A limit-cycle model of leg movements in cross-country skiing and its adjustments with fatigue

Research on Low-Cycle Fatigue Engineered Hybrid Sandwich Ski Construction

This research is aimed at evaluating the effect of low-cycle fatigue on a newly designed hybrid sandwich ski structure to determine the changes that may occur due to cyclic loading and thus affect its use. This is primarily concerned with the fatigue behavior of the tested ski over different time intervals simulating its seasonal use and its effect on the mechanical properties of the ski, i.e., the durability and integrity of the individual layers of the sandwich ski structure. The ski was subjected to 70,000 deflections by moving the crossbar by 60 mm according to the ski deflection calculation in the arch. The results of the cyclic tests of the engineered ski design showed no significant changes in the ski during loading. The average force required to achieve deflection in the first 10,000 cycles was 514.0 ± 4.2 N. Thereafter, a secondary hardening of the structure occurred during relaxation and the force required increased slightly to 543.6 ± 1.7 N. The required force fluctuated slightly during the measurements and in the last series the value was 540.4 ± 0.8 N. Low-cycle fatigue did not have a significant effect on the mechanical properties of the ski; there was no change in shape or visual delamination of the individual layers of the structure. From the cross-section, local delamination was demonstrated by image analysis, especially between the Wood core and the composite layers E-Glass biaxial and Carbon triaxial.

The effect of exhaustive exercise on the choice of technique and physiological response in classical roller skiing

PURPOSE

The aim of this study was to investigate the effect of exhaustive exercise on technique preference and the accompanying physiological response during classic skiing at constant workload, but with varying incline-speed combinations.

METHODS

Seven male competitive cross-country skiers performed four tests, each lasting 23 min, at constant 200 W workload roller skiing on a treadmill using classic style, three in unfatigued state, and one after exhaustion. The incline and speed combination (that determined the 200 W) were altered each minute during the tests. The athletes were allowed to change sub-technique at free will. Physiological variables and cycle rate were recorded continuously as well as the incline-speed combinations at which the sub-technique was changed.

RESULTS

Exhaustive exercise did not (or hardly) affect cycle rate and choice of technique. The physiological response was most prominent in slight incline-high speed conditions, independent of exercise duration. Exhaustive exercise affected the physiological response in a differentiated manner. HR and RER remained, respectively, higher and lower after fatigue, while [Formula: see text] (and thereby GE) were affected only during approximately the first 8 min of post-exhaustion exercise.

CONCLUSIONS

Exhaustive exercise has a minimal effect on choice of technique in classic cross-country skiing with free choice of sub-technique, even though physiological stress is increased.

Central and peripheral fatigue in knee and elbow extensor muscles after a long-distance cross-country ski race

Although elbow extensors (EE) have a great role in cross-country skiing (XC) propulsion, previous studies on neuromuscular fatigue in long-distance XC have investigated only knee extensor (KE) muscles. In order to investigate the origin and effects of fatigue induced by long-distance XC race, 16 well-trained XC skiers were tested before and after a 56-km classical technique race. Maximal voluntary isometric contraction (MVC) and rate of force development (RFD) were measured for both KE and EE. Furthermore, electrically evoked double twitch during MVC and at rest were measured. MVC decreased more in KE (-13%) than in EE (-6%, P = 0.016), whereas the peak RFD decreased only in EE (-26%, P = 0.02) but not in KE. The two muscles showed similar decrease in voluntary activation (KE -5.0%, EE -4.8%, P = 0.61) and of double twitch amplitude (KE -5%, EE -6%, P = 0.44). A long-distance XC race differently affected the neuromuscular function of lower and upper limbs muscles. Specifically, although the strength loss was greater for lower limbs, the capacity to produce force in short time was more affected in the upper limbs. Nevertheless, both KE and EE showed central and peripheral fatigue, suggesting that the origins of the strength impairments were multifactorial for the two muscles.

A minimal limit-cycle model to profile movement patterns of individuals during agility drill performance: Effects of skill level

Identification of control strategies during agility performance is significant in understanding movement behavior. This study aimed at providing a fundamental mathematical model for describing the motion of participants during an agility drill and to determine whether skill level constrained model components. Motion patterns of two groups of skilled and unskilled participants (n=8 in each) during performance of a forward/backward agility drill modeled as limit-cycles. Participant movements were recorded by motion capture of a reflective marker attached to the sacrum of each individual. Graphical and regression analyses of movement kinematics in Hooke's plane, phase plane and velocity profile were performed to determine components of the models. Results showed that the models of both skilled and unskilled groups had terms from Duffing stiffness as well as Van der Pol damping oscillators. Data also indicated that the proposed models captured on average 97% of the variance for both skilled and unskilled groups. Findings from this study revealed the movement patterning associated with skilled and unskilled performance in a typical forward/backward agility drill which might be helpful for trainers and physiotherapists in enhancing agility.