• exercise testing
• fatigue
• muscle metabolism

Alpine ski racing is a demanding and multifaceted sport requiring high levels of physical and technical competence.1 Because of the complexity of the sport, the selection of useful sport-specific tests of physiological capacities is challenging.1 Aerobic capacity is commonly measured using incremental cycle ergometer exercise, while anaerobic capacity is commonly tested using metabolically highly demanding all-out Wingate cycling.2 High aerobic/anaerobic capacity of an elite alpine skier is considered an important physiological determinant of competitive success.2 However, new findings on the molecular basis for exercise-induced fatigue do not support this assertion.3–6

The contractile function of skeletal muscle declines during intense or prolonged physical exercise, that is, fatigue develops.3 Within the muscle fibres, fatigue is generally related to increased energy demands, in which effective ATP resynthesis is needed to match the dramatically increased ATP consumption during contractions.3 In contracting muscle fibres, ATP is mainly consumed by actomyosin cross-bridges and the sarcoplasmic reticulum Ca²⁺ pumps.3 Adequate ATP delivery to the ATP-consuming proteins is essential for normal cell function and integrity.3 Obviously, mechanisms to prevent these catastrophic consequences of ATP depletion exist within the muscle fibres.3 These mechanisms involve, on the one hand, effective metabolic systems to resynthesise ATP and, on the other hand, a fatigue-induced decline in ATP consumption.3 The latter fatigue mechanisms, which inhibit contraction-dependent ATP consumption, are a major focus of a recent review.3 Examples of exercises in which different fatigue mechanisms might limit performance are given in table 1 of the review by Cheng et al.3 Importantly, these different fatigue mechanisms might limit performance in metabolically demanding exercises.

For example, increased production of reactive oxygen/nitrogen species (ROS) and impaired cellular Ca²⁺ handling are implicated in prolonged force depression observed in skeletal muscle after metabolically highly demanding all-out Wingate cycling.4 Moreover, muscle biopsies taken 24 hours after high-intensity cycling exercise show an extensive fragmentation of the sarcoplasmic reticulum Ca²⁺ channels, the ryanodine receptor 1 (RyR1).4 Interestingly, elite endurance athletes develop a prolonged force depression after metabolically highly demanding all-out Wingate cycling, but no ROS-dependent RyR1 fragmentation.4

By contrast, prolonged force depression after mechanically demanding eccentric contractions (100 drop jumps from a height of 0.5 m) is largely independent of Ca²⁺ and ROS, and RyR1 fragmentation is observed in only some recreationally active elderly subjects.5 However, the force depression was not more marked in these subjects.5 Moreover, force depression after mechanically demanding eccentric contractions are similar in both recreationally active subjects and endurance trained athletes, despite the antioxidant capacity being higher in endurance trained muscles.5 Thus, the mechanisms underlying prolonged force depression after mechanically demanding eccentric contractions are dissimilar to those after metabolically highly demanding all-out Wingate cycling.

In addition, eccentric muscle activity is a titin based and not an O₂ ATP coupled contraction form. Shortly, when the sarcomeres of a skeletal muscle are stretched, for example, by gravitational forces, the titin immunoglobulin (Ig) domain segments and the PEVK region (The PEVK region is a titin spring element, which is rich in proline (P), glutamate (E), valine (V), and lysine (K) residues and is considered to be an intrinsically disordered protein region) extend.6 Titin domain folding against a force represents a potential source of work production in muscles, which presumably acts synchronously with the actomyosin contractile mechanism.6 This way, titin is an active component in the sarcomere that helps to maximise work output without consuming ATP.6 Titin as a force generating muscle protein is a major focus of a recent review.6 Titin Ig domain refolding under force as a source of work production speaks against ATP depletion as a central factor underlying the impaired contractile function after mechanically demanding eccentric contractions.

Alpine skiing can arguably be characterised as the only sport in which well-coordinated eccentric muscle action is the decisive element.7 Eccentric muscle contraction is essential for opposing the high centrifugal forces experienced by skiers in carved turns.7 Carved turns of successful skiers are characterised by a short and distinct eccentric steering phase.7 Well-coordinated eccentric muscle activation is thus a key feature for success in competitive alpine skiing.7 Consequently, the mechanisms underlying force depression after ski racing are similar to those after mechanically demanding drop-jump exercise.5

Fatigue develops rapidly during physical activities requiring a rate of ATP production that exceeds the aerobic capacity of the muscle fibres.3 This type of fatigue is closely related to the need for ATP production by anaerobic metabolism,3 for example, metabolically highly demanding all-out Wingate cycling. Anaerobic metabolism leads to accumulation of lactate and hydrogen ions and increased blood lactate concentrations were measured after alpine ski racing without affecting competitive performance.1 3 However, the high metabolic loads experienced by alpine skiers essentially stem from the simultaneous activation of a plethora of trunk muscles necessary to maintain core stability and balance.7 Moreover, a recent study shows that recreational alpine skiing is associated with prolonged eccentric quadriceps and hamstring fatigue.8 Interestingly, concentric types of endurance training (metabolically demanding exercises) (eg, biking) do not prevent fatigue during eccentric (ie, skiing) types of endurance exercise (mechanically demanding exercises).8

Based on these findings and in agreement with a recent study, we suggest that preseason aerobic and anaerobic tests are of limited use for prediction of alpine skiing performance.1 A valid and reliable test battery that can predict performance in alpine skiing seems to be lacking. Therefore, future research directed towards screening for valid components of athletic performance is required.1