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

Introduzione La tecnica di duble poling (DP) ha recentemente assunto una crescente importanza durante le gare di sci di fondo in tecnica classica, divenendo la tecnica dominante o esclusiva durante alcune tipologie di gara. Il DP \ue8 un tipo di locomozione molto particolare, poich\ue9 le azioni propulsive vengono trasmesse al suolo attraverso i bastoncini, grazie ad azioni di spinta sinergiche di arti superiori e tronco. L\u2019obiettivo principale di questo progetto \ue8 quello di verificare se gli sciatori di fondo utilizzino strategie biomeccaniche per facilitare o aumentare l\u2019impiego delle forze di spinta e l\u2019economicit\ue0 della locomozione, o per fronteggiare l\u2019insorgenza della fatica. Per soddisfare tali proposte, \ue8 stata eseguita un\u2019analisi integrata della tecnica di DP, includendo valutazione metaboliche, biomeccaniche ed elettromiografiche di tale locomozione. Studio 1 L\u2019obiettivo del primo studio \ue8 stato una valutazione delle relazioni esistenti tra costo energetico e spostamento del centro di massa (COM) durante il DP. Otto sciatori di alto livelli (HLG) e otto di livello regionale (RLG) hanno eseguito una prova sottomassimale in DP, sciando con ski roll per 5 minuti su treadmill a 14 km\u2022h-1 e 2\ub0 di pendenza. Le variabili analizzate sono state il costo energetico (ECDP), il range di spostamento verticale del COM, l\u2019inclinazione del corpo (\u3b8, ovvero l\u2019angolo esistente tra la linea verticale e la linea passante per il COM ed un punto fisso identificato a livello dei piedi) ed il lavoro meccanico associato al movimento del COM. Inoltre, sono state misurate la cinematica articolare e dei bastoncini, le forze di spinta e la tempistica del ciclo di movimento. Una forward multiple regression analysis \ue9 stata eseguita per identificare quali parametri relativi al movimento del COM possono predire il ECDP. HLG hanno mostrato un minor ECDP rispetto a RLG (3.37 \ub1 0.16 vs. 4.83 \ub1 0.57 J\u2022m-1\u2022kg-1), un minor spostamento verticale del COM ed un minor lavoro meccanico. In HLG, \u3b8 durante la prima parte della fase di spinta \ue8 stata maggiore, l\u2019inclinazione dei bastoni elevata, le forze di spinta maggiorate e la durata del ciclo pi\uf9 lunga. Considerando tutti gli sciatori, il massimo valore di \u3b8 (\u3b8max) ed il minimo valore di spostamento verticale del COM sono risultati prenditori significativi di ECDP (AdjR2 = 0.734; P < 0.001). Inoltre, \u3b8max \ue9 risultato positivamente correlate con gli integrali di forza di spinta e con la durata del ciclo. Durante il DP, un movimento meccanicamente vantaggioso del COM nelle direzioni verticale e antero-posteriore gioca un ruolo importante nella determinazione del range di spostamento verticale del COM, dell\u2019inclinazione dei bastoni, della generazione delle forze di spinta e della durata del ciclo, influenzando ultimamente il costo energetico della locomozione. Studio 2 L\u2019obiettivo del secondo studio \ue9 stato una valutazione delle modificazioni biomeccaniche nel gesto di DP che si verificano dopo un esercizio specifico ad alta intensit\ue0. Otto sciatori di alto livello hanno eseguito una prova sottomassimale di 5 minuti (20 km\u2022h-1 e 1\ub0 di pendenza) prima (PRE) e dopo (POST) un test massimale ad esaurimento in DP, utilizzando ski roll su treadmill. Sono stati considerati i parameteri metabolici, lo spostamento verticale del COM, l\u2019inclinazione del corpo (\u3b8), la cinematica articolare e dei bastoncini, le forze di spinta e la tempistica del ciclo. Inoltre, \ue8 stata misurata la fatica muscolare nei muscoli triceps brachii, latissimus dorsi and teres major, considerando l\u2019indice di fatica proposto da Dimitrov (FInms5) durante segmenti specifici di segnale elettromiografico, registrati durante le fasi di spinta. Il progressivo aumento di FInms5 nei muscoli latissimus dorsi and teres major durante cicli consecutivi di DP, l\u2019elevata concentrazione di acido lattico (P = 0.001), l\u2019aumento di percezione dello sforzo (P = 0.005) e la riduzione e della capacit\ue0 di esprimere forza di spinta (P = 0.020) hanno delineato uno stato di fatica durante la prova POST. Ciononostante, non sono state trovate differenze tra le prove PRE e POST nello spostamento verticale del COM (P = 0.968), nell\u2019inclinazione del corpo (P = 0.087), nella cinematica articolare e dei bastoncini (P = 0.415). La fatica ha portato ad una riduzione nella durata della fase di recupero (P < 0.001) e dell\u2019intero ciclo di movimento (P = 0.001). E\u2019 stata confermata una correlazione positive tra \u3b8max e gli integrali di forza di spinta (P = 0.06), nonch\ue9 tra \u3b8mac e la durata del ciclo (P = 0.02). Durante il DP, la cinematica del corpo non varia prima e dopo un esercizio affaticante ad alta intensit\ue0, in sciatori di fondo di alto livello. La ridotta capacit\ue0 di esprimere forza di spinta dopo l\u2019affaticamento porta ad eseguire cicli di DP pi\uf9 corti e frequenti, e deriva dallo stato di affaticamento muscolare localizzato piuttosto che da un\u2019alterazione nel pattern cinematico del corpo. Studio 3 L\u2019obiettivo del terzo studio \ue9 stato una valutazione dell\u2019efficacia dello stretch-shortening cycling (SSCEFF) in alcuni muscoli estensori degli arti superiori, durante la tecnica di DP. A questo scopo, SSCEFF \ue9 stata analizzata in relazione alla velocit\ue0 di DP ed al livello di performance degli atleti. Undici atleti d\u2019elite di sci di fondo hanno eseguito un test incrementale massimale per determinare la massima velocit\ue0 di DP (Vmax). Dopodich\ue9, le caratteristiche del ciclo, la cinematica dell\u2019angolo al gomito e le forze di spinta sono state monitorate durante l\u2019esecuzione del DP su treadmill con ski roll, a due velocit\ue0 sottomassimali e ad una velocit\ue0 di gara (85% of Vmax). E\u2019 stata calcolata l\u2019attivit\ue0 elettromiografia media registrata nei muscoli triceps brachii e latissimus dorsi durante le fasi di flessione ed estensione del gomito dell\u2019intera fase di spinta (EMGFLEX, EMGEXT), e durante la fase di pre-attivazione, identificata prima dell\u2019inizio della fase di spinta (EMGPRE). Per ogni muscolo, \ue9\u2019 stato definito SSCEFF il rapporto tra EMGFLEX e aEMGEXT. EMGPRE and EMGFLEX sono aumentati incrementando la velocit\ue0 di DP in entrambi I muscoli (P<0.01), cos\uec come SSCEFF (da 0.9\ub10.3 a 1.3\ub10.5 per il muscolo triceps brachii and da 0.9\ub10.4 a 1.5\ub10.5 per il muscolo latissimus dorsi) e la forza di spinta (da 253\ub133 a 290\ub136 N; P<0.05). Inoltre, SSCEFF ha mostrato una correlazione positiva con Vmax, EMGPRE e EMGFLEX (P<0.05). Gli adattamenti neuromuscolari che si verificano durante il DP a velocit\ue0 pi\uf9 elevate, quando una maggiore forza di spinta deve essere applicata al suolo, esercitano una grande influenza sulla performance degli sciatori di alto livello. Conclusione generale Questo progetto ha dimostrato che le strategie biomeccaniche e neuromuscolari adottate dagli sciatori di fondo durante la tecnica di double poling hanno una grande importanza nel determinare la capacit\ue0 di esprimere forza di spinta, il costo energetico della locomozione e la stessa performance Questi risultati portano ad utili considerazioni nell\u2019ottimizzazione dei metodi per allenare la forza specifica, la tecnica e le procedure di valutazione, specialmente in sciatori di fondo di alto livello.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\u2022h-1 and 2\ub0 inclination. Energetic cost (ECDP), COM vertical displacement range, body inclination (\u3b8, 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 \ub1 0.16 vs. 4.83 \ub1 0.57 J\u2022m-1\u2022kg-1), smaller COM vertical displacement range and mechanical work, higher \u3b8 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 \u3b8 (\u3b8max) and the minimum value of COM vertical displacement resulted significant predictors of ECDP (AdjR2 = 0.734; P < 0.001). Furthermore, \u3b8max 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\u2022h-1 and 1\ub0 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 (\u3b8), 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\u2019 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 \u3b8max and integrals of poling force (P = 0.06), as well as between \u3b8mac 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\ub10.3 to 1.3\ub10.5 for the triceps brachii and from 0.9\ub10.4 to 1.5\ub10.5 for the latissimus dorsi) and poling force (from 253\ub133 to 290\ub136 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

Gait models and mechanical energy in three cross-country skiing techniques.

Fluctuations in mechanical energy of the body center of mass (COM) have been widely analyzed when investigating different gaits in human and animal locomotion. We applied this approach to estimate the mechanical work in cross-country skiing and to identify the fundamental mechanisms of this particular form of locomotion. We acquired movements of body segments, skis, poles and plantar pressures for eight skiers while they roller skied on a treadmill at 14 km h 121 and a 2 deg slope using three different techniques (diagonal stride, DS; double poling, DP; double poling with kick, DK). The work associated with kinetic energy (KE) changes of COM was not different between techniques; the work against gravity associated with potential energy (PE) changes was higher for DP than for DK and was lowest for DS. Mechanical work against the external environment was 0.87 J m 121 kg 121 for DS, 0.70 J m 121 kg 121 for DP and 0.79 J m 121 kg 121 for DK. The work done to overcome frictional forces, which is negligible in walking and running, was 17.8%, 32.3% and 24.8% of external mechanical work for DS, DP and DK, respectively. The pendulum-like recovery (R%) between PE and KE was ~45%, ~26% and ~9% for DP, DK and DS, respectively, but energy losses by friction are not accounted for in this computation. The pattern of fluctuations of PE and KE indicates that DS can be described as a \u2018grounded running\u2019, where aerial phases are substituted by ski gliding phases, DP can be described as a pendular gait, whereas DK is a combination of both

Biomechanical and energetic determinants of technique selection in classical cross-country skiing.

Classical cross-country skiing can be performed using three main techniques: diagonal stride (DS), double poling (DP), and double poling with kick (DK). Similar to other forms of human and animal gait, it is currently unclear whether technique selection occurs to minimize metabolic cost or to keep some mechanical factors below a given threshold. The aim of this study was to find the determinants of technique selection. Ten male athletes roller skied on a treadmill at different slopes (from 0\ub0 to 7\ub0 at 10 km/h) and speeds (from 6 to 18 km/h at 2\ub0). The technique preferred by skiers was gathered for every proposed condition. Biomechanical parameters and metabolic cost were then measured for each condition and technique. Skiers preferred DP for skiing on the flat and they transitioned to DK and then to DS with increasing slope steepness, when increasing speed all skiers preferred DP. Data suggested that selections mainly occur to remain below a threshold of poling force. Second, critically low values of leg thrust time may limit the use of leg-based techniques at high speeds. A small role has been identified for the metabolic cost of locomotion, which determined the selection of DP for flat skiing

Nordic walking versus walking: energy expenditure and muscle activation

Nordic Walking has increased in popularity in the last decades as a form of exercise that can be adapted for various populations

The effectiveness of stretch-shortening cycling in upper-limb extensor muscles during elite cross-country skiing with the double-poling technique.

This investigation was designed to evaluate the effectiveness of stretch-shortening cycling (SSCEFF) in upper-limb extensor muscles while cross-country skiing using the double-poling technique (DP). 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±36N; 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