In cross-country skiing (XCS), the double poling (DP) technique is getting more and more important in classical races. Nowadays, an increasing number of classic sprint competitions are performed completely with the DP technique. Furthermore, the DP technique, used at the finish line often decides the outcome in classical technique mass start competitions. This development can be explained by enhanced upper-body strength capacities, combined with enhanced technical aspects in elite skiers, as well as better track preparation, and superior equipment (Holmberg et al., 2005).
Therefore, it is important to understand the basic mechanisms to gain maximal DP velocity. Technical aspects, such as longer cycle length, play an important role (Stoggl and Holmberg, 2011a; Stoggl et al., 2007), but also depend on strength parameters. Whilst leg and trunk strength are known to be important factors to increase DP performance (Rud et al., 2014; Zoppirolli et al., 2017), it has been suggested that upper-body power is a decisive factor in XCS sprint (Stoggl et al., 2007). However, most of the previous studies concerning strength in DP were performed as XCS specific strength tests on a cable pulley (Hoff et al., 1999; Osteras et al., 2002), a DP ergometer (Nilsson et al., 2004) or a rollerboard (Stoggl et al., 2007) and referred to the DP performance on an ergometer or a treadmill. Hence, there is a lack of studies relating general strength, i.e., strength measured separately for different body parts to direct field-test parameters like pole force and maximal velocity (Vmax) in sprint distance. Only a few studies dealing with general strength in DP, using correlation and regression analysis to reveal that upper-body strength parameters have a profound impact on Vmax (Mikkola et al., 2010; Ng et al., 1988; Osteras et al., 2016; Stoggl et al., 2011b). However, there is, to date, only one previous study that has recorded both pole force and strength parameters in concert (Stoggl et al., 2011b).
For many winter sport disciplines, several concepts for general strength tests have been developed. Olympic Training Centers (e.g. Olympic Training Center of Bavaria) regularly apply trunk and leg strength tests for elite athletes (e.g. hockey, bobsleigh) on a motor-driven dynamometer. To the best of the authors' knowledge, in XCS these tests rarely exist, although they could give e.g. insights into the muscular status of an athlete using a high-standardized setup or allow cross-sectional and longitudinal comparisons among different athletes.
It is hypothesized that the resulting pole forces and general maximum strength in the upper-body segments relate to each other strongly and predict DP performance at maximal velocity.
Thirteen recreational cross-country skiers volunteered to participate in the present study. The study population consisted of two females and eleven males participants, who performed XCS races in their adolescence regularly. The median of self-reported fitness of the athletes was 2 (3-4 non-specific training session/week), on a scale from 1 (very good) to 6 (very poor). The skiers' mean [+ or -] SD age was 29 [+ or -] 4 years, height was 1.82 [+ or -] 0.06 m, weight was 77 [+ or -] 6 kg, classical pole length was 1.55 [+ or -] 0.06 m. The subjects were instructed to carry out only light training two days before the measurements. Participants were informed about the design of the study, with special information on possible risks and benefits, and subsequently voluntarily signed an informed consent document before the start of the study. The ethic statement for this study was approved by the Dean of the Faculty of Sports and Health Sciences of the Technical University Munich. All tests were conducted according to the Declaration of Helsinki.
Two pairs of classic roller skis (same model) were used for the test (Marwe Oy, Hyvinkaa, Finland) and skiers used their own ski boots. One pair of classic roller skis had a NNN (Rottefella) binding system (n = 9), and the other had a SNS (Salomon) binding system (n = 4). Prior to the test, the roller skis were pre-warmed by the skiers on the tartan track for 15 minutes, to ensure the same rolling resistance for each skier during the measurements.
One-dimensional piezoelectric force transducers (Kistler Instrumente AG, Winterthur, Switzerland, length 4.5 cm, diameter 2.2 cm, weight 95 g were firmly attached (screw system) on the proximal end of the poles, underneath the grip in line with the long axis of the poles. To avoid the influence of signal drifting, the force transducers were reset immediately prior to each trial. The signals were recorded at a sampling rate of 256 Hz and stored via cable on a data logger (MSR165, MSR Electronics GmbH, Seuzach, Switzerland) placed in a belt bag. Four pairs of poles (LEKI-Sport, HM-Carbon, Kirchheim/Teck, Germany) were available to host the force transducer. Each pair was adjustable in length (7 cm) and was adapted to each individual's classic pole length (85.0 [+ or -] 1.5% of body height).
The torque of the different joints was measured with a sampling rate of 200 Hz on a motor-driven dynamometer (IsoMed 2000, D&R Ferstl GmbH, GER). During all tests, subjects were seated (backwards inclination 70[degrees] for the arm strength tests) and fixed by straps to avoid redundant movements (Figure 1 a-c).
The DP test and the strength test were performed during a 14-day period with a minimum break of 24 hours in between. For familiarization of the isokinetic strength tests, all subjects did a complete test-setup in advance.
The DP test took place on an outdoor tartan track. In a first step, subjects performed a 15 min lasting DP warm-up at a self-selected effort with their own poles. Subsequently, the skiers had a passive recovery (10-15 min), to get the sensor poles and the wires adapted. The second warm-up lasted 5 min including a 60 m test sprint with the sensor poles. For the main test which started right after the second warm-up, subjects had to accelerate 40 m to reach maximal speed, immediately followed by a 60 m DP sprint. The minimum break between the two maximum trials was 3 minutes. The mean velocity of the 60 m DP sprint is called Vmax in the following manuscript. At first, part of the subjects complained about the slow surface of the tartan track, but quickly adapted. Some of the subjects actually trained regularly on tartan track.
The strength tests started with a 15 min warm-up on a rowing ergometer at a self-selected resistance. The test session included an isometric and concentric test each consisting of trunk flexion and extension as well as left and right elbow and shoulder extension measurements. For both maximum voluntary isometric and concentric tests, the subjects were advised to develop force as quickly as possible and try to maintain the strength. During each test, the instructor gave maximum verbal encouragement. The isometric contraction lasted approximately 5 s. The concentric tests were performed as an extension-flexion motion, three...