity used for measurement were the vastus lateralis (VL), biceps femoris (BF), tibialis anterior (TA), and gastrocnemius (GA) of the left lower extremity, and the muscle of upper extremity was the triceps brachii (TB) of the right upper extremity.
A one-step cycle during walking (gait-cycle) was measured by a strain-gauge board attached to the heels of both shoes. A gait-cycle was defined as the amount of time from a heel strike to the next heel strike on the same foot. A sample recording from a subject is provided in Figure 1.
Analog signals of the EMG instrument and the strain-gauge were sampled using an A/D converter (MP100; Biopac System Inc., Goleta, CA, USA) at 1 kHz, and were stored in a personal computer (iMac; Apple Corp, Cupertino, CA, USA). Digitized EMG signals were integrated over a 30-second period at each walking speed during NW and LW. Each iEMG signal was expressed as a percentage of the iEMG reading obtained during LW at 60 m/min for comparison.
All data are expressed as mean ± SD.,, HR and iEMG data obtained during the final 30 seconds at each walking speed were used to compare differences between the two walking methods (NW and LW). All statistical analyses were conducted using Statview software (version 5.0; SAS Institute Inc., Cary, NY, USA). Two-way analysis of variance (ANOVA) analyzing walking speeds and walking with or without Nordic poles, was used to determine the significant differences between walking methods at different speeds. If a significant F ratio was indicated, Fisher’s protected least squares difference was used as a post hoc test to determine the significance of differences. P < 0.05 was considered significant.