Osteoporosis is a disease that reduces bone mineral content (BMC) and leads to deterioration of bone structure. The lower bone strength leads to a higher risk of fracture (National Institute of Health (NIH), 2000). After menopause, bone fragility increases and fractures can occur, which can limit daily activity and reduce quality of life, particularly in the older population. The NIH has reported that hip and vertebral fractures are a problem for women in their late 70s and 80s, while wrist fractures are more common in patients in their late 50s to early 70s (NIH, 2000). The distal radius is also a common site of fracture.
Nordic walking (NW) was first described in Scandinavia as a Nordic skiing training method during the summer of 1930. NW was further developed and was later introduced in Europe as a modern sport around 1990 (Tschentscher et al., 2013). NW is a walking fitness exercise that uses specially designed poles. When walking using poles, as in cross-country skiing, the pole push-off movements involve placing pressure on the ground through the poles while swinging the arms. There is evidence that 12 weeks of NW improves overall health and cardiovascular metabolism (Church et al., 2002), increases high-density lipoprotein concentration and reduces low-density lipoprotein and triglyceride concentrations (Hagner et al., 2009) and increases the walking distance during a 6-min walking test in patients with cardiovascular disease (Girold et al., 2017). The driving force against the ground produced by the pole push-off movement during NW increases walking speed (Church et al., 2002) and stimulates muscle activity in both arms (Shim et al., 2013; Sugiyama et al., 2013; Pellegrini et al., 2018).
Adaptation to bone mechanical stress is site-specific (Kohrt et al., 1997; Bass et al., 2002; Kontulainen et al., 2002; Dowthwaite et al., 2012; Duckham et al., 2014; Umemura et al., 1997; Umemura, 2016; Hart et al, 2017) and mechanical stress that acts as a reaction force in the direction of gravity increases bone strength (Kohrt et al., 1997; Umemura et al., 1997). Unlike the femur, the radius does not have a gravitational mechanical compression load when ground reaction forces are transmitted from the ground during daily activities (Kohrt et al., 2009; Robling, 2009). We therefore wished to focus on the efficacy of NW pole push-off for stimulating osteogenesis in the distal radius. In the present study, we aimed to determine the effects of a 6-month NW program on the BMC and areal bone mineral density (aBMD) of the distal radius and the cross-sectional area of the muscle at the mid-humeral and mid-femoral levels, measured using dual-energy x-ray absorptiometry (DXA) and magnetic resonance imaging (MRI), respectively.
Participants and exercise program
Ethics approval for this study was obtained from the ethics committee at the Mie Prefectural College of Nursing (reference number 110401) and the procedures were conducted in accordance with the institutional guidelines. The purpose and all experimental procedures were carefully explained to each participant and written informed consent was obtained prior to their enrollment in the study. One hundred and twenty-seven healthy female college students were asked to take part in the study and 41 students (21.5 [+ or -] 1.8 yrs) volunteered to participate. The inclusion criteria were: no contraindications to exercise, no pregnancy, eumenorrhea, non-smoker, no regular training except swimming and walking and no medical or surgical conditions likely to affect bone metabolism. We used questionnaires that have previously been used in similar age groups (Kato et al., 2006; 2015). The 41 healthy female college students were randomly allocated to two groups, using the random number function of MS Excel 2013: NW (n = 21) and control (n = 20) groups. A menstrual cycle questionnaire was completed twice, at baseline and immediately after the 6-month NW training period. At the end of the training period, two participants were excluded from the NW group because of irregular menstruation (menstruation occurred on fewer than nine occasions during the 12 months preceding the end of the NW training period). Two participants were also excluded from the control group because they began regular resistance training after the NW training intervention, which included arm and wrist curls using dumbbells three times a week. Therefore, data from 19 NW and 18 control participants were analyzed.
We prepared the NW poles (Leki, Supreme titanium) by adjusting the pole length and exchanging the replacement rubber grip (Leki, walking rubber chip), following the instructions of an International NW Federation coach. The NW group participants used the same poles throughout the intervention periods. The NW group participants walked for at least 30 min, but not more than 60 min, using the NW poles three times a week for 6 months at a target rating of perceived exertion of 10-11. The NW group participants walked for 30 min during the first month, for 5-to-10 min longer during the second month and for a further 5-to-10 min longer during the third month. The NW group participants walked in groups as much as possible, along sidewalks near the university they attended and selected the walking routes themselves. These routes were relatively flat and even and the surfaces were frequently hard, so that there was relatively little slip when the NW pole came into contact with the walking surfaces. Prior to the intervention, participants in the NW group were instructed and trained by an International NW Federation coach. All 21 of the NW group completed 6 months of NW training. The participants in the control group were asked to perform similar normal daily activities to those they performed before the study and all 20 completed the measurements at baseline and after the intervention period.
Dietary intake was assessed using a 3-day food diary 1 month before initiating the NW training, across 2 weekdays and 1 weekend day. The participants recorded their menus on each day, listing each ingredient and its approximate mass. These food records were checked by a dietitian and nutrient intake was analyzed using Eiyokun version 4.0 software (Kenpakusha, Tokyo, Japan) and a standard Japanese food database. Participants who consumed less than 650 mg/day (the recommended calcium intake; Japanese Ministry of Health, Labour and Welfare, 2015) of calcium received nutritional counseling and were subsequently reassessed using a second 3-day food diary. Participants who consumed less than 500 mg/day were administered calcium carbonate supplements as 300 mg tablets to be taken with meals during the intervention.
Dual energy x-ray absorptiometry (DXA) measurement
aBMD, BMC and muscle cross-sectional area (CSA) were measured by a single nationally registered and qualified radiological technologist, both before and after the intervention, who was blinded to the group each individual belonged to. A DXA (DCS-3000, Aloca, Japan; analysis software ver. 5.00) scanner was used to make measurements 1/10, 1/6 and 1/3 of the length of the radius of the nondominant arm from the distal end, according to the manufacturer's instructions (Figure 1). The BMC (g) and area ([cm.sup.2]) were...