INTRODUCTION
Nordic walking (NW) is a fitness activity that uses specially designed poles to engage the upper body's musculature and provides both a cardio and strength workout simultaneously (Nottingham, 2002). NW requires only three basic functional capacities: walking, trunk mobility and handgrip. In addition, it is easy to learn, can be done throughout the year and can be enjoyed at any age and any fitness level. Originally, there were two main ways to walk with poles: ExerstridingTM, almost exclusively in united States from the 1980s, and NW proper, developed in Finland in the 1990s and identified with the International Nordic Walking Association (INWA). Over the years, several other educational organisations have emerged along with variations in NW techniques. That is the case with the International Nordic Fitness Organisation (INFO) with the ALFA technique (Svensson, 2009).
In recent years, several reviews had reported the positive effects of NW as a form of rehabilitation, as a well-tolerated and safe exercise even as a primary and secondary prevention instrument (Martínez-Lemos, García-García and Serrano-Gómez, 2011; Morgulec, 2011; Fritschi, Brown, Laukkanen and uffelen, 2012; Tschentscher, Niederseer and Niebauer, 2013. One of these reviews concluded that NW is an emerging field of research in which the people without diagnosed conditions were underrepresented, and suggested an exploration of this topic on these populations (Fritschi, Brown, Laukkanen and uffelen, 2012). Up until now, few studies have been conducted on the non-clinical population and only four were truly experimental designs. They were focused in middle-aged women with health problems, such as obesity (Figard-Fabre, Fabre, Leonardi and Schena, 2011) or sedentary behaviour (Kukkonen-Harjula et al., 2007) and a special health situation such as menopause (Hagner, Hagner-Derengowska, Wiacek and Zubrzycki, 2009; Saulicz et al., 2015). The remaining studies including healthy females-all of them-were observational designs and the most frequently examined physical outcomes were cardiorespiratory measures, mainly VO2max,through a submaximal test.
upper body engagement is one of the most promising characteristics of NW, and several studies have reported increased upper body muscle involvement performing NW (Shim et al., 2013; Sugiyama, Kawamura, Tomita and Katamoto, 2013; Pellegrini et al., 2015). All were cross-sectional studies conducted with electromyography; therefore, there was no possibility to report muscular changes. Recently, a new, non-invasive and valid technique (tensiomyography-TMG) has been proposed for assessing mechanical and neuromuscular response (Šimunič, 2012). TMG gives an assessment of the contractile properties of superficial muscles. This technique overcomes the difficulty of determining the performance of each muscle within a muscle group and provides relevant information on several aspects such as muscle tone and changes in the diameter of the muscle fibre (Pišot et al., 2008), distribution of muscle fibre types (Dahmane, Djordjevič, Šimunič, Valenčič, 2005; Šimunič et al., 2011), assessment of sporting populations, muscle fatigue, recovery and symmetry (Macgregor, Hunter, Orizio, Fairweather & Ditroilo, 2018), muscle injuries control (García-García, Hernández-Mendo, Serrano-Gómez, Morales-Sánchez, 2013) or the relationship between TMG parameters and potential predictors of performance (García-García, Cuba-Dorado, Fernández-Redondo & López-Chicharro, 2018) and the contractile properties of women muscles (García-García, Serrano-Gómez and Martínez-Lemos, 2011; García-García, Cancela-Carral y Huelin-Trillo, 2015), However, an assessment of the contractile properties of the upper body muscles engaged in NW is lacking.
Therefore, there is still limited evidence regarding muscular effects after NW training in healthy youths, especially in females. In addition, to our knowledge tensiomyography response to NW have never been reported. In this context, the purpose of this study was to assess selected upper-body muscular changes following a short-term NW training in healthy young females.
MATERIAL AND METHODS
Participants
Twenty-four young females were selected for this study. To ascertain the random sampling, we recruited the volunteers via an advertisement on a University website for two weeks. They had to meet the following eligibility criteria: female, non-smoking, body mass index (BMI) < 30 kg/m2; not pregnant or breastfeeding; no previous experience in NW practice; no participation in any other current supervised exercise programme and no prescribed drugs affecting HR. After two weeks 125 contacts answered the advertisement of which 83 met the inclusion criteria and passed pre-screening to assess the homogeneity of participants. This process was conducted using three self-report questionnaires online to determine health, readiness for exercise and physical activity (PA) related eligibility: (1) PA readiness questionnaire (PAR-Q), (2) stages of change for PA behaviour questionnaire and (3) health self-report by means of SF-12 questionnaire. Readiness for exercise was defined as having answered no to all PAR-Q questions. A ‘physically active subject' was defined as being in one of the three active stages of changes PA behaviour (preparation, action or maintenance). A ‘healthy subject' was defined as being in excellent, very good or good self-reported health. Fifty-nine persons were excluded due to their not meeting the eligibility criteria, and 24 females were finally accepted to participate in the study. They were randomly assigned to an NW group (NWG) (n =12) that performed the 6 weeks NW program, or the control group (CG) (n =12), that followed your daily habits. Participants were informed of the purpose of the study and the associated risks before providing their informed consent. The study protocol was conducted in accordance with the ethical principles of the Declaration of Helsinki and bibrroved by the institutional ethics committee.
Instruments
Height and weight were measured with the participant wearing light clothing and with their shoes off using a portable stadiometer (SECA®, Mod. 217, Germany) and digital scales (SECA®, Mod. 899, Germany), respectively. BMI was calculated by dividing body weight (kg) by squared height (m2). Heart rate in beats per minute (bpm) was controlled with a monitor (Polar S810, Polar Electro Oy, Kempele, Finland) and was recorded continuously during the entire intervention. Maximal heart rate (MHR) was estimated with a specific equation for females [MHR= 206-(0.88·age)] because there is a strong, linear relationship between age and peak HR (p<0.001) achieved with exercise stress testing (Gulati et al., 2010). The participants were instructed to record the supine resting HR on three successive mornings at the same time also to calculate HR reserve [HR reserve = (MHR - Resting HR)]. In addition, VO2max was estimated indirectly with equation [VO2max=15· (HRmax/ HRresting)] from maximum and resting HR, which has shown accuracy comparable with other common VO2maxtests (uth, Sørensen, Overgaard and Pedersen, 2004). They were advised not to follow the HR monitor's display during training. They were also instructed on how to rate exertion on the Borg's Rating of Perceived Exertion (RPE) scale ranging from 6 to 20 (Borg, 1982), separately, for every one of the three 30-min set sessions, and the weighted mean of three measures was calculated.
Neuromuscular and mechanical responses of triceps brachii (TB) and deltoideus (DE) were assessed using TMG following the protocol described by García, Cancela and Trillo (2015), Radial displacement of the muscle belly was evaluated using a digital transducer (GK 30 Panoptik doo, Ljubljana, Slovenia), which was placed perpendicular to the thickest part of the muscle belly and was marked using a skin marker, in accordance with the protocol suggested by Perotto, Delagi, Lazzeti and Morrison (2005). TB: Immediately posterior to the insertion of deltoid or deltoid tubercle. DE: Halfway between the tip of the acromion and the deltoid tubercle. Self-adhesive electrodes (5x5 cm, AB Cefar-Compex Medical Co., Ltd., Malmo, Sweden) were placed symmetrically spaced 5 cm sensor (Figure 1).
Time-displacement curves were obtained through electrical stimulation lasting 1 ms. A total of 19 curves were obtained for each measured muscle of each participant. Only the maximal radial displacement curve of muscle evaluated was selected for further analysis. In each measurement the following parameters were obtained: maximal radial displacement of the muscle (Dm) measured in mm; time contraction (Tc) determined between 10 and 90% maximal response measured in ms; time delay (Td) determined between 0 and 10% of the maximum response; Radial displacement velocity (Vc) as the rate (mm·s-1) between the radial displacement occurring during the time period of Tc(Dm80) and Tc[Dm80/Tc]. In order to determine the reproducibility of the data, two measurements of the two muscles were tested for each participant. The time between the first and second measurement was between 10 and 15 min. All participants were healthy and did not perform strenuous exercise 72 hours prior to each assessment. An experienced researcher with 10 years of TMG practice performed the neuromuscular and mechanical responses assess.

Figure 1. Image showing triceps brachii neuromuscular and mechanical response assess by tensiomyography (TMG). The initial stimulus intensity was 20mA, 5mA on increasing until the maximum intensity of the electro -S2 TMG (EMF- FuRLAN & Co. doo, Ljubljana, Slovenia), located at 110mA. The deltoideus and triceps brachii muscles of both arms were assessed static and relaxed, with the participant seated, the back supported and the elbow flexed to 100º.
Procedures
Participants came to the laboratory regularly for ten weeks: the first week for baseline measures (T1), the second and third to learn NW technique on the treadmill (3/week), the fourth to the ninth to train (2/week), and the final week for post-training measurements (T2). After baseline measures, the NWG were tutored by a NW master instructor for 90 -min sessions using a proven and structured teaching method based on the guidelines of INFO. The four main principles of the ALFA functional NW technique are the following (Wilhelm, Neureuther and Mittermaier, 2009): (a) walk upright; (b) long arm movements; (c) the triangle; and (d) adapt step. All sessions of the training period were performed at the same time of day for each participant (± 30 min) in a climate-controlled laboratory. Participants in the NWG performed twice weekly on non-consecutive days for 6 weeks. The training session lasted 135 min and included: 15 min warm-up off the treadmill, 90 min NW training on the treadmill (30 min for 3 sets with 5 minute rests between sets) and 15 min of cooling down off the treadmill.
Training intensity was defined by treadmill speed and gradient. This setting was based on subjective perception of exertion by means of RPE and HR monitoring during the learning period. The final speed setting (4.6 km·hour -1= 76.6 m/min) and gradient (2%) constant were considered as endurance without the risk of participants falling. The motorised treadmill (Jaeger LE 300 C, Germany) had a 92 cm wide by 160 cm long walking deck. NW poles (Leki, Speed Pacer Vario, Leki Co., Germany) weighing bibrroximately 0.45 kg with an adjustable strap, ergonomic grip and asphalt paw. The length of the NW poles was adjusted for all participants by multiplying the subject's height by 0.7 (Wilhelm, Neureuther and Mittermaier, 2009). During the study, the participants were asked not to change any lifestyle habits, nutrition and physical activity routines. Before and after intervention, resting HR and MNR were measured using standardised procedures. During the intervention (6 weeks), intensity was controlled using both subjective (RPE) and objective (HR) measures.
Statistical analysis
The normality distribution of the data was checked by means of the Kolmogorov-Smirnov test. The criterion reference for statistical significance was set at p < 0.05 and results were expressed as means ± SD. The intra-class correlation coefficients (ICC) analysis, using single rater measurement, two-way mixed effects model and absolute agreement of TMG parameters were assessed, using two measurements of each participant with a confidence interval (CI) of 95%. ICC > 0.8 was interpreted as good reliability, while ICC < 0.8 reflected insufficient reliability (Atkinson and Nevill, 1998). To determine the difference between left and right sides a t test for paired samples was conducted. Mixed-design factorial analysis of variance (mixed ANOVA) was used to detect changes in the mechanical and neuromuscular characteristics after a NW training. Two factors were included in the mixed-design ANOVA model. Time (changes detected between assessment point 1 and 2) was used as the within-subjects variable and group (NWG vs CG) was used as the between-subjects variable. Bonferroni post hoc tests with adjustment for 95% confidence intervals were used to compare the main effects and identify significant individual differences. The effect sizes in mixed-design ANOVA were reported as partial eta square (η2) and interpreted as small (0.01), moderate (0.06) or large (0.14) (Cohen, 1988). The percentage differences in TMG parameters between the two assessments points in the muscles were also calculated and interpreted based on detectable change in the parameters. An alpha level of p<.05 was considered statistically significant. All data were analysed using SPSS v19.0 for Windows (SPSS Inc., Chicago, IL, uSA).
RESULTS
All of the 24 women completed the study and there no were dropouts or exclusions for any reason. No participant on NWG reported negative effects of exercise (muscle pain or fatigue) in the course of the training. Characteristics of the study population are shown in Table 1. No significant differences were observed between groups, and therefore, they were considered as homogeneous. During NW training means and range for objective intensity were HR (115.83 ± 15.86 bpm), maximal HR (49.87 ± 16.10%) and reserve HR (36.94 ± 10.65%), whereas for subjective intensity the RPE was 10.25 ± 1.05 points based on an original scale of 6-20 (Table 2). ICC scores reported (0.81-0.97; 95% CI) for TMG, were interpreted to reflect good reliability. The highest ICC (0.90-0.97) was obtained for Tcand Dmwhile the lowest ICC (0.89) was obtained for Td. No significant differences were found in any TMG parameter of deltoideus and triceps brachii for either side, so the values shown correspond to the mean of both arms. After 6 weeks, as shown in Table 3, there were no statistically significant differences intra-subjects (before vs. after) or inter-subjects (NWG vs. CG) for deltoideus. For triceps brachii, a moderate increase (6.25%, p=0.02, η2=0.06) after training was observed in delay time (Td) for NWG. However, Tcand Dmof both muscles have shown an increase with a low-moderate effect.
Table 1. Baseline characteristics of the participants (data are presented as mean ± SD).

Standard deviation (SD); Body mass index (BMI); Health self-perceived (HSP) describe five stage; 1=wrong, 2=regular, 3=good, 4=very good and 5=excellent. Stage of change for physical activity (SOCPA) describe five stage of change behaviour for PA; 1=Precontemplators: inactive and not thinking of becoming active, 2=Contemplators: inactive but are thinking about becoming active, 3=Preparers: intend to be physically active in the next month or have unsuccessfully taken action in the past year , 4= Action: physically active at the recommended levels but have been active for less than six months, and 5=Maintenance: physically active at the recommended levels and have been active for six or more months.; NWG ( Nordic walking group); CG (control group). 1“t” Test for compare means.
Table 2. Subjective and objective intensity report during NW training as variable control

1HRmax= %[(206-0.88)· age]
2HRmax= % [ (Resting HR) – (220-age )]
3Rate of perceived exertion
Table 3. Results of mechanical muscle response assess. TMG parameters values for both arms, pre and post NW training (mean ± SD)

Contraction time (Tc), maximal radial displacement (Dm), delay time (Td), radial displacement velocity (Vc). Mean, SD, % differences and effect size (η2), Mixed Analysis of Variance (ANOVA) p<0.05(*)
DISCUSSION
The present study indicates that 6-week light-intensity NW training was not sufficient to achieve changes in MNR of selected upper-body muscles in healthy and young females.
To our knowledge, this is the first study conducted with ALFA-functional NW technique and the first to assess muscular properties with TMG after NW training. NW is an aerobic activity for outdoor training; nevertheless, the use of treadmill has made it possible to control critical variables such as training load, dose and walking surface. However, the NW technique on a standard treadmill could be not representative of that used by most people in recreational over-ground NW (Church, Earnest and Morss, 2002). To clarify this topic, a recent study, also conducted with young and healthy people, aimed to compare the physiological response between NW on a specially designed treadmill and NW on a level over-ground surface. The conclusion was that over-ground NW created a greater physiological stress than treadmill NW performed at the same speed and distance. This difference may be due to the relatively narrow walking and poling decks on the treadmill, which made it difficult for the participants to place their poles correctly and maintain a consistent walking pattern. Increasing the width of the decks could eliminate the discrepancy (Dechman, Appleby, Carr and Haire, 2012). This previous evidence advises us to use a comfortable treadmill regarding walking area, and to include a long learning period with a supervised teaching method by a master NW instructor.
There are also serious differences between NW techniques, and therefore, the training effects may differ depending on techniques and poles used, something that was reported by a recent review (Fritschi, Brown, Laukkanen and uffelen, 2012). As all modify motor pattern, NW must be learning, because take a poles and walking with them spontaneously is not equal to perform NW, something that has already been reported by one of the few studies that included details of the technique used as well as learning and evaluation process (Figard-Fabre, Fabre, Leonardi and Schena, 2010). In our study we have tried to control these three critical variables: treadmill, NW technique and learning process.
Taking into account that means of RPE and reserve HR (%) in our study were within range of light exercise intensity (10-11 points RPE and 20-59% reserve HR) following the standard classification for healthy adults, we can conclude that this kind of workout is close to the recommendation for health-enhancing physical activity for adults (Haskell et al., 2007).
A good reproducibility in TMG parameters has been reported using the ICC with 95% confidence interval (CI) by Martín-Rodríguez, Loturco, Hunter, Rodríguez-Ruiz & Munguía-Izquierdo (2017). Considering an ICC of 0.8 reflecting a good reproducibility, and a lower value as insufficient reproducibility (Atkinson and Nevill, 1998), it can be said that in this case, good reproducibility is confirmed, on the same day, for the TMG assessment. For NWG, the training programme bibrears to have produced a slight increase in TMG parameters, although not significant, after the intervention at triceps brachii and deltoideus. The values of Tcand Tdthat are not significant, except for the triceps brachii Td(6.25%; p = 0.02), and a small or large effect size (η2between 0.02 and 0.15). One possible explanation is that the training programme caused a slight increase in motor units of slow fibres type I (slow), which occurred as a result of the NW training as aerobic activity; this is because the Tchas been linked to the distribution of muscle fibre types (Dahmane, Djordjevič and Šmerdu, 2006), specifically the percentage of slow fibres type I (Šimunič et al., 2011; Travnik et al., 2013; Dahmane et al., 2005). That is, a higher Tc indicates a greater percentage of slow fibres type I.
Furthermore, a slight increase in the values of Dm of both muscles (η2= 0.04 and 0.02, respectively) has also been found, which has not proven to be significant. This is in line with the increase that occurred in athletes after 10 days of endurance training (Kerševan, Valenčič, Djordjevič and Šimunič, 2002); however, professional road cyclists have not experienced any change in their lower limbs Dm(García-García, Cancela-Carral, Martínez-Trigo and Serrano-Gómez, 2013), and Dmdecreased with different lower limb strength training protocols correlated with decreases in maximal voluntary isometric contraction (de Paula et al., 2014). The Dmis related to the changes in pitch and diameter of the muscle and to changes in the mechanical properties of the tendon (Pišot et al., 2008) particularly, an increase of Dmvalue is associated with a decreased muscle tone. In fact, the TMG parameters for deltoideus are similar to woman elite kayakers (García-García et al., 2015), except in Dmvalues, where women elite kayakers have a lower Dm(4.4 ± 0.8 mm), that is, a higher muscular tone, than NWG and GC groups in all assessments. However, Dmhas also been associated with neuromuscular peripheral fatigue, because although their values tend to decrease, increasing muscle stiffness, after use of force in the biceps brachii (García-Manso et al., 2012), they tend to increase their values during highly stressed resistance such as ultra-endurance triathletes (García-Manso et al., 2011). Both facts are perfectly compatible, because the neuromuscular system reacts differently depending on the stimulus to which it is subjected, but both can also be taken as an immediate response after effort, not as an acute effect caused by these stimuli. A more plausible explanation for this case is that the Dmexplains, to some extent, the accumulative fatigue in NW training period in active young women. However, this question still needs to be clarified.
It has been suggest that the activity of NW could add value in the use of the muscles of the shoulder and elbow, which would make the NW prove to be a more complete activity. In fact, the use of walking poles significantly increases muscle activity, measured by iEMG, in deltoideus and triceps brachii (Shim et al., 2013). This muscular activity of the triceps brachii, during NW, increases with enhanced walking speed (Sugiyama, Kawamura, Tomita y Katamoto, 2013). Judging by the results, it bibrears that the duration of activity (2 sessions for 6 weeks), training volume (1.5 hours per session) and intensity (49.87% maximal HR) has not been enough to cause a marked change in the contractile properties of the muscles of the shoulder and elbow. Furthermore, it has been suggested that in order to detect an actual change in the Tc, Dm, and Tdparameters, differences should exceed 20% (Ditroilo, Smith, Fairweather y Hunter, 2013). This finding indicates that a longer duration, volume and/or intensity of the NW activity would be required to cause a sharp change in muscle contractile properties in physically active young women.
This study has several limitations such as the relatively small size of the sample, no direct measures of maximal physiological parameters (VO2) or the lack of follow-up measures to assess the evolution of the changes over time and, specifically, to estimate any possible recoil.