Search

This study focused on resolving the differences in economy between two common sit-skiing postures used by disabled athletes, suspected to be the most and least effective. Ten experienced non-disabled male cross-country skiers went through an incremental testing protocol with an ergometer simulating double poling in two sitting postures “kneeing” and “knee-high.” The protocol consisted of 3 × 4 min steady-state stages (13, 22, and 34% of maximal sprint power output). Subjects' respiratory gases and heart rate were measured and blood lactate concentrations were determined. In addition, pulling forces and motion capture recordings were collected. Oxygen consumption was 15.5% (p < 0.01) higher with “knee-high” compared to “kneeing” at stage three. At stage three cycle rate was 13.8% higher (p < 0.01) and impulse of force 13.0% (p < 0.05) and hip range of motion 46.6% lower (p < 0.01) with “knee-high” compared to “kneeing.” “Kneeing” was found to be considerably more economical than “knee-high” especially at 34% of maximum sprint power output. This might have been due to higher cycle rate, lower impulse of force and smaller hip range of motion with “knee-high” compared to “kneeing.” This indicates that sit-skiers should adopt, if possible, posture more resembling the “kneeing” than the “knee-high” posture. Combining such physiological and biomechanical measurements and to further develop them to integrated miniature wearable sensors could offer new possibilities for training and testing both in the laboratory and in the field conditions. peerReviewed

Arm swing has been shown to lead to greater maximal speed and movement economy in cross -country skiing. The current study aimed to investigate how arm swing alters skiing mechanics and contributes to performance and acceleration of the athlete’s centre of mass (COM). While skiing on snow seven highly skilled cross -country skiers simulated V2 -alternate skating without using ski poles and with double or single arm swing and without arm swing . During leg push -off the linear momentum of the body increased due to arm swing. Simultaneously, linear momentum of the arm(s) decreased in arm swing trials, indicating a transfer of momentum from arms to the rest of the body and being more prevalent with double arm swing compared to single arm swing (all P < 0.05). Greater maximal skiing speeds were reached with single and double arm swing, while the forward lean angle, the force leading to acceleration of COM in skiing direction and the force effectiveness increased (all P < 0.05). The effects of less mass moving in single arm swing could be compensated by carrying out the arm swing faster, almost aligned in skiing direction and with a “long arm” pattern, indicating how arm swing can be conducted efficiently. peerReviewed

This study investigated how the fatigue caused by a 20-km simulated skating cross-country skiing race on snow affects the final spurt performance from a biomechanical perspective. Subjects performed a 100-m maximal skiing trial before and at the end of the simulated race. Cycle characteristics, ground reaction forces from skis and poles, and muscle activity from eight muscles were recorded during each trial. Results showed that subjects were in a fatigued state after the simulated race manifested by 11.6% lower skiing speed (P

Cross country sit skiing is a Paralympic discipline in which athletes due to physical impairment ski sitting on a sit-ski. The impairment influences performance directly and also through sitting position. Athletes with a better trunk control usually adopt a sitting position called “kneeing” in which the hip joints are higher than the knee joints. In contrast, athletes with high impact of impairment prefer a sitting position called “knee high” in which the hip joints are lower than the knee joints. Able bodied athletes skiing on the ergometer in these two sitting positions showed different performance. However, to the best of authors’ knowledge, no studies have examined performance, force production, and cycle characteristics of athletes with physical impairment who ski using different sitting positions. Therefore, the aim of the present study is to assess if athletes with physical impairment sitting in “kneeing” and “knee high” position perform differently while skiing. To obtain this purpose, a k-means cluster analysis was used to group ten male athletes according to their skiing performance in terms of maximal speed, generated force, and cycle characteristics. Cluster analysis results showed athletes aggregation in two clusters and a very high agreement between clusters analysis outcome and real athletes sitting position. Maximal speed and generated force were the most relevant variables for grouping athletes. Despite some limitations, these results suggest that the cluster analysis can be used to discriminate athletes according to their performance.

The punctum fixum-punctum mobile model has been introduced in previous publications. It describes general principles of intersegmental neuromuscular succession patterns to most efficiently generate specific movement intentions. The general hypothesis of this study is that these principles-if they really do indicate a fundamental basis for efficient movement generation-should also be found in intramuscular coordination and should be indicated by "longitudinal sequencing" between fibers according to the principles of the punctum fixum-punctum mobile model. Based on this general hypothesis an operationalized model was developed for the rectus femoris muscle (RF), to exemplarily scrutinize this hypothesis for the RF. Electromyography was performed for 14 healthy male participants by using two intramuscular fine wire electrodes in the RF (placed proximal and distal), three surface electrodes over the RF (placed proximal, middle, and distal), and two surface electrodes over the antagonists (m. biceps femoris and m. semitendinosus). Three movement tasks were measured: kicking movements; deceleration after sprints; and passively induced backward accelerations of the leg. The results suggest that proximal fibers can be activated independently from distal fibers within the RF. Further, it was shown that the hypothesized function of "intramuscular longitudinal sequencing" does exist during dynamic movements. According to the punctum fixum-punctum mobile model, the activation succession between fibers changes direction (from proximal to distal or inversely) depending on the intentional context. Thus, the results seem to support the general hypothesis for the RF and could be principally in line with the operationalized "inter-fiber to tendon interaction model".

Since trunk function plays a major role in propulsion in sit-skiers and athletes belonging to distinct classes have different abilities to control trunk muscles, the aim of this study is to evaluate how athletes personalized their skiing strategy to compensate their impairment and improve skiing effectiveness. Sixteen Paralympic sit-skiers belonging to different classes volunteered as participant. Each athlete executed two double poling tests on a ski ergometer at the maximal speed he/she can reach and the best, identified as the fastest, was considered in the analysis. A set of two passive reflective markers was used for kinematic analysis: the first marker was placed on the athlete shoulder, while the second was placed on the sledge in correspondence of the hip joint. A Vicon motion capture system with 8 cameras was used to collect the athlete's motion during the double poling tests. Forces were recorded by sensors attached to the pulling cables. For the analyses athletes were grouped concerning to the individual impairment level into: group A (LW10-LW10.5), group B (LW11-LW11.5), and group C (LW12). Per each athlete the trunk maximal forward and trunk maximal backward, the range of motion, the start and finish time of trunk movement with respect to the beginning of the poling phase were measured during each cycle, and the time between the start and the finish of trunk movement were calculated. The main results of this study were: 1) greater trunk flexion and range of motion for group C compared to group A; 2) group A started trunk movement earlier compared to group C; 3) no differences in range of motion timing. Since greater trunk flexion and range of motion allow generating higher propulsion force with low level of fatigue, it is possible to conclude that thanks to their ability in control trunk muscles athletes in group C can develop a skiing strategy which is more effective than athletes that cannot control abdominal and lumbar extensor muscles.

The purpose of this paper was to present and evaluate a methodology to determine the contribution of bilateral leg and pole thrusts to forward acceleration of the centre of mass (COM) of cross-country skiers from multidimensional ground reaction forces and motion capture data. Nine highly skilled cross-country (XC) skiers performed leg skating and V2-alternate skating (V2A) under constant environmental conditions on snow, while ground reaction forces measured from ski bindings and poles and 3D motion with high-speed cameras were captured. COM acceleration determined from 3D motion analyses served as a reference and was compared to the results of the proposed methodology. The obtained values did not differ during the leg skating push-off, and forcetime curves showed high similarity, with similarity coefficients (SC)[0.90 in the push-off and gliding phases. In V2A, leg and pole thrusts were shown to contribute 35.1 and 65.9% to the acceleration of the body, respectively. COM acceleration derived from ground reaction forces alone without considering the COM position overestimated the acceleration compared to data from motion analyses, with a mean difference of 17% (P\0.05) during leg push-off, although the shapes of forcetime curves were similar (SC = 0.93). The proposed methodology was shown to be appropriate for determining the acceleration of XC skiers during leg skating push-off from multi-dimensional ground reaction forces and the COM position. It was demonstrated that both the COM position and ground reaction forces are needed to find the source of acceleration. (VLID)1660682

Das Kontraktionsverhalten im Dehnungs-Verkürzungs-Zyklus (DVZ) ist der grundlegende Aspekt reaktiver Bewegungsformen. Durch eine muskuläre Voraktivierung kann einer anschließenden Dehnungsphase ein größerer Widerstand entgegengesetzt werden was durch energetische Speicherprozesse zu erklären ist. Voraussetzung dafür aber ist, dass ein zentral generiertes Bewegungsprogramm die motorischen Befehle über das ZNS an die Muskulatur leitet. Das Ziel der vorliegenden Arbeit war es, den Einfluss von sensorischen Informationen auf die Bewegungsprogrammierung bei fehlendem visuellem Feedback und variierenden Dehnungsbelastungen zu untersuchen. Hierzu führten Probanden reaktive Tief-Hochsprünge aus 24 cm, 34 cm, 44 cm und 54 cm ohne visuelle Kontrolle aus. Durch Variation in den Absprunghöhen und der, den Probanden zur Verfügung gestellten, verbalen Höheninformation wurden unterschiedliche Aspekte der Bewegungssteuerung und Bewegungskontrolle beleuchtet. Für eine quantitative und qualitative Darstellung der Bewegungsausführung wurden Bodenreaktionskräfte und die muskuläre Aktivierung mittels Oberflächenableitungen registriert. Die Ergebnisse zeigen, dass fehlende visuelle Informationen kompensiert werden können wenn aus vorhergehenden Sprüngen kinästhetische Informationen zur Verfügung stehen. Ein Transfer von gespeicherten Bewegungserfahrungen über die Bewegungsparameter erscheint nur eingeschränkt möglich. Stehen diese nicht zur Verfügung erfolgt ein Wechsel in der Bewegungsstrategie dahingehend, dass primär eine Stabilisation des Körperschwerpunktes angestrebt wird um damit das individuelle, gespeicherte Körperschema zu sichern., In reactive drop jump conditions information about external and internal aspects of the task are necessary for programming an adequate movement pattern. For this, sensory information has to be collected and evaluated. If a sensory cue is blocked either alternative sensory information or knowledge about the movement can compensate this lack. To investigate the lack of visual information in the case of drop jumping 14 physical education students performed reactive drop jumps from 4 different heights with randomized height variations. In the experimental conditions also the verbal information about the dropping height was presented either after each jump or after a block of jumps or nor information was given. To analyse the neuromuscular activation patterns surface electromyography from different leg and trunk muscles were registered and synchronized with the vertical ground reaction forces. It could be demonstrated, that the lack of visual information can be compensated by tactile and propriozeptive information. The verbal information alone seems insufficient to do this. The functional role of a prolonged contact time must be seen in a stabilizing effect on the center of gravity. For this the ankle flexor activation prevents falling down backwards. The dividing into two parts of the ankle extensor must be interpreted such that in the case of insecurity, the motor programming changed to two separate programs, one for landing and one for jumping. The time between these two parts is necessary to collect additional information about the individual body scheme which defines the position and movement of the body in time and space.