Double Poling Incross-Country Skiing : Biomechanical and Physiological Analysis of Sitting and Standing Positions

Sammanfattning: Double poling (DP) is a sub-technique in cross-country skiing that has increased in interest over the last decades, e.g. athletes in cross-country skiing have increased their utilisation of double poling during competitions. In cross-country sit-skiing athletes with impairments in legs and/or trunk sit in a sledge and utilise DP to propel themselves. Technique (i.e. movement pattern) is one key factor determining performance but also a factor that may affect the risk of overuse injuries in sports.Therefore, the overall aim of the thesis was to improve the understanding of the human movement technique in cross-country skiing DP, in both standing (paper I-II) and sitting positions (paper III-IV, Thesis A-B) using biomechanical and physiological measurements and inverse dynamics simulations. All studies were carried out on a double poling ergometer in laboratory. Three experimental studies were performed with able-bodied participants (papers I-II, IV-VI), one study with one participant with growth defect in the legs (paper III), and one study (Thesis B) with one participant with complete spinal cord injury at thoracic vertebra 4.In paper I the first full-body simulation of DP was performed and results were comparable to results found in literature when the kinematics and external kinetics were similar. Paper II showed how increased leg utilisation increased performance (forward impulse) but reduced skiing efficiency (output work divided by metabolic muscle work). These results indicate that both high performance (power output) and efficiency may not be achieved in the same technique.In sitting DP many different sitting positions are utilised. Athletes with full muscle control in hip and trunk mainly sit with their knees lower than their hips (KLnoS). Athletes with paralysis in lower trunk and legs need trunk stability from the sit-ski. Most often, this is achieved by adopting a knees higher than hips (KH) position together with a support for the lower back. However, this position might induce large flexion in the spine, which is hypothesised to affect injury risk in the shoulders and lower back. This thesis has enabled the knees low sitting position for athletes with paralysis in the lower trunk and legs by supporting the anterior trunk with the sledge (KL). In sitting DP in athletes with full hip and trunk muscle control, high performance was achieved through proximal-distal sequencing from the hips through the trunk to the arms, and large muscle work in spine and legs (IV, V, Thesis A). In order of performance, KLnoS utilised muscles in the hips-spine-arms, compared with utilisation of spine-arms in KH, and mainly arms in KL. Higher amount of activated muscle mass resulted in lower relative anaerobic metabolism during submaximal exercise (IV).The lower back joint reactions were higher for the sitting position with larger spinal flexion, KH compared to KL (VI). These results suggest that there is an increased risk of injury in the lower back for the sitting position KH. Athletes with paraplegia generally have a high risk of injuries in the shoulders. The results of this thesis showed higher shoulder joint reactions in the sitting position with larger shoulder-arm muscle work, in KL compared to KH.For the case study with one participant with thoracic spinal cord injury (Thesis B) highest performance was achieved in the KH sitting position where spinal flexion occurred at the beginning of the poling phase. When comparing the fixed trunk positions KL and KHS, higher performance was achieved in KHS. It was speculated that the difference between KL and KHS was due to the impairment of the vasoconstriction in paralysed muscles. The effect of gravity on venous pooling is probably larger when the legs are lower down as in KL. This effect was not present for individuals without paralysis (III), where KL was more economical than KHS.Parasport classification needs evidence of how impairment affects sporting performance (Tweedy et al., 2014, Tweedy and Vanlandewijck, 2011). Classification might benefit from simulations as performed in this thesis. The musculoskeletal simulations of seated DP in paper V and the KLnoS position presented in the thesis have showed the relative contribution of different muscle groups on performance. These results are novel and might contribute to improvement of the classification system.