To our knowledge, only two studies have examined the effects of wearing commercially available US on VO2 while walking at one or two speeds (Gjøvaag et al., 2011; van Engen et al., 2010). Therefore, earlier studies were able to identify only fragments of walking speed vs. VO2, which presumably have a quadratic curve relation. For example, van Engelen et al., 2010 reported that wearing US caused an increase of about 10% in metabolic energy cost compared with WS when walking at a fixed speed (4.5 km·h-1). Gjøvaag et al. (2011) measured VO2 during walking at a self-selected speed (4.5 km·h-1) and a fixed higher speed (5.8 km·h-1) while wearing US and WS. The VO2 at the self-selected speed for US and WS were 11.7 ± 1.3 and 11.4 ± 1. 2 ml·min-1·kg-1, respectively, and the corresponding values for the higher speed were 15. 3 ± 1.2 and 15.1 ± 0.8 ml·min-1·kg-1. Although the VO2 for US was 1-2% higher than that for WS, the difference was not significant. This study is the first to show that the quadratic relationship between walking speed and VO2 shifted to a higher value in subjects wearing US, and that VO2 increased by 3-5% in subjects wearing US while walking compared with subjects wearing WS. Optimum speed and RPE did not differ according to shoe type, but a decreased cadence and increased step length were significantly associated with wearing US while walking. The general features of US, compared with WS, are that the sole is rounded, thicker, and softer, and the shoe mass is greater. These features might have changed the walking motion, thus leading to a change in muscle activity and/or VO2. The largest difference between US and WS was the shape of the sole. The flat area of the sole as a percentage of the total area on the bottom of the shoe was less for US (60%) than for WS (80%), and the radius of the curve was smaller for US (35 cm) than for WS (60 cm), indicating that the US had a pronounced curvature compared with the WS. It is possible that the increased instability attributable to the sole shape increased the activities of the thigh and calf muscles during standing, thereby contributing to increased energy consumption (Nigg et al., 2006). It has also been reported that wearing US, compared with WS, while walking at a self-selected speed decreased both cadence and step length, and increased muscle activity of the lower extremities (Romkes et al., 2006). Wearing US while walking at a fixed speed (5.0 km·h-1) increased medial gastrocnemius muscle activity, but the changes in muscle activity varied among different muscles (Nigg et al., 2006). In contrast, it has been suggested that the increased curvature of US may decrease oxygen consumption. Hansen et al. (2011) made shoes with soles of different curvatures and compared the VO2 during walking at a self-selected speed (4.03 km/h). The VO2 (ml·min- 1·kg-1) was 11.9 for flat-soled shoes, 10.9 for shoes with R25 curvature (25% of leg length), 10.7 for shoes with R40 curvature (40% of leg length), and 11.0 for shoes with R55 curvature (55% of leg length). These results indicate that it was easier to walk in curved-sole shoes, as VO2 decreased by 10% compared with flat-soled shoes (p < 0.05). However, no significant difference in VO2 was observed among the different curvatures (R25-R55). In the present study, the ratios of the radius to leg length for US and WS corresponded to about 40% (R40) and 70% (R70), respectively. We cannot directly compare the results obtained in our experiment using commercially available shoes and the results obtained using purely experimental shoes. Nevertheless, the findings that the flat area of the US (60%) was less than that of the WS (80%) and that the curvature of the US (R40) was greater than that of the WS (R70) suggest that the curved shape of the shoes would not affect VO2. Therefore, the 4% greater VO2 for US cannot be explained by their curved sole. The thickness of the sole differed between US (fore foot, 2.5 cm; rear foot, 4.3 cm) and WS (fore foot, 1.5 cm; rear foot, 3.3 cm), making the leg length 1 cm longer when wearing US compared with WS. This represents only 1% increase of a subject’s leg length in the present study. Meanwhile, the step length increased 2 to 5 cm when wearing US compared with WS corresponding to 3-13% increases of the step length. A previous study by van Engelen et al., 2010 demonstrated that step length during walking on the ground at 4.5 km·h-1 was 1cm longer for US than WS, which appeared much less than the 3-13% increase observed in the present study, and therefore makes it difficult for only 1% increase of the leg length to explain the entire magnitude of increase in step length. One of the possible mechanisms may be that the backward force produced by the treadmill may have influenced increased step length in the present study. In the present study, as the shoe mass differed between US (1.0 kg) and WS (0.54 kg), the total weight of shoes was 1kg higher in US than WS, equivalent to about 1% body weight. This increase might cause 1% increase in energy expenditure if they walked at the same speed. Therefore it is possible that this difference might have influenced muscle activity and VO2. However, it is well known that a difference of 0.5 kg in shoe mass had little effect on VO2 during walking at normal speed (Hettinger and Muller, 1953). In addition, one foot must be on the ground during walking. Therefore, the greater mass of US may not directly influence muscle activity or VO2 very much. Thus, the 4% greater VO2 for US cannot be clearly explained by sole curvature, sole thickness, or shoe mass, leaving sole softness (or hardness) as a possible key factor underlying the increased VO2. The US sole was softer than the WS sole (US hardness: fore foot, 2.5 ± 0.2 N and rear foot, 1.4 ± 0.2 N; WS hardness: fore foot 3.7 ± 0.3 N and rear foot, 4.3 ± 0.2 N). During normal walking, the heel contacts the ground with dorsiflexion, which then becomes plantar flexion at toe off (Romkes et al., 2006). van Engelen et al., 2010 reported that the force developed for propulsion is attenuated before it is conveyed to the ground when the sole hardness is lower, resulting in increased metabolic energy cost during walking at the later supporting phase. Stewart et al., 2007 demonstrated that lower sole hardness of an US results in increased plantar pressure on the fore foot during walking. In the present study, the iEMG of the lower leg was higher (soleus, 7-23% higher; medial gastrocnemius, 6-16% higher) when walking while wearing US compared with WS. These suggest that because the US sole material was softer than that of WS, the propulsion force of the push-off phase was reduced for US. Therefore, to maintain a given walking speed, the medial gastrocnemius and soleus muscles must develop greater activity when wearing US. Although the increased iEMG of these muscles could be caused by an increase of the contact time rather than the intensity of muscle contraction, there was no significant difference in the percentage of contact time between US and WS over a given time period (Table 1). This suggests that the increased iEMG with US could be largely accounted for by increased intensity of muscle activity. Taken together, our results indicate that the increased step length and increased muscle activity per unit time in the lower leg, particularly the calf, during walking at all speeds while wearing US contributed to the increase in VO2 and oxygen cost. Nevertheless, these observations are insufficient to explain the entire increase in VO2. The present study measured muscle activity only in the lower body. Increased activity of muscles in the body trunk or changes in the motion of the upper extremities might have occurred because of changes in posture, or unknown factors might have contributed to the increase in VO2. A previous study showed that when step length was extended by 20% while walking at a constant speed, the mechanical work of the ankle joint increased, the vertical movement of the center of mass increased by 24%, and metabolic power increased by 36% (Gordon et al., 2009). The present study demonstrated an increase of 3-13% in step length during walking at various speeds while wearing US compared with WS, which might have led to the increase in VO2. Furthermore, because walking propulsion force is developed by muscle tension and utilization of the elastic energy of the tendinous tissues (Fukunaga et al., 2001; Ishikawa et al., 2005), the effect of tendinous tissue behavior on VO2 during walking should be investigated in the future. We observed little change in RPE despite the significant increase in VO2 in subjects wearing US. Gjøvaag et al. (2011) reported that while wearing US, RPE increased only at the fastest walking speed with a 10% inclination of the treadmill. Wang and Hansen, 2010 reported the possibility that wearing curved-sole shoes instead of flat-sole shoes may bring a sense of easier walking because of the changes in ankle joint movement during walking. In the present study, although heart rate and VO2 increased, an increase in RPE might have been suppressed by the feeling of easier walking while wearing US. |