Journal 
Ⅰ. Introduction
Walking is one of the most basic forms of exercise in our daily lives, and it has a crucial relationship with healthcare (Lee & Buchner, 2008). Many studies have demonstrated that walking strengthens muscles and bones, as well as improves cardiopulmonary functions across various age groups (Kelly, Murphy & Mutrie, 2017). Furthermore, It also has a positive effect on mental health, helping to reduce symptoms of depression and anxiety (Robertson et al., 2012).
However, in some cases, walking has limitations as an exercise. First, if balance or walking ability is impaired, the risk of falls increases. Second, if the exercise intensity is too high, there is an increased risk of lower extremity muscle damage, fatigue and injury. For these reasons, walking exercise requires caution for special populations such as individuals with disabilities or the elderly (McCulloch et al., 2010).
From this perspective, walking with poles was developed in Europe and is known as Nordic walking (Zurawik, 2016). And through many previous studies, the health benefits of Nordic walking have been validated. Representatively, Nordic walking has been shown to increase oxygen consumption, heart rate, and energy expenditure during walking, and clinically prove positive effects on chronic diseases such as obesity and diabetes (Figard-Fabre et al., 2011;Tschentscher, Niederseer & Niebauer, 2013).
However, as Nordic walking originated in Europe, where there are many flat terrains such as hills, it is not easily accessible in South Korea, which mainly consists of narrow roads and mountainous terrain. In addition, it takes time to acquire the skills required to perform the exercise correctly, and there is a risk of injury due to the large range of motion of the upper body. Furthermore, a previous study reported that Nordic walking did not have a significant effect on reducing weight-bearing in older adults with knee osteoarthritis (Hansen et al., 2008).
Therefore, this study was aimed to investigate whether a newly developed full-body walking exercise called Polking, which was designed to be more accessible in terms of walking technique and allows for greater upper body involvement for stability and prevention of injury while walking, has a similar positive impact on muscle activation and plantar pressure during walking as Nordic walking.
Ⅱ. Methods
1. Participants
A total of 16 participants (mean age : 21.81 ± 1.83 yrs, height : 169.16 ± 8.80 m, body weight : 62.79 ± 15.28 kg) with no health problems to conduct the walking task participated in this study. All participants had no prior experience with walking exercise using poles.
2. Experimental protocols & Measurements
As shown in Figure 1, all participants performed two different walking tasks, (1) natural walking and (2) Polking, three times each along a 10m walking path. Plantar pressure plate (Freemed, Sensor Medica srl, Italy) was placed in the middle of the walking path. The participants were instructed to walk at a comfortable pace. While performing tasks, muscle activation and ten parameters related to plantar pressure were collected by EMG system with sampling frequency of 1500 Hz (MiniDTS, Noraxon, USA) and plantar pressure plate with sampling frequency of 100 Hz. The areas where EMG was collected were the middle deltoid (AD), biceps brachii (BB), triceps brachii (TB) and Latissimus dorsi (LD) for the upper extremity, and gastrocnemius (GC), tibialis anterior (TA), Vastus medialis (VM) and adductor longus (AL) for the lower extremity. The EMG electrodes were attached according to SENIAM guidelines. Prior to attaching the EMG device, the skin was cleaned with alcohol and hair removal was performed if necessary to reduce skin resistance.
3. Polking exercise
Polking is a walking exercise designed for seniors, disabled individuals, and those with joint pain, and it should be performed while adhering to 4 basic posture principles (Figure 2). First, the body should be held straight, second, legs should be walked with full knee extension as much as possible, and arm joints should be relaxed while consciously taking wider strides than usual habit. While Nordic walking aims to improve walking speed and then encourage longer strides, Polking focuses on increasing stride length first, and then gradually increasing exercise intensity afterwards, making it easier to adjust the exercise intensity for each individual.
4. Data & Statistical analysis
The EMG signal was filtered using a 10-500Hz band-pass filter and then rectified. Finally, each participant's muscle activity has been normalized on a scale from 0 to 1 based on the activation while standing position (Fransson, Gomez, Patel & Johansson, 2007). The normalized data for each muscle was trimmed and averaged by gait cycle. All plantar pressure data from both natural walking and Polking were statistically compared using a two-way MANOVA (Walking type x Leg side). Post hoc analysis was conducted using the Tukey technique. Muscle activation was statistically compared between the two walking types for each muscle using paired t-tests. Statistical analysis was performed using SPSS (Ver. 23), with a significance level set at α = 0.05.
Ⅲ. Results
1. Muscle activation during Polking compared to walking
As shown in Figure 3, the results of comparing the activation of upper and lower limb muscles between normal walking and Polking revealed a significant increase in all normalized and averaged muscle activities (i.e. AD, BB and TB for the upper extremity; GC, VM and AL for the lower extremity) except for LD and TA during Polking (P <.01).
Figure 3
muscle activity of the upper and lower limb muscles on the dominant side during normal walking and Polking. A represents the activation of upper and lower limb muscles during the gait phase of normal walking, while B represents muscle activation during Polking.
2. Plantar pressure during Polking compared to walking
As shown in Table 1, significant differences were found in the averaged plantar pressure in gr/cm² (P = .01) and Delta CoF (P = .008) between normal walking and Polking, regardless of leg side. No significant differences were found in the other variables.
Ⅳ. Discussion
The purpose of this study is to compare the muscle activity of upper and lower extremities and plantar pressure between normal walking and a newly devised Polking exercise method. First, the analysis of upper and lower limb muscle activity showed that Polking exercise method resulted in significantly higher muscle activation in all muscles except for the LD in the upper limb and the TA in the lower limb, compared to normal walking. Secondly, among the 10 variables related to plantar pressure during walking, there was a significant increase in the averaged plantar pressure in gr/cm² and Delta CoF, regardless of the leg side during Polking when compared to normal walking.
The use of poles in walking activates upper limb muscles, increases walking speed, enables longer distance travel, and improves cardiovascular fitness (Church, Earnest & Morss, 2002). Additionally, Foissac et al. (2008) reported that the use of poles during walking increased muscle activity by an average of 95%.
On the other hands, some studies reported that the muscle activity in the lower limb was significantly lower when using poles during walking than when not using poles (Seo, Kim & Yoon, 2007). Similarly, Knight and Caldwell's (2000) study also reported that muscle activity was significantly lower with pole use than with normal walking. These results contradicts the findings of previous studies. This discrepancy in results is attributed to the fact that previous studies performed Nordic walking as a way of walking.
Generally, in Nordic walking, the opposite hand presses the pole onto the ground at the same time as the foot contacts the ground and moves forward. Therefore, using poles before and after the moment of foot-ground contact affects muscle activity in the leg, reducing muscle activity and causing reliance on the poles. Thus, Nordic walking have been considered a very effective exercise that reduces knee stress compared to normal walking and uses the upper body, reducing the burden of joint arthritis (Schwanbeck, 2014;Wellsandt & Golightly, 2018).
However, in normal walking, muscle activity increases during the movement between when the foot contacts the ground and when the same foot contacts the ground again in order to increase the stability of the leg. On the other hand, it can be interpreted that Polking can produce greater effects as an exercise than Nordic walking by increasing the activation of both upper and lower limb muscles in the long term because the inherent purpose of exercise is to strengthen muscles, which can reduce joint stress and improve pain.
The increase in the averaged plantar pressure and delta CoF is also thought to be related to the increase in lower extremity muscle activity (Nurse & Nigg, 2001;Telfer et al., 2013). In many previous biomechanics studies, the increase in plantar pressure has been interpreted as an increase in joint load, and the increase in CoF has been interpreted as an increase in balance instability. However, in this study, the increase in plantar pressure is a natural phenomenon induced by the increased activation of the lower leg muscles, and the increase in CoF is better interpreted as an increase in walking stability due to the support of the poles, rather than an increase in balance instability. Therefore, it is more desirable to interpret the results as an increase in exercise variability among the participants, rather than an increase in instability.
Ultimately, Polking can be seen to have the effect of increasing variability in the control of exercise by utilizing the increased activation of lower limb muscles, thereby dispersing high plantar pressure applied to the foot.
Although this study validated the potential of Polking as an exercise by comparing muscle activation and plantar pressure, it is limited in generalizability due to the fact that the study only included young adults and did not observe actual clinical effects. Therefore, there are limitations in generalizing the results.
Ⅴ. Conclusion
In conclusion, this study examined the full body effects of Polking as a combined aerobic and anaerobic exercise exercise and a potential gait training method by comparing muscle activation and plantar pressure between Polking and normal walking. Polking showed significantly higher muscle activation in both upper and lower extremities compared to normal walking and had the effect of increasing the variability of plantar pressure modulation during the exercise. Future studies are needed to verify the clinical efficacy of Polking as a joint pain relief method for people with arthritis, the elderly, and children with reduced physical fitness.
