At 1° inclination on a treadmill. Pole and plantar ground reaction forces, joint angles (elbow, hip, knee, and ankle), cycle characteristics, and electromyography (EMG) of upper and lower body muscles were analyzed. Results: 1) Pole force pattern with initial impact force peak and the following active force peak (PPF) correlated to V85%, (r = 0.66, P < 0.05); 2) active flexion–extension pattern in elbow, hip, knee, and ankle joints with angle minima occurring around PPF, correlated to hip angle at pole plant (r = −0.89, P < 0.01), minimum elbow angle (r = −0.71), and relative poling time (r = −0.72, P < 0.05); 3) two different DP strategies (A and B), where strategy A (best skiers) was characterized by higher angular elbow- and hip-flexion velocities, smaller minimum elbow (P < 0.01) and hip angles (P < 0.05), and higher PPF (P < 0.05); 4) EMG activity in trunk and hip flexors, shoulder, and elbow extensors, and several lower body muscles followed a specific sequential pattern with changing activation levels; and 5) EMG activity in lower body muscles showed DP requires more than upper body work. Conclusions: DP was found to be a complex movement involving both the upper and lower body showing different strategies concerning several biomechanical aspects. Future research should further investigate the relationship between biomechanical and physiological variables and elaborate training models to improve DP performance....

Key points: In human skeletal muscles, the current view is that the capacity for mitochondrial energy production, and thus endurance capacity, is set by the mitochondria volume. However, increasing the mitochondrial inner membrane surface comprises an alternative mechanism for increasing the energy production capacity. In the present study, we show that mitochondrial inner membranes in leg muscles of endurance‐trained athletes have an increased ratio of surface per mitochondrial volume. We show a positive correlation between this ratio and whole body oxygen uptake and muscle fibre mitochondrial content. The results obtained in the present study help us to understand modulation of mitochondrial function, as well as how mitochondria can increase their oxidative capacity with increased demand. Mitochondrial energy production involves the movement of protons down a large electrochemical gradient via ATP synthase located on the folded inner membrane, known as cristae. In mammalian skeletal muscle, the density of cristae in mitochondria is assumed to be constant. However, recent experimental studies have shown that respiration per mitochondria varies. Modelling studies have hypothesized that this variation in respiration per mitochondria depends on plasticity in cristae density, although current evidence for such a mechanism is lacking. In the present study, we confirm this hypothesis by showing that, in human skeletal muscle, and in contrast to the current view, the mitochondrial cristae density is not constant but, instead, exhibits plasticity with long‐term endurance training. Furthermore, we show that frequently recruited mitochondria‐enriched fibres have significantly increased cristae density and that, at the whole‐body level, muscle mitochondrial cristae density is a better predictor of maximal oxygen uptake rate than muscle mitochondrial volume. Our findings establish an elevating mitochondrial cristae density as a regulatory mechanism for increasing metabolic power in human skeletal muscle. We propose that this mechanism allows evasion of the trade‐off between cell occupancy by mitochondria and other cellular constituents, as well as improved metabolic capacity and fuel catabolism during prolonged elevated energy requirements.

Cross-country (XC) skiing is one of the most demanding of endurance sports, involving protracted competitions on varying terrain employing a variety of skiing techniques that require upper- and/or lower-body work to different extents. Through more effective training and extensive improvements in equipment and track preparation, the speed of cross-country ski races has increased more than that of any other winter Olympic sport, and, in addition, new types of racing events have been introduced. To a certain extent this has altered the optimal physiological capacity required to win, and the training routines of successful skiers have evolved accordingly. The long-standing tradition of researchers working closely with XC-ski coaches and athletes to monitor progress, improve training, and refine skiing techniques has provided unique physiological insights revealing how these athletes are approaching the upper limits of human endurance. This review summarizes current scientific knowledge concerning the demands involved in elite XC skiing, as well as the physiological capacity and training routines of the best athletes.

To determine whether conditions for O₂utilization and O₂off-loading from the hemoglobin are different in exercising arms and legs, six cross-country skiers participated in this study. Femoral and subclavian vein blood flow and gases were determined during skiing on a treadmill at ∼76% maximal O₂uptake (V̇o_{2 max}) and at V̇o_{2 max}with different techniques: diagonal stride (combined arm and leg exercise), double poling (predominantly arm exercise), and leg skiing (predominantly leg exercise). The percentage of O₂extraction was always higher for the legs than for the arms. At maximal exercise (diagonal stride), the corresponding mean values were 93 and 85% ( n = 3; P < 0.05). During exercise, mean arm O₂extraction correlated with the Po₂value that causes hemoglobin to be 50% saturated (P₅₀: r = 0.93, P < 0.05), but for a given value of P₅₀, O₂extraction was always higher in the legs than in the arms. Mean capillary muscle O₂conductance of the arm during double poling was 14.5 (SD 2.6) ml·min⁻¹·mmHg⁻¹, and mean capillary Po₂was 47.7 (SD 2.6) mmHg. Corresponding values for the legs during maximal exercise were 48.3 (SD 13.0) ml·min⁻¹·mmHg⁻¹and 33.8 (SD 2.6) mmHg, respectively. Because conditions for O₂off-loading from the hemoglobin are similar in leg and arm muscles, the observed differences in maximal arm and leg O₂extraction should be attributed to other factors, such as a higher heterogeneity in blood flow distribution, shorter mean transit time, smaller diffusing area, and larger diffusing distance, in arms than in legs.

That muscular blood flow may reach 2.5 l kg⁻¹ min⁻¹ in the quadriceps muscle has led to the suggestion that muscular vascular conductance must be restrained during whole body exercise to avoid hypotension. The main aim of this study was to determine the maximal arm and leg muscle vascular conductances (VC) during leg and arm exercise, to find out if the maximal muscular vasodilatory response is restrained during maximal combined arm and leg exercise. Six Swedish elite cross‐country skiers, age (mean ±s.e.m.) 24 ± 2 years, height 180 ± 2 cm, weight 74 ± 2 kg, and maximal oxygen uptake 5.1 ± 0.1 l min⁻¹ participated in the study. Femoral and subclavian vein blood flows, intra‐arterial blood pressure, cardiac output, as well as blood gases in the femoral and subclavian vein, right atrium and femoral artery were determined during skiing (roller skis) at ∼76% of and at with different techniques: diagonal stride (combined arm and leg exercise), double poling (predominantly arm exercise) and leg skiing (predominantly leg exercise). During submaximal exercise cardiac output (26–27 l min⁻¹), mean blood pressure (MAP) (∼87 mmHg), systemic VC, systemic oxygen delivery and pulmonary (∼4 l min⁻¹) attained similar values regardless of exercise mode. The distribution of cardiac output was modified depending on the musculature engaged in the exercise. There was a close relationship between VC and in arms (r= 0.99, P < 0.001) and legs (r= 0.98, P < 0.05). Peak arm VC (63.7 ± 5.6 ml min⁻¹ mmHg⁻¹) was attained during double poling, while peak leg VC was reached at maximal exercise with the diagonal technique (109.8 ± 11.5 ml min⁻¹ mmHg⁻¹) when arm VC was 38.8 ± 5.7 ml min⁻¹ mmHg⁻¹. If during maximal exercise arms and legs had been vasodilated to the observed maximal levels then mean arterial pressure would have dropped at least to 75–77 mmHg in our experimental conditions. It is concluded that skeletal muscle vascular conductance is restrained during whole body exercise in the upright position to avoid hypotension.