Downhill skiing is an activity enjoyed by millions of people around the world. The number of skiing days increased from 44 to 51 million (+16%) from 1999 through 2004 in Austria (7). The National Ski Areas Association (13) reported that nearly 7 million skiers made 58.9 million visits to ski areas in the United States during the 2005 ski season.
Alpine skiing can be characterized as continuous activity as the skier maneuvers down the slope. It can also be characterized as an intermittent activity within the run because leg muscles contract more during the turn and then have a semirelaxation period between turns. An individual ski run can last anywhere from 1 to 10 minutes or longer. After a run, skiers usually have a 10- to 15-minute recovery period while they ride a chair lift that takes them to the top of a run. On a typical ski day, skiers will ski approximately 3 hours in the morning, take a lunch break, and then ski for 2-3 more hours in the afternoon.
Numerous authors have reported that muscle contraction forces can reach upwards of 100-150% of maximal voluntary contraction when making a turn (2,4,15,24). Szmedra et al. (21) also added that skiers experienced significant levels of muscle ischemia and hypoxia when they completed short radius slalom-type turns. The combined effects of high forces, ischemia, and hypoxia leads to increased muscle stress during skiing, as noted by the increase in creatine kinase (CK) levels. Seifert et al. (18) noted a 93% increase in CK levels in recreational skiers after 3 hours of self-paced skiing.
Heart rate (HR), blood lactate (LA), myoglobin, and cortisol have also been used to assess the acute and overall stress levels of an activity. Krautgasser et al. (6) and Scheiber et al. (17) recently reported on the acute load effects of skiing. These authors reported that recreational skiing results in a blood LA of approximately 2 mmol·L−1. These data support previously reported average blood LA of 2.7 mmol·L−1 in younger (18-45 years old) recreational skiers after 3 hours of self-paced skiing (18). Those authors also reported a skiing HR of approximately 80% maximal of HR (HRmax) in older recreational skiers (6,17). In contrast, race training significantly increases skiing HR more than that of recreational skiing. Burtscher et al. (3) reported that HR during controlled giant slalom skiing was approximately 87% HRmax in elite skiers, whereas Seifert et al. (19) reported that elite collegiate racers trained at approximately 97% HRmax during giant slalom training.
With thousands of repeated contractions occurring during a day of skiing, some level of fatigue is inevitable. From a training point of view, it can be argued that a certain amount of fatigue or stress is required to enhance physiological capacity. However, too much fatigue exposes the skier to increased risk of injury and reduces the pleasure of the activity (1,5,9,10,12). Identifying and understanding the physiological responses to fatigue, improvements in the sports equipment used in the activity, changing work to rest ratios of the activity, and using nutritional interventions are helpful in not only improving the comfort and enjoyment of skiing, but also enhancing the training and safety associated of this activity.
It is important to understand how a specific activity can change the various physiological indices associated with fatigue. For example, in activities such as cycling and running, HR and blood LA levels are positively correlated to and good predictors of an acute training load and resulting fatigue. As the training load increases, HR and blood LA also increase (11,20,22). As a result, common fatigue indices and muscular stress increase with increasing training loads. It is not known if HR and blood LA change in a similar manner when skiing because of the influence of external factors, such as snow conditions, changing terrain, and changing ski edge pressure application, all of which increase muscular stress. Little is known about how HR and blood LA levels interact with the indices of muscle stress during recreational skiing.
Therefore, the aim of this study was to investigate if acute load variables, represented by HR and blood LA during alpine skiing, may serve as predictors and correlates of chronic stress, as assessed by the cumulative fatigue indices. Consequently, chronic stress was induced in a group of recreational skiers by requiring them to ski at a consistent manner in standardized skiing conditions.
Experimental Approach to the Problem
Subjects were instructed to refrain from exercise the day before skiing. All subjects arrived the night before testing and slept in a hotel at the resort. Timing was standardized from waking to the beginning of testing. Subjects consumed a standardized breakfast the morning of their test.
Ten healthy women (age, 22.7 ± 4.0 years) gave informed consent to participate in this study after institutional review board approval. All subjects were healthy, university sport science students who were physically active but not engaged in competitive athletic training. To get a homogenous sample, subjects were selected according to their skiing ability and amount of skiing days per year. All subjects were of the intermediate level based on the Austrian Ski Teaching Concept (23). Intermediate level skiers are able to perform short and long radii turns on prepared terrains. In flat terrain, intermediate skiers are able to execute carved turns but perform mostly skid turns on steep terrain.
All subjects followed a standardized warm-up by completing a 15-minute warm-up ride on a cycle ergometer and 2 warm-up runs on the ski slope. Peak isometric force was measured with a force plate mounted on an upright, stationary seat. The right leg was used for testing with a knee angle of 100 degrees. Three leg press attempts were conducted to achieve peak force, with the highest value used for statistical analysis. The protocol was made with 3 seconds to attain peak force with 12 seconds in between attempts. The isometric endurance test was completed 5 minutes after the peak force test was completed. The endurance test was performed at 45-50% of the preskiing maximal isometric force with a knee angle of 100 degrees. The test was terminated when sustained force decreased to less than the 45% of peak force for a total of 1.5 seconds. Peak force and isometric endurance test were measured 10 minutes before and 10 minutes after skiing. Verbal encouragement was not provided to the subjects during these tests. Visual feedback on force output was made available to the subjects; however, they did not receive information on elapsed time during the endurance test.
Data collection occurred at Hinterreit Ski Area in Maria Alm, Austria during the month of March. Each run took approximately 100 seconds to complete. Figure 2 shows each individual skier's average run times. After the isometric endurance test, muscle fatigue was induced by skiing 24 runs during a 3-hour period. Total ski time for the 24 runs was approximately 40 minutes of the 3 hours total time. Within each run, there were 3 pitch changes: 21, 29, and 13 degrees. Subjects performed an average of 22 turns on the 21-degree pitch, 10 turns on the steep pitch, and 28 turns on the flat section. Total elevation change for the run is 300 m vertical elevation, with the bottom of the run at 890 m above sea level. This run is classified as an intermediate level run. Intermediate level runs are classified as moderate difficulty, moderate length, and a moderate level of risk. To control the length of turns and distance skied across the fall line, subjects skied through a standardized corridor on groomed ski terrain. Although they could ski as they preferred, they were instructed to maintain similar finishing times and HR for their individual runs to ensure a standardized load throughout their skiing. To reach this goal, verbal feedback on HR and finishing time were provided to each skier at the end of each run. Skis were standardized according to body size and consisted of 150 or 160 cm recreational slalom skis (Atomic, Inc., Altenmarkt, Austria).
Salivary and earlobe blood samples were collected after breakfast (time 0) and after the 2nd, 12th, and 24th run. Salivary samples were analyzed for cortisol (Diagnostic Products, Inc., Ft. Lauderdale, Fla.), whereas the 20-μL blood samples were analyzed for LA (Biosen 5140; EKF-Diagnostic GmbH, Magdeburg, Germany). An additional blood sample was collected at time 0 and 3 hours after skiing and analyzed by reflectance photometry at 25oC for CK (Reflotron; Roche Diagnostics, Basel, Switzerland).
Heart rate was collected at the end of each run (Polar Electro Oy, Kempele, Finland). Heart rate was not collected at the 13th run to allow subjects a short break to empty their bladder. Percent of HRmax was calculated of 220 by age.
The cumulative stress variables were CK, cortisol, and isometric endurance performance. A chronic stress score (Cstress) was then calculated from how the individual ranked among our group of skiers for the cumulative stress variables. For example, the skier with the greatest percent change in CK received 10 points, the skier with the second largest change received 9 points, and so forth. One goal of the study was to determine the Cstress and compare it to acute load variables. Acute load variables are described as per run loads and include HR and LA.
Data were analyzed with an analysis of variance and t-tests (EXCEL; Microsoft Inc., Redmond, Wash.). All data and figures are expressed as mean ± SD. Alpha level of p ≤ 0.05 was accepted as significant. Regressions and correlations were performed between Cstress and the load variables and between Cstress and chronic stress variables. Analysis was performed on data collected after the 2nd, 12th, and 24th runs. These data are expressed by run number rather than time because skiers completed their run with different finishing times. In a time-point reference, data were collected at approximately 15 minutes (2nd run), 90 minutes (12th run), and 180 minutes (24th run) into skiing.
Figure 1 depicts the HR responses from 3 skiers on a typical ski run cycle. This cycle includes skiing the run in approximately 1:40 minutes, then a slight pause on the hill for the investigators to record data, and then to ski the 20 m to the lift house, riding the 4:30 minute ski lift to the top of the run, and preparing to execute the run.
Average run time for the group from runs 2, 12, and 24 was 101.5 ± 5.5 seconds (Figure 2). The fastest average finishing time for an individual skier for those 3 runs was 95.0 seconds by subjects 6 and 9, whereas subject 8 had the slowest time at 108.7 seconds (Figure 2). Although the between-subjects range was 13.7 seconds, the within-subject range in finishing times was consistent. The average finishing time for all the subjects and all 24 runs was 102.9 ± 7.1 seconds.
The group average for HR during runs 2, 12, and 24 was 166.7 ± 15.5 b·min−1. The 166.7 b·min−1 represents 84.5 ± 7.7% of estimated HRmax (Figure 2). Skier 5 skied with an average of 68.5 % HRmax over the 3 runs, whereas skier 8 skied with the highest %HRmax at 92.8% (Figure 2). Heart rate was an insignificant predictor (p = 0.65) and was poorly correlated (r 2 = 0 .03) to the Cstress. Thus, only 3% of the variability in the Cstress could be accounted for by HR. Over the 24 runs, subjects skied at an average of 82.5 ± 8.8% of HRmax.
Average blood LA was 2.7 ± 1.1 mmol·L−1 after the second run (Figure 3). Lactate decreased significantly by the 12th and 24th runs to 1.8 ± 0.7 and 2.0 ± 1.0 mmol·L−1, respectively. Individual LA values ranged from 1.24 to 4.45 mmol·L−1 after the 2nd run and from 0.9 to 3.63 mmol·L−1 after the 24th run (Figure 4). Blood LA was a significant predictor of the Cstress (p = 0.05) with an r 2 value of 0.40. Thus, LA accounted for approximately 40% (moderate level) of the variability in the Cstress.
Changes in the first 3 salivary cortisol collections reflect what would be expected in the diurnal changes (Figure 3). Cortisol concentration decreased significantly from the preskiing level to the 12th run (4.5 ± 1.4 to 2.9 ± 0.8) collection point. However, cortisol levels increased significantly by 16% in the final 1.5 hours of skiing.
Creatine kinase significantly increased during the skiing. The preskiing CK concentration was 40.4 ± 19.3 U·L−1. Three hours after skiing, CK concentration had increased to 57.3 ± 25.4 U·L−1, an increase of 42% (p = 0.000). Individual CK changes ranged from 1 to 42 U·L−1.
Peak force did not statistically change during the skiing (p = 0.62). Preskiing peak force was 1151.6 ± 202.1 N compared to the postskiing peak force value of 1112.4 ± 187.7 N. Skiers experienced a significant decrease of 12% in isometric endurance time (p = 0.02). Preskiing isometric endurance time was 106.1 ± 29.6 seconds, whereas postskiing endurance time was 93.2 ± 24.0 seconds. Eight of 10 skiers experienced a reduction in endurance time performance (the largest individual decrease was 38 seconds), 1 skier had the same time, and 1 skier increased endurance time from before to after skiing by 10 seconds.
Although the variables to calculate chronic stress are dependent variables, a regression was performed to get a sense of how each of the 3 impacted chronic stress. The most important regression variable in assessing this stress was isometric endurance (r 2 = 0.81), whereas CK had an r 2 of 0.34, and salivary cortisol was at 0.32. The regression formula for cumulative stress was chronic stress = 8.021 ± 0.337 (change in isometric time) + 0.072 (percent change CK) + 0.085 (percent change cortisol) (p = 0.000; r 2 = 0.961).
The aim of this study was to investigate how well the acute markers of stress, HR and LA, predict and correlate with chronic stress when skiing at a recreational level intensity. This study did not use an extreme exercise but a half-day session of skiing at an intensity that is indicative of recreational skiing. The importance of this study was to assess the viability of typical field measures of LA and HR in predicting chronic stress.
We used a novel approach of assessing stress during skiing. The cumulative stress variables, isometric contraction time, CK, and cortisol were used to calculate a chronic fatigue score. These markers were used because each one would not change appreciably over a single run. In contrast, HR and LA were used as acute markers because these variables are frequently used and could change from run to run.
Isometric contraction performance time was the most important single factor in the regression equation accounting for 82% of the variance in the Cstress, whereas CK and salivary cortisol were moderate factors in accounting for 34 and 32% of the variance in Cstress, respectively. Based on the changes of the selected measures, LA was a significant predictor of and correlated with chronic stress. However, HR was not a significant predictor and did not correlate with chronic stress during skiing. We postulate that HR is not sensitive to overall fatigue because of the influence of multiple factors during skiing.
Environmental and mechanical factors exert a profound influence in the physiological responses to skiing intensity. For example, the environmental factors of snow conditions and changing terrain may force the skier to alter their skiing style accordingly. An individual's fitness level and skiing style, such as ski edge angulation during a turn, changing pressure distribution to the ski edge, intensity and type of muscle contraction (or efficiency of movement), and turn radii affect acute physiological responses, and hence, muscle stress. We controlled for snow conditions by having skiers ski a groomed course in similar temperatures. Turn radii were controlled by skiers completing turns within a standard corridor, skis were waxed each day, and skiers were of similar skiing ability. Although we controlled run time and HR during skiing, skiers could still change edge angles, pressure distribution on the ski, and muscle contraction mechanisms based on their individual style and preferences.
As the data demonstrate, individual ski styles leads to variability in the skiers' physiological responses (Table 1). Verbal feedback was given to the skiers to ski consistently from run to run. Although finishing times were approximately 103 seconds, the range of physiological responses in that subjects skied to achieve this time was substantial (Figure 2). Skier 8 demonstrated a low Cstress, achieving a HRmax at 93%, a very low LA of approximately 1.0 mM·L−1, finished the runs with an average time of 102 seconds, possessed a small change in CK, and lost 4 seconds in the isometric endurance test. By comparison, skier 1 demonstrated a high level of stress with the highest LA, lost the most time in the endurance test at 38 seconds, had an average run time of 105 seconds, had a large increase in CK, and skied at a HRmax of 86%. Subject 9 also had a high Cstress, as noted by an increase in LA, losing 19 seconds in the endurance test, had an average finishing time of 97 seconds, had a large increase in CK, and skied at an average percent of HRmax of 74%. Although the between-subject range of these variables was substantial, the within-subject range in the data over the 24 runs was rather small but consistent.
Blood LA and HR are frequently used indicators of fatigue and muscle stress during physical activity. Using these two criteria, recreational skiers often select skiing intensities that are moderate in nature. Average LA during skiing in the present study was approximately 2 mmol·L−1, similar to what was has been reported previously (6,17) and slightly less than what Seifert et al. (18) reported (2.7 mmol·L−1 vs. 2.0 mmol·L−1).
The dichotomy between LA and HR in the present study may arise from changing pace within the turn or different skiing styles. The change in pace occurs when performing a turn simply by changing edge angle and edge pressure, having to respond to variable snow conditions, such as hard pack vs. soft snow, and responding to changes in terrain or pitches. Thus, there are many options to change the steering variables within a turn, and consequently, the physiological responses will change accordingly. Changing the variables is difficult in activities such as cycling and running where HR and LA are acceptable indicators of acute stress and fatigue.
It is interesting to observe the 30% decrease in average LA from run 2 to 12, after only 1.5 hours on snow (20 minutes of actual skiing) and then little change in LA from run 12 to 24. We cannot explain why this occurred, given that HR and time to complete each run was held relatively constant. It is possible that skiers changed their style or pace of skiing to make sure they could make all 24 runs, using more of the terrain to aid in steering, changing edge angles or pressure distribution of the skies, or a change in muscle recruitment patterns to use more type I muscle.
Kröll et al. (7) reported a distinct shift in mean power frequency, indicating a change in fiber recruitment patterns during skiing-induced fatigue. These authors reported a shift from a predominant signal of fast twitch frequencies (IIa) to a signal dominated by slow twitch frequencies in the vastus lateralis because this muscle fatigues. These findings support those of Sadoyama and Miyano (16) who noted a shift from type II fibers to type I during fatigue. The consistently low LA levels in the last 12 runs in the present study, along with Kröll et al.'s (7) findings, indicate that there is a preferential recruitment pattern toward the type I muscle fiber as fatigue occurs during recreational skiing.
There are different ways that fatigue may be manifested during skiing. Consequently, the indices to assess fatigue may vary in response to the method used to induce muscular fatigue. Whereas total vertical meters skied in the present study was similar to that reported by Seifert et al. (18), the type, or severity, of muscular stress seems to be different. Skiers averaged approximately 103 seconds per run in the present study, whereas run times were approximately 5 minutes in the Seifert et al. (18). Vertical meters per run and, consequently, time per run was quite different between the 2 reports. The run time difference may have implications on the type of metabolic and contraction requirements placed on the active muscles.
Skiing is an activity with a high degree of eccentric muscle contraction (2). Skiers may experience increased levels of stress and muscle damage on a longer vs. a shorter run. Seifert et al. (18) noted a significant increase in CK of approximately 93% (from 126 to 243 U·L−1) in a group of recreational skiers. Creatine kinase concentration increased by an average of 42%, from 40 to 53 U·L−1, in the present study.
The apparent disparity in CK concentration between the present study and Seifert et al.'s (18) may be explained by differing CK analysis methods (Reflotron® vs. Johnson & Johnson Vitros II®; Johnson and Johnson, Langhorne, Pa.) or, most likely, that skiers skied twice as many runs (24 vs. 12) but less vertical distance per run in the present study (300 vs. 600 vertical meters). With less vertical skiing per run in the present study, the active muscles may have been subjected to less contractile stress during any run. Given the fact that the lift time was approximately 5 minutes, there may have been enough time to recover between runs. It also stands to reason that contractile stress in the muscle was less per run in the present study. Thus, it may have been the combined effect of a run time with recovery on the lift that contributed to the apparent CK difference in the 2 studies.
It is well known that salivary cortisol is an indicator of stress and follows a circadian rhythm throughout the day. As physical stress increases, so does salivary cortisol concentration (8,14). Under normal, unstressed conditions, cortisol typically reaches a peak during the early morning hours and then progressively decreases throughout the day to reach a nadir sometime around midnight. Although we did not collect 24-hour samples from our subjects, the assumption is that cortisol would have continued to decrease under resting conditions. O'Connor and Corrigan (14) reported salivary cortisol levels increased after 30 minutes of continuous cycling at 75% maximum oxygen uptake. However, cortisol levels decreased with time during the resting control trial in that study. Cortisol increased by 16% from the 12th to the 24th run (approximately 1.5 hours on the hill and 18 minutes of actual skiing) as skiers fatigued in the present study. The lower intensity of exercise and short duration of the runs (approximately 100 seconds) would explain why the magnitude of change is less in the present study than other reports. These results indicate that cortisol did increase under stress, that it could be used as training aid, or to aid future studies in fatigue and stress.
Skiing is an atypical sport where many external and internal variables act upon the skier. Individual compensation mechanisms and skiing style contributed to highly variable responses during skiing. It is important for the practitioner to be able to discern acute from chronic stress in terms of physiological responses, especially in sports such as downhill skiing. Measuring only one variable of stress may not be indicative of the true stress of the activity. Whereas HR is an indicator of acute stress within a given run, it was not a good indicator of chronic stress and fatigue. The use of LA as a practical on-hill marker of chronic stress is, however. Blood LA should be measured to assess skiing load. This information can help the instructor, guide, or coach give direction to the skier regarding skiing intensity to minimize chronic stress. This may be particularly helpful during multiday skiing.
When the results of the present study and Seifert et al. (18) are compared, it seems that intermediate level skiers should ski shorter runs to minimize muscle stress and optimize recovery on the chair lift. The present study's results indicate that skiers don't feel as fatigued with shorter runs; however, objective data indicate that fatigue and stress still occurred.
Future research may include the training effect of alpine skiing and identifying proper training zones for skiing. From a safety perspective, physical preparation for skiing should not neglect, and possibly emphasize, type I muscle fiber development because this is the fiber the body relies on during muscular fatigue and improving the methods of quantifying overall stress during different types of interventions (dietary manipulation, materials, etc.). With this knowledge, determining training loads for positive physiological adaptations and minimizing negative stressors that may lead to greater enjoyment of skiing and improved safety is required.
The authors would like to thank Atomic Skis and Pacific Health Laboratories for financial support, to Stefan Lindinger, Alex Kösters, Peter Scheiber, Jürgen Birklebauer, Monika Stadlman, Gerhard Stricker, Christian Schiefermüller, Martin Gimpl, Gernot Wagner, and Thomas Finkenzeller for their technical assistance, and to our subjects for their efforts and cooperation.
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Keywords:© 2009 National Strength and Conditioning Association
muscle stress; muscle damage; performance