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Corresponding author R. A. Ferguson: School of Sport, Exercise and Health Sciences, Loughborough University, Leicestershire LE11 3TU, UK. Email: R.Ferguson@lboro.ac.uk
Alpine skiing is characterized by high-intensity exercise of between 90 and 120 s duration that requires repeated phases of high-force isometric and eccentric contractions. The nature of these contractions, during which all fibre types are active, results in restricted blood flow to the working muscle, thereby reducing oxygen delivery and increasing metabolite accumulation. The consequence of this will be skeletal muscle fatigue, through both central and peripheral mechanisms, and a potential loss of motor control which will ultimately limit skiing performance.
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Alpine skiing has been contested at every Winter Olympics since 1936 and consists of the four traditional events of downhill (DH), super giant slalom (SG), giant slalom (GS) and slalom (SL), as well as events such as moguls and snowboarding. The events are approximately 90 s in duration for the GS and SL and up to 2 min for the DH and SG. All events require the skier to accelerate as quickly as possible from the starting gate to full speed before maintaining proper form and technique for the remainder of the race. Elite ski racing is performed at speeds that can approach 160 Km h−1 (DH), or takes place on extremely steep terrain (SL). Therefore, there are immense physical requirements for elite skiers to anchor the whole body in a streamlined and aerodynamic position (DH and GS) or make tight turns in rapid succession (SG and SL). Given that the margins between gold and silver medals in the Winter Olympic alpine skiing events are measured to the hundredth of a second, understanding any of the factors that limit performance is of importance.
The physical characteristics of skiers and requirements for the various events have been previously described (Turnbull et al. 2009). The energy demand of skiing is often measured in terms of maximal oxygen uptake . Tesch et al. (1978) observed energy demands equivalent to ∼80–90% during DH skiing, whereas Saibene et al. (1985) reported energy demands equivalent to 120% during GS skiing. Veicsteinas et al. (1984) observed much higher demands equivalent to ∼160–200% during SL and GS events, in which the oxygen cost of anaerobic sources was accounted for through measurements of postexercise blood lactate concentration. What have been less well explored, however, are the specific responses that occur within the skeletal muscle during skiing and how these responses may influence skiing performance. The focus of this brief review, therefore, is to discuss some of the fundamental aspects of skeletal muscle physiology in relation to the available skiing-specific knowledge in order to understand what the main factors might be that limit performance in alpine skiing.
Contractile activity and muscle fibre recruitment during alpine skiing
In order to appreciate the limitations to performance in alpine skiing events, the type of contraction and level of activity of the main muscle groups involved and the subsequent physiological consequences of their involvement must be considered. Berg et al. (1995) and Berg & Eiken (1999) observed high levels of EMG activity and a predominance of high-force, slow-velocity eccentric contractions of the quadriceps muscle group during SG, GS and SL events. This is probably due to the low posture assumed during these events, as well as the downward displacement of the body, especially during turns. Hintermeister et al. (1995) observed high levels of EMG activity in the majority of leg (including quadriceps, hamstrings and calf) and trunk muscles studied during GS and SL events. They also observed evidence of co-contraction and suggested that there is a significant isometric component to skiing.
The high level of EMG activity measured during skiing events would suggest that a significant proportion of the muscle is active. The studies by Tesch et al. (1978) and Nygaard et al. (1978) set the benchmark for understanding which fibre types may be active during skiing. Both studies used the histochemical measurement of glycogen, using the periodic acid–Schiff (PAS) reaction, where cross-sections of frozen muscle biopsies obtained before and after skiing activity were stained according to different levels of glycogen content, providing a semi-quantitative assessment of glycogen utilization. Serial cross-sections were also stained to characterize the fibres as either slow-twitch (ST) or fast-twitch (FT) fibre types. The consensus from these studies was that a greater utilization of glycogen occurred in ST fibres, suggesting a greater use of these fibres during skiing. An important limitation to this technique, however, is that exercise of a relatively long duration (>10 min) is necessary to detect a decline in the PAS staining intensity. Moreover, the glycogen depletion patterns in these studies were monitored during a whole day of skiing and are therefore not representative of a single bout of skiing activity. Given these methodological limitations, together with the short duration of ski races and the high level of EMG activity (Berg et al. 1995; Hintermeister et al. 1995; Berg & Eiken, 1999), it is probable that the pattern of muscle fibre recruitment may not be as described.
Physiological experiments conducted over the past 15 years have enhanced our knowledge of muscle fibre recruitment patterns during brief (<10 s) intense exercise (e.g. Beltman et al. 2004a,b; Gray et al. 2008) and longer duration (1 h) exercise (e.g. Altenberg et al. 2007). These studies examined muscle fibre activity by measuring the ATP and phosphocreatine (PCr) content (Gray et al. 2008) or the PCr/creatine (Cr) ratio (Beltman et al. 2004a,b; Altenberg et al. 2007) in single muscle fibres dissected from freeze-dried muscle biopsies which were obtained before and immediately after the defined exercise. Fibres are characterized either histochemically as type I and type II or according to their relative expression of myosin heavy chain isoform (type I, type IIA and type IIX). These studies demonstrated that with increasing exercise intensity, fibres are recruited in an orderly heirerarchical pattern, i.e. type I followed by type IIA and then type IIX, which conforms to the size principle defined by Henneman & Mendell (1981). However, the size principle is not the only mechanism of muscle fibre recruitment. Force can be modulated by varying the frequency of motor unit stimulation, known as rate coding, which occurs during submaximal exercise (Ivy et al. 1987; Altenberg et al. 2007) and very short-duration isometric contractions of the quadriceps (Beltman et al. 2004a).
It is difficult to exactly translate the muscle fibre recruitment patterns observed in the studies described above to the possible changes that might occur during skiing. Furthermore, the activation strategies during eccentric contractions are not fully understood. The neural commands controlling eccentric contractions are unique compared with those required for isometric and concentric contractions (Enoka, 1996). For example, Nardone et al. (1989) found that high-threshold motor units (i.e. FT fibres) in the gastrocnemius muscle were selectively activated when performing eccentric contractions compared with concentric contractions. However, Beltman et al. (2004c) did not find evidence for selective recruitment of FT fibres during a brief series of eccentric contractions of the quadriceps in which, using the PCr/Cr ratio as an indicator of muscle fibre activity, it was observed that all fibre types had been active.
A consequence of the high-force contractions performed during skiing is an increased intramuscular pressure (Sejersted et al. 1984), which will restrict, or even stop, blood flow to the working skeletal muscle (Bonde-Petersen et al. 1975; Sjøgaard et al. 1988). This will reduce the delivery of oxygen to the working muscles, resulting in reduced tissue oxygenation and the muscle becoming ischaemic. The inflow of oxygen-saturated arterial blood to the quadriceps muscle group, measured using the non-invasive technique of near-infrared spectroscopy, has been shown to stop at relatively low torques (∼35% maximal torque capacity) during isometric knee extensor contractions (De Ruiter et al. 2007). In well-trained junior skiers, a reduction in oxygen satuaration of the quadriceps muscle (also measured using near-infrared spectroscopy) has been observed during GS and SL events (Szmedra et al. 2001). The reduction seen during GS was greater than that observed during SL, which may have been related to a greater static load imposed by the lower posture during GS.
The impact of muscle ischaemia may also influence muscle fibre activity. Krustrup et al. (2009) investigated the degree of single-fibre activity during single-leg dynamic knee-extensor exercise in combination with blood flow occlusion. During 90 s exercise, blood flow occlusion to the quadriceps increased the activity of both type I and type II fibres compared with the non-occluded free-flow condition, despite the workload being the same (Fig. 1). Similar findings were also observed by Greenhaff et al. (1993), who demonstrated a greater rate of glycogenolysis in type I and type II fibres during intermittent isometric contractions with occluded circulation compared with the non-occluded condition. Therefore, although further research is warranted to establish the exact muscle fibre recruitment patterns during skiing events, the combination of sustained high-force contractions within the ischaemic environment suggests that a significant proportion of the muscle, including all fibre types, will be active during skiing.
Muscle fatigue during alpine skiing
The ischaemic environment and high activity of all muscle fibre types during skiing will undoubtedly result in muscle fatigue and a consequent decline in performance, with a range of mechanisms having been identified to contribute to the fatigue process. The accumulation of metabolic byproducts is a possible mechanism which will have an impact on many aspects of the muscle contractile processes that are distal to the neuromuscular juntion, i.e. ‘peripheral fatigue’. For example, cross-bridge cycling (Fitts, 2008), calcium (Ca2+) release and binding (Allen et al. 2008b), action potential propogation and metabolism (Allen et al. 2008a) have all been attributed to the fatigue process. However, the exact role of specific metabolites has not been fully established.
During intense exercise, H+ accumulation, resulting in a decline in muscle pH to as low as 6.5 (Spriet et al. 1989), has traditionally been considered to impair the function of contractile proteins. However, this is continually being challenged, since it has been observed that fatigue can still occur without any substantial decrease in pH (Bangsbo et al. 1996). Furthermore, it has been shown that the effects of pH on contractile function are very dependent on muscle temperature. In mouse muscle, the effect of acidity on isometric force was signifiant at very low tempeatures (12°C), whereas at more physiological temperatures (32°C) the pH had little effect (Westerblad et al. 1997). Therefore, in normal conditions the pH may not necessarily have a direct effect on force production. Alternative mechanisms are now focussed on the effects of ionic changes of the action potential and failure of sarcoplasmic reticulum (SR) Ca2+ release. For example, the accumulation of inorganic phosphate (Pi; Cady et al. 1989) leads to a reduced release of Ca2+ from the SR because the Pi enters the SR and precipitates to form calcium phosphate, which reduces the amount of free Ca2+ available for release (Allen et al. 2008b). What is clear, however, is that there is no single metabolite that can cause peripheral fatigue, and it is more likely to be a combination of metabolic changes within the muscle that affect the contractile process.
As well as the ‘peripheral’ factors involved in the muscle fatigue process, ‘central’ factors involving spinal and supraspinal processes are also involved (Gandevia, 2001). This stems from the fact that the metabolic byproducts produced during contractions stimulate group III and IV afferent receptors (e.g. Kaufman et al. 1984; Light et al. 2008). As well as modulating the cardiovascular and respiratory responses to exercise (the exercise pressor reflex; Kaufman & Forster, 1996), the afferent stimulation also acts to inhibit motor unit firing rate and thus voluntary activation of muscle (Gandevia, 2001; Fig. 2). Evidence for this comes from the observation that muscle ischaemia maintained after contraction sustains the discharge of group III and IV afferents (Kaufman et al. 1984). Furthermore, Woods et al. (1987) observed that motor-unit firing rate declined during maximal voluntary contractions of biceps brachii and did not recover during 3 min rest while the arm remained ischaemic, during which afferent stimulation would be maintained. Firing rate had recovered by 3 min after restoration of blood flow and a consequent removal of the metabolic afferent stimulation. It is therefore suggested that the central nervous system monitors the peripheral state of the working muscles to either adjust the neural activation or impair the ability to sustain high levels of voluntary activation in order to prevent the development of peripheral fatigue to a ‘catastrophic’ intracellular state (Gandevia, 2001).
Therefore, the manifestation of both ‘central’ and ‘peripheral’ fatigue (Fig. 3) during a skiing event must have major implications for motor control and performance. The motor control necessary to perform highly skilled and well-rehearsed movements requires small alterations in force at very precise moments as well as extremely large forces to react to sudden external perturbations. Failure of the peripheral mechanisms of force generation together with central nervous system motor output is likely to result in an impaired task performance, and in some cases a high-speed fall, often seen in the latter stages of a ski race.
Some of the distinguishing characteristics of elite skiers include greater strength and a slight preponderence of type I fibres compared with non-athletes (e.g. Thorstensson et al. 1977). Given the influence of the reduced muscle blood flow in setting the intramuscular environment during skiing activity, it is intriguing to consider whether the peripheral vasculature in elite skiers is adapted to provide an enhanced blood flow during the brief periods where there is little activity of some muscles. There is evidence to suggest that athletes who are conditioned to perform high-force intermittent exercise with short rest periods have a highly adapted vasculature to enhance the blood flow response. For example, Ferguson & Brown (1997) demonstrated an enhanced blood flow capacity in the forearms of elite rock climbers, who perform high-force intermittent isometric contactions with short rest intervals, compared with normal sedentary subjects. Forearm blood flow responses were greater during a postocclusion reactive hyperaemia, as well as following sustained isometric handgrip exercise (Fig. 4). More recently, it has been demonstrated that resistance exercise in combination with circulatory occlusion enhances the increase in peak postocclusion blood flow compared with resistance training at the same relative load with unrestricted blood flow (Patterson & Ferguson, 2010). The mechanisms responsible for the enhanced blood flow capacity are not clear, but may include functional changes in the vasculature, such as an altered myogenic response or endothelial control, or structural changes, such as increased capilliarity, with the ischaemic environment providing an enhanced stimulus for these adaptations. In any case, a greater blood flow capacity in skiers would allow an enhanced washout of the accumulating metabolic byproducts, which would reduce the negative influence of these metabolites on both the peripheral causes of fatigue and the stimulation of the afferent receptors involved in inhibiting neural drive. This would ultimately result in the ability of muscle to keep contracting and the maintainance of appropriate motor control during skiing, enabling an optimal performance.
Prolonged changes in muscle function associated with alpine skiing
So far, this review has considered the acute responses to alpine skiing performance. However, in situations where repeated bouts of intense skiing activity are performed, perhaps over several days, there may be a subsequent impact on the ability for muscle to function properly due to the manifestation of low-frequency fatigue (LFF; Edwards et al. 1977). This type of fatigue is characterized by reduced tetanic force at low frequencies of stimulation, while force is less reduced at high frequencies. The most likely cause of LFF is a disturbance of excitation–contraction coupling (Jones, 1996). Low-frequency fatigue has been observed after several types of exercise, including isometric (Ratkevicius et al. 1998), concentric and eccentric contractions (Newham et al. (1983); Smith & Newham (2007)). Using a rat isolated gastrocnemius model to control for difference in recruitment patterns between different modes of contraction, Rijkelijkhuizen et al. (2003) observed that LFF is more pronounced after eccentric muscle activity. Furthermore, the glycolytic portion of the muscle was more susceptible to LFF than the oxidative part of the muscle, suggesting a fibre-type-related effect. Thefore, LFF may be relevant for elite skiers if intense prequalifying is an issue. However, it is likely to be more significant for the recreational skier during a ski holiday, especially since Tesch et al. (1978) and Nygaard et al. (1978) observed a greater utilization of FT fibres in recreational skiers compared with well-trained skiers over 1 day of skiing. Given the fact that skiing requires precise, well-practised motor programmes which rely on relatively low-frequency firing to generate small correcting forces, it is perhaps not surprising that recreational skiers typically find that their legs fail to work effectively a few days into their ski holiday.
Alpine skiing is characterized by repeated phases of high-force isometric and eccentric contractions in which all fibre types are likely to be active. The ishaemic nature of the contractions, brought about by high levels of intramuscular pressure and reduction in blood flow, will inevitably result in an accumulation of metabolic byproducts that will impact on the force-generating properties of muscle though various central and peripheral mechanisms. Peripheral fatigue will result through the inhibition of the contractile process. At the same time, central fatigue will result in the stimulation of afferent receptors within the muscle, which will inhibit voluntary activation of the muscle and cause a reduction in force output, as well is impacting on the tightly regulated motor control patterns. Through training, it is possible that elite skiers have a highly adapted peripheral vasculature that allows an enhanced washout of metabolic byproducts and thus reduces the negative influence on muscle activity.
The environmental conditions and local terrain make skiing-specific research extremely difficult to undertake. However, future research should consider a more accurate examination of the potential mechanisms that cause fatigue during the various skiing events. Given the importance of the ability to reperfuse the ischaemic muscle, further work could examine the blood flow characteristics of elite skiers and identify training techniques that may enhance the capability of the peripheral vasculature to maintain or restore blood flow during high-force contractions.