Biomechanical and energetic aspects of cross-country skiing double poling technique

Introduction Double poling (DP) is increasingly used during classic cross-country skiing races, becoming the dominant or exclusive technique used over the entire track, in some typology of competitions. This technique is a very particular type of locomotion, since the propulsion is mainly generated through the poles, thanks to the synergic poling actions of upper-limbs and trunk, exerted during the poling phase. The principal aim of this project was to verify weather biomechanical strategies are adopted by cross-country skiers to enhance the poling force exertion, the economy of locomotion, or to face fatigue. To accomplish these purposes, an integrative analysis of the technique was performed, including metabolic, biomechanical end EMG evaluations. Study 1 The aim of the study 1 was to evaluate relationships between energetic cost and COM displacement in DP cross-country skiing. Eight high-level (HLG) and eight regional-level (RLG) cross-country skiers performed a 5-min sub-maximal DP trial while roller skiing on a treadmill at 14 km•h-1 and 2° inclination. Energetic cost (ECDP), COM vertical displacement range, body inclination (θ, i.e. the angle between the vertical line and the line passing through COM and a fixed pivot point identified at feet level) and mechanical work associated to COM motion were analyzed. Pole and joint kinematics, poling forces and cycle timing were also considered. A forward multiple regression analysis was used to identify the COM-related parameters better predicting ECDP. HLG showed lower ECDP than RLG (3.37 ± 0.16 vs. 4.83 ± 0.57 J•m-1•kg-1), smaller COM vertical displacement range and mechanical work, higher θ during the early part of the poling phase. In HLG, pole inclination was higher, poling forces greater and cycle duration longer. Considering all skiers, the maximum value of θ (θmax) and the minimum value of COM vertical displacement resulted significant predictors of ECDP (AdjR2 = 0.734; P < 0.001). Furthermore, θmax was positively related to the integrals of poling force and to the cycle duration. During DP, a mechanically advantageous motion of COM in vertical and antero-posterior dimensions plays an important role in determining COM vertical displacement range, pole inclination, generation of poling force and cycle duration, finally influencing the energetic cost of locomotion. Study 2 The aim of the study 2 was to evaluate the biomechanical changes occurring in the DP technique after a high-intensity DP skiing exercise. Eight high-level cross-country skiers performed a 5-min sub-maximal DP trial (20 km•h-1 and 1° inclination) before (PRE) and after (POST) a maximal DP test to exhaustion, while roller skiing on the treadmill. Metabolic parameters, COM vertical displacement, body inclination (θ), pole and joint kinematics, poling forces and cycle timing were considered. Furthermore, muscle fatigue was measured in triceps brachii, latissimus dorsi and teres major muscles, by considering the Dimitrov’ fatigue index (FInms5) of specific EMG-signal segments recorded during the poling phase. An increasing trend of FInms5 across consecutive DP cycles in latissimus dorsi and teres major muscles, higher blood lactate concentration (P = 0.001), elevated rate of perceived exertion (P = 0.005), together with a reduction of poling force exertion (P = 0.020) delineated a state of fatigue during POST. However, no statistical differences were found in COM vertical displacement (P = 0.968), body inclination (P = 0.087), joint and pole kinematics (P = 0.415) between PRE and POST. Cycle characteristics were affected by fatigue, showing a reduction in recovery phase duration (P < 0.001) and cycle duration (P = 0.001). A positive relationships between θmax and integrals of poling force (P = 0.06), as well as between θmac and cycle duration (P = 0.02) were confirmed. While DP skiing, body kinematics is maintained unaltered in high-level skiers, before and after a high-intensity fatiguing exercises. The reduced poling force exertion capacity after fatigue lead to more short and frequent DP cycles. However, it seems due to the state of localized muscle fatigue rather that to an alteration in the body kinematic pattern. Study 3 The aim of the study 3 was to evaluate the effectiveness of stretch-shortening cycling (SSCEFF) in upper-limb extensor muscles while cross-country skiing using the DP technique. To this end, SSCEFF was analyzed in relation to DP velocity and performance. Eleven elite cross-country skiers performed an incremental test to determine maximal DP velocity (Vmax). Thereafter, cycle characteristics, elbow joint kinematics and poling forces were monitored on a treadmill while skiing at two sub-maximal and racing velocity (85% of Vmax). The average EMG activities of the triceps brachii and latissimus dorsi muscles were determined during the flexion and extension sub-phases of the poling cycle (EMGFLEX, EMGEXT), as well as prior to pole plant (EMGPRE). SSCEFF was defined as the ratio of aEMGFLEX to aEMGEXT. EMGPRE and EMGFLEX increased with velocity for both muscles (P<0.01), as did SSCEFF (from 0.9±0.3 to 1.3±0.5 for the triceps brachii and from 0.9±0.4 to 1.5±0.5 for the latissimus dorsi) and poling force (from 253±33 to 290±36 N; P<0.05). Furthermore, SSCEFF was positively correlated to Vmax, to EMGPRE and EMGFLEX (P<0.05). The neuromuscular adaptations made at higher velocities, when more poling force must be applied to the ground, exert a major influence on the DP performance of elite cross-country skiers. General conclusion The present project demonstrated that the biomechanical and neuromuscular strategies adopted by cross-country skiers during the double poling skiing are of great importance in determining poling force exertion capacity, energetic cost of locomotion and double poling performance. These findings lead to useful considerations to optimize strength training methods, technical training sessions and testing procedures, especially for high-level cross-country skiers.