Mechanical and physiological effects of varying pole weights during Nordic walking compared to walking

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1 DOI /s ORIGINAL ARTICLE Mechanical and physiological effects of varying pole weights during Nordic walking compared to walking Thorsten Schiffer Axel Knicker Melissa Montanarella Heiko K. Strüder Accepted: 10 November 2010 Ó Springer-Verlag 2010 Abstract The study investigated the effect of varying pole weights on energy expenditure, upper limb muscle activation and on forces transmitted to the poles during Nordic walking (NW). Twelve women [age = 21 (2) years, body mass = 60.8 (6) kg, height = 1.71 (0.06) m] participated in five 7-min walking tests randomly chosen without poles (W), with normal NW poles (NW) or with added masses of 0.5 kg (NW? 0.5) 1.0 kg (NW? 1.0) or 1.5 kg (NW? 1.5) at a speed of 2 m s -1. Heart rate (HR), relative oxygen uptake (r VO _ 2 ), blood lactate (La) and rate of perceived exertion (RPE) were registered along with surface EMG (SEMG) from biceps brachii, triceps brachii, trapecius and deltoideus muscles. Inbuilt force transducers measured reaction forces along the long axes of the poles. NW? 0.5 and NW? 1.5 showed significant increases for r VO _ 2 and RPE compared with W (p \ 0.05) but with no respective differences within NW. SEMG revealed higher activation of biceps brachii for all NW tests plus added masses compared to W (p \ 0.05). Additionally the activation of biceps brachii was higher for NW? 1.5 compared to NW (p \ 0.05). The contribution to overall activation duration of triceps brachii became lower but increased for biceps brachii with heavier poles. The increased energy expenditure during NW can be attributed to intensified muscle activation during forward swing of the poles. Heavier poles have no effect on energy expenditure compared to NW with usual poles but Communicated by Jean-René Lacour. T. Schiffer (&) A. Knicker M. Montanarella H. K. Strüder Institute of Movement and Neurosciences, German Sport University Cologne, Am Sportpark Müngersdorf 6, Cologne, Germany t.schiffer@dshs-koeln.de enhance muscular activity. Since there are no benefits concerning physiological and biomechanical parameters we do not recommend the use of heavier NW poles. Keywords Capillary blood lactate Oxygen uptake Pole reaction forces Endurance EMG Introduction Nordic walking (NW) is an established discipline among endurance sports, which is widely used in health and leisure time sports. NW is a suitable discipline to gain health benefits from regular participation in endurance sports (ACSM 1998; Lee and Paffenbarger 1998). Even though it is accepted that NW increases the metabolic and cardiovascular demands compared to walking (W) (Church et al. 2002; Porcari et al. 1997; Rodgers et al. 1995; Schiffer et al. 2006), the extent and cause for the increase is still controversial. A measurable parameter for the registration of metabolic responses associated with W and NW is oxygen uptake (r VO _ 2 ). r VO _ 2 showed differences between NW and W of 8% at 1.8 m s -1 and 7% at 2.1 m s -1 (Schiffer et al. 2006), which supported earlier findings by Rodgers et al. (1995), measuring increases of 12% at 1.86 m s -1. However, they differed markedly from the data published by Church et al. (2002) and Porcari et al. (1997) who measured differences from 20.6% (at 1.65 m s -1 ) to 23% (at 1.69 m s -1 ), respectively. The reasons for the differences between the study results were attributed to different study designs, outdoor versus treadmill exercise, different movement speeds and the skills and training status of the subjects. Porcari et al. (1997) explained their higher oxygen uptake compared to Rodgers et al. (1995) mainly with the difference of the weight of the

2 walking poles used in each study, which was 0.11 kg. Concerning the latter aspect, it is known that the use of hand weights can increase oxygen consumption during walking and running (Auble et al. 1987; Graves et al. 1987; Owens et al. 1989). In contrast, lighter hand weights have been shown to be inappropriate to increase the oxygen consumption during running and walking (Evans et al. 1994; Owens et al. 1989). Increased metabolic and hemodynamic responses to hand weights have been reported to result in higher heart rates, energy expenditure, blood pressure and higher rating of perceived exertion (RPE) (Auble et al. 1987; Bryant et al. 1993; Caroll et al. 1991; Evans et al. 1994; Graves et al. 1987; Thompson et al. 1991). The causes have been speculated to lie in higher muscular activation during arm swing. One decisive movement pattern of NW compared to running and walking is the enforcement of a pronounced arm swing, which in turn increases the work of the upper extremities. Thus lower loads can be expected to have higher influence on energy expenditure than during running and walking where additional loads can diminish the range of the arm swing. The physiological effects from NW are said to originate from additional recruitment of skeletal muscles of the upper extremities. However, there are only a few studies examining acute or training effects of NW on the upper extremities with contradictory results (Karawan et al. 1992; Kukkonen-Harjula et al. 2007). Schiffer et al. (2009) have recently reported the forces acting along the longitudinal axes of the walking poles, showing a typical force time history of pole reaction forces and proposed that the work of the upper extremities is rather a luxury effort for experienced Nordic walkers with a proper technique rather than a NW-technique-embedded characteristics which was able to contribute to overall propulsion. However muscular work must be generated to move poles and arms alike during proper NW technique. Electromyographic measurements (EMG) are appropriate means to detect muscle activations during exercise in poling disciplines (Lindinger et al. 2009; Stöggl et al. 2006) but have not yet been applied to detect muscle activation patterns during NW. The purpose of the present study was to investigate the effect of varying pole masses on energy expenditure, on arm and shoulder muscle activation patterns and on forces transmitted to the poles. Methods Subjects Twelve women with an average age, body mass and height of 21 (2) years, 60.8 (6) kg and 1.71 (0.06) m, respectively participated in this study. They were all experienced in NW on an instructor level and thus familiar with the proper usage of NW poles. All the subjects completed medical examination and physical activity questionnaires before the beginning of the experimental protocol and gave their written consent. The study was approved by the local Ethics Committee. Experimental procedure and protocols All participants carried out five 7 min walking tests on the 400 m artificial track and the field arena of the German Sport University Cologne under each of the following conditions: (W) (NW) (NW? 0.5) (NW? 1.0) (NW? 1.5) Walking without poles Nordic Walking with normal poles Nordic Walking with loaded poles (?0.5 kg) Nordic Walking with loaded poles (?1.0 kg) Nordic Walking with loaded poles (?1.5 kg) The poles were loaded with lead foil coiled around the poles directly below the strain gauge force transducer (see below). The order of the performed test was randomised. All tests were performed at a speed of 2 m s -1. In order to provide a constant speed, we used pylons which divided the track into 50 m sections. An electronic time transmitter delivered an acoustic signal with constant interval time for every 50 m, which was adjusted to the necessary speed. Food intake was standardized 2 days before the test and all participants were not allowed to be physically active the day prior to the test. Measurements and analysis Heart rate (HR) was recorded continuously with a digital heart rate monitoring system (Polar Vantage XL, Polar Electro, Kempele, Finnland). r VO _ 2 was continuously measured with a portable indirect calorimetry system (K4b 2, Version 7.4b, Cosmed, Rome, Italy). Before every test the device was calibrated according to the manufacturer s description. HR and r VO _ 2 were analysed from the last 30 s of every test. Capillary blood lactate (La) was analysed using blood samples taken from the ear-lobe (BIOSEN C line, EKF-diagnostic GmbH, Barleben, Germany) at the end of every test. After each test, the subjects were asked for their rating of perceived exertion (RPE) on a scale from 6 to 20 (Borg 1982). The forces acting along the longitudinal axes of the poles were measured directly and over a representative number of strides. For that purpose, a strain gauge force transducer was built into each of the walking poles close to the handgrips. (For details see Schiffer et al. 2009). The strain gauges were calibrated in a Z-020 Zwick material tester (Zwick Roell AG, Ulm, Germany). The output signal

3 was transferred to a data logger System (Biovision, Wehrheim, Germany) consisting of a pocket PC (HP 5450, HP Inc., Houston, TX, USA) holding a DAQP-12 32bit AD conversion card (Quatech Inc., Hudson, OH, USA) and stored in ASCII format using a PLAB data acquisition software (Stiegele Datasystems, Rothenburg o.d.t. Germany). The pocket PC and power supply for the force transducers had to be carried out by the subjects in a small backpack (weight \ 1 kg). Muscle activation was registered with bipolar EMG recordings from biceps brachii muscle, triceps brachii muscle, trapecius pars descendens muscle and deltoideus pars clavicularis muscle. Self-adhesive silver/silver chloride electrodes (Medicotest Blue Sensor N-00-S, Denmark) were positioned along the principal line of muscle action above the most prominent part of the muscle belly between the respective innervation zones and the distal end of the muscle with an interelectrode distance of \2 cm. Interelectrode resistance was reduced below 5 kx by shaving and cleansing the skin with 70% isopropanyl. EMG signals were amplified directly behind the detection electrodes by a BIOVISION (61273 Wehrheim, Germany) differential amplifier. After 12 bit AD conversion, the signals were stored synchronously together with the force transducer signals onto a personal digital assistant at a sampling rate of 1,024 Hz. Signal processing The EMG raw signals were bandpass filtered at Hz. A root mean square (RMS) algorithm was used to estimate the amplitude of muscle activation. Mean maximum muscle activation was determined as the activation RMS averaged over ten consecutive walking strides. The individual activation level found for NW without added loads was taken for normalisation and set to 100%. Muscle activation onset was set at RMS above 3 SD of baseline noise and synchronised to the initial pole ground contact identified in the pole reaction force data. Statistics The statistical evaluation was performed with SPSS (Version 13.0 for MAC OSX, SPSS Inc., Chicago, IL, USA). Statistical differences between parameter patterns were assessed by repeated measures analysis of variance (ANOVA). Bonferroni post hoc test for homogenous variances and Tamhane-T2 test for inhomogeneous variances were applied for multiple comparisons. Data is expressed as mean ± SD. The significance level for all analyses was set at a \ Results NW? 0.5 and NW? 1.5 resulted in significant increases for RPE (Fig. 1) compared with W (p \ 0.05). Similarly, r VO _ 2 during NW? 0.5 (32.0 ± 2.7 ml min -1 kg -1 ) and NW? 1.5 (32.4 ± 3.9 ml min -1 kg -1 ) was significantly higher than during W (27.5 ± 4.1 ml min -1 kg -1 ). There were no differences for these parameters between NW (29.7 ± 3.8 ml min -1 kg -1 ) and the NW tests with additional weight. RPE ranged from 11 to 17. La was significantly lower during W and NW compared with NW? 1.5 (p \ 0.05). HR was not significantly influenced by the test conditions (Table 1). The only mechanical effect, showing significant differences were the forces at the second peak representing the initial push during pole ground contact (p \ 0.05, Table 2). The force at NW? 1.5 was higher compared with NW (p = 0.46) and NW? 0.5 (p = 0.49). First and third peak forces of the pole ground contact time as well as ground contact time and total impulse showed no significant differences due to the added weight (Table 2). EMG data showed increased mean activation of biceps brachii muscle for all NW tests plus added weight compared to W (p \ 0.05) and for NW compared to NW? 1.5 (p \ 0.05). No differences were found for muscle Fig. 1 Rate of perceived exertion (RPE) and relative oxygen uptake (r VO _ 2 ) during walking (W) and Nordic walking (NW) without and with additional pole weight of 0.5 kg (NW? 0.5), 1 kg (NW? 1.0) and 1.5 kg (NW? 1.5) at a constant speed of 2 m s -1 (mean ± SD). Asterisk RPE is significantly higher with pole weights compared to W. Asterisk r VO _ 2 is significantly increased between W and NW with 0.5 and 1.5 kg additional weight. *p \ 0.05

4 Table 1 Heart rate, capillary blood lactate and MET during W and NW without and with additional pole weight of 0.5 kg (NW? 0.5), 1 kg (NW? 1.0) and 1.5 kg (NW? 1.5) at a constant speed of 2 m s -1 W NW NW? 0.5 NW? 1 NW? 1.5 Heart rate (bpm) 149 ± ± ± ± ± 11 Lactate (mmol l -1 ) 1.3 ± 0.4*,NW?1,5 1.4 ± 0.5*,NW?1,5 1.8 ± ± ± 0.6*,W,NW Data are presented as mean ± SD * Significantly different (p \ 0.05) Table 2 Forces at the first, second and third peak, impulse and contact time during pole ground contact during Nordic walking (NW) without and with additional pole weight of 0.5 kg (NW? 0.5), 1 kg (NW? 1.0) and 1.5 kg (NW? 1.5) at a constant speed of 2 m s -1 NW NW? 0.5 NW? 1 NW? 1.5 1st peak (N) 33.9 ± ± ± ± 9.0 2nd peak (N) 56.8 NW?0.5,NW?1.5 ± NW,NW?1.5 ± ± NW,NW?0.5 ± rd peak (N) 37.1 ± ± ± ± 13.6 Impulse (N s) 9.6 ± ± ± ± 7.8 Contact time(s) 0.37 ± ± ± ± 0.20 Data are presented as mean ± SD Table 3 Mean RMS values (mean ± SD) of muscle activation under test conditions Mean maximum activation (mv) W NW NW? 0.5 NW? 1 NW? 1.5 Biceps brachii muscle ± ± W ± W ± W,NW ± 0.17 Percentage of NW Triceps brachii muscle ± W ± W ± W ± W ± 0.15 Percentage of NW (=100%) Trapecius muscle 0.37 ± ± ± ± W ± 0.85 Percentage of NW Deltoideus muscle 0.12 ± ± W ± W ± W ± 0.15 Percentage of NW Upper case letters depict significant differences (p \ 0.05). Activation height relative to NW (=100%) activation of triceps brachii muscle, deltoideus pars clavicularis muscle and trapecius pars descendens muscle, except when compared to W without poles (Table 3). There is clearly a stronger activation of all muscles for Nordic walking conditions compared to walking without poles. Especially, biceps brachii muscle and triceps brachii muscle are more actively involved into movement execution during poling which becomes even more evident when pole weight is increased (Table 3). Table 4 reveals that also innervation timing is influenced by the additional loads with decreasing contribution of the triceps brachii muscle and increasing activation time for the biceps brachii muscle. Figure 2 depicts the agonist antagonist timing pattern of these two muscles over a series of stride cycles without co-contraction periods. Interestingly the period of activation of the triceps brachii muscle prior to the contact of the pole with the ground for the push becomes smaller with increasing additional loads, but it increases again under heaviest load conditions above the level of preactivation for NW. Discussion Oxygen uptake, capillary blood lactate and heart rate Data for r VO _ 2 during NW with and without weight ( ml min -1 kg -1 ) are in accordance with values of former studies with NW experienced participants (Schiffer et al. 2009). Although not statistically significant, the 8% increased r VO _ 2 during NW compared to W confirm with former study results (Rodgers et al. 1995; Schiffer et al. 2006). HR ( bpm) and La ( mmol l -1 ) were within moderate ranges throughout all tests (Bransford and

5 Table 4 Innervation timing characteristics showing total innervation time (mean ± SD) for biceps brachii muscle and triceps brachii muscle during stride cycles, and the relation between activation periods of biceps brachii muscle:triceps brachii muscle NW NW? 0.5 NW?1.0 NW? 1.5 Triceps brachii muscle preactivation time (ms) 96 ± NW ± NW ± NW,NW?0.5,NW?1.0 ± 134 Triceps brachii muscle innervation time (ms) 574 ± ± ± NW,NW?0.5 ± 83 Biceps brachii muscle innervation time (ms) 408 ± ± ± ± 81 Biceps brachii muscle:triceps brachii muscle 1:1.66 ± :1.53 NW ± 0.8 1:1.3 ± :1.03 NW ± 0.28 Fig. 2 Activation patterns (RMS) of biceps brachii muscle and triceps brachii muscle for NW? 0.5 Howley 1977; Morgan et al. 1995), demonstrating that our subjects were physiologically not overloaded by the additional loads despite the relatively high-movement speed (2 m s -1 ). Significantly increased r VO _ 2, La and RPE (p \ 0.05) between NW? 1.5 compared with W indicate a real increased exercise intensity. However, the absence of differences between NW with and without loads suggests that there is no physiological health benefit for the use of heavier poles during NW. Our results are in accordance with data from other endurance disciplines like running, walking and aerobic dance with additional hand weights. Indeed, additional weights of 2.25 and 1.36 kg in older people were suitable to increase the oxygen uptake during walking and running, respectively, but lighter weights (0.45 kg) were not appropriate (Evans et al. 1994; Owens et al. 1989). Also, during aerobic dance the use of 0.9 kg hand weights failed to increase the oxygen uptake. In this regard our data reject the suggestion by Porcari et al. (1997), who explained a higher oxygen uptake in their study compared to Rodgers et al. (1995) with different walking pole weights of 0.11 kg. EMG measurements revealed that biceps brachii muscle was the only muscle, which responded to the altered weight bearing during NW with higher activation (p \ 0.05), reflecting the need to lift and hold additional weights during the forward swing phase. These alterations appear between all NW tests and W (p \ 0.05), indicating a high importance of biceps brachii muscle for maintaining an adequate movement technique with loads. Exclusively during NW 1.5 kg, a higher exercise stress for biceps brachii muscle was found when comparing with the other NW situations (p \ 0.05). Vice versa lighter walking poles had no effects. These results are in line with our data for r VO _ 2 (increased metabolic costs for NW? 0.5 and NW? 1.5 compared to W), which can be explained by increased muscle efforts. However, even the use of 1.5 kg was not sufficient to cause increased energy costs compared to NW. With increasing loads, the biceps brachii muscle needs to produce more muscular tension to carry and accelerate the loaded pole during the pole swing phase. This is supported by a biomechanical validation of a specific upper body testing device for cross-country skiers, where EMG measurements revealed a higher activation of biceps brachii muscle in the rollerboard situation compared with double poling. Even though the movement technique in cross-country skiing (double poling) and NW are distinct from each other, as in our study no further muscles of the upper extremities were affected (Stöggl et al. 2006). TBM is mainly active during pole ground contact with decreasing contribution within the pole movement cycle as it can be seen in biceps brachii muscle:tbm ratio (Table 4). In accordance with the total impulses remaining unaffected by the pole weights, it is also consistent that the mean activation height of TBM showed no significant differences between the load situations. As in former studies the measurements of the pole reaction forces displayed three peaks (Schiffer et al. 2009). The second peak (P2) represents the initial push during the ipsilateral leg s forward swing phase. The slightly higher force at P2 compared to earlier studies under almost identical conditions (55 vs. 42 N) also results in slightly higher impulses (9.6 vs 7.7 N s -1 ) but identical contact times (0.37 s) of the poles on the ground during NW. In the present study values at P2 were significantly higher for NW? 1.5 compared to NW. Even though these differences are significant, they do not affect overall impulse generation. Higher P2 values might have occurred due to

6 the demand to control the higher moment of inertia of the heavier poles. This is also reflected in a significantly longer preactivation time of the TBM for NW? 1.5, which needs to decelerate the heavy pole before ground contact. Thus, the higher forces probably induce changes in the poling technique due to the 1.5 kg pole load. This is also indicated by the increased activation of the biceps brachii muscle and the high score for the rate of perceived exertion (Borg 1982), which cannot be explained by physiological exhaustion, since La, HR and r VO _ 2 are still in moderate zones. Moreover, the participants reported that it was difficult to control the heavier poles, especially during the forward swing phase. As the effects of added masses on energy expenditure was insignificant the altered RPE values according to the pole weight conditions must be attributed to proprioceptive inputs in muscles, joints, tendons or ligaments. Conclusions The increased energy expenditure during NW compared with walking can be attributed to increased muscle activation during the forward swing of the poles. The use of heavier poles during NW up to 1.5 kg do not have any effect on energy expenditure compared to NW with usual poles, which is in accordance with other endurance disciplines without poles, where the effects of additional loads have been studied. In general increased oxygen uptake during NW has to be attributed to increased muscular activity of shoulder and arm muscles. Using heavier poles mainly lead to enhanced involvement of biceps brachii muscle during the forward swing of the pole. It appears that the forces transmitted to the poles remain unaltered by the use of additional loads. As there are no major effects on physiological and biomechanical parameters we do not recommend the use of additional weights with NW poles. Acknowledgments We thank the subjects who devoted time and effort to the study. The study was supported by the German Track and Field Association, Deutscher Leichtathletik Verband (DLV). The performed experiment complies with the current laws of Germany. Conflict of interest We declare that there are no conflicts of interests concerning our relationship to the German Track and Field Association, Deutscher Leichtathletik Verband (DLV, governing body of NW in Germany) References ACSM written by Pollock ML, Gaesser GA, Butcher JD, Després JP, Dishman RK, Franklin BA, Garber CE (1998) Position stand. The recommended quantity and quality of exercise for developing and maintaining cardiorespiratory and muscular fitness, and flexibility in healthy adults. Med Sci Sports Exerc 30: Auble TE, Schwartz L, Robertson RJ (1987) Aerobic requirements for moving handweights through various ranges of motion while walking. Phys Sportsmed 15: Borg G (1982) The rating of perceived exertion scale. 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