Secondary Logo

Journal Logo

Competitive Sports: Section Articles

Cross-Country Skiing Injuries and Training Methods

Nagle, Kyle B. MD, MPH

Author Information
doi: 10.1249/JSR.0000000000000205
  • Free

Abstract

Introduction

Cross-country, or Nordic, skiing encompasses many different variations of skiing and equipment. For the purposes of this review, we considered cross-country skiing to be skiing on groomed cross-country ski trails using primarily lightweight skis, boots, and poles for the purpose of racing or fitness. Within cross-country skiing, there are two styles: classic, where the skis are aligned parallel to the direction of travel using a specific kind of ski wax to aid propulsion, and skate, where the skis move at an angle to the overall direction of travel, similar to ice skating. Within each style, there are multiple techniques employed, depending on the speed of the skier and the terrain. In the classical style, techniques include (used from slower to faster conditions) herringbone (used on steep uphills, with the contralateral arm and leg moving forward with the skis angled out to the side to prevent slipping down the hill but with no gliding on the ski), diagonal stride (contralateral arm and leg moving forward, skis parallel, with a distinct gliding phase to each ski), kick double pole (one leg kick followed by a double pole motion with both poles landing at the same time), and double pole (skis remain parallel and relatively in line with each other while the propulsion comes from both poles hitting the snow at the same time). Common skating techniques (in order of use from slowest to fastest conditions) include V1 (both poles hit the ground at the same time as one ski, without poling with the other leg stride), V2 (both poles hitting the snow just before the leg push, poling with each leg stride), V2 alternate (both poles hitting the snow just before the leg push, poling only with leg strides on one side), and free skate (skating without poling). While traditionally viewed primarily as an aerobic sport, cross-country skiing is unique in that it regularly involves races varying from 1 km to over 50 km, requires high force development and strength over a long period, and uses both the upper and lower body to provide propulsive forces.

Over the past two decades, there have been many changes in the sport of cross-country skiing. There have been new race formats introduced, such as sprints consisting of multiple heats over a short course and the skiathlon, or continuous pursuit, which requires skiers to ski in classic style, then transition to skate, often with a change of skis and poles and the transition time being taken into account in the timing, much like triathlon. There also have been changes in the techniques employed during races. For example, many classic marathons, typically around 50 km long, are now being skied exclusively using double poling on skis without kick wax. In the context of these changes in the sports, it would not be surprising to see changes in injury patterns and training strategies. This review will look at recent advances and debates in the cross-country ski community regarding injury occurrence and long-term health effects of cross-country skiing as well as recent literature that can be applied to training methods to improve cross-country ski performance.

Epidemiology

Cross-country skiing remains a relatively low-risk sport in terms of injury incidence. Cross-country ski injury rates generally are reported from 0.1 to 0.8 injuries per 1,000 skier days (1,71,7). A recent study found an injury rate of 0.02 to 0.09 per 1,000 skier days for competitive skiers and 0.51 per 1,000 skier days for recreational cross-country skiers, with 0.65 per 1,000 for women and 0.40 per 1,000 for men (21). Injury rates appear to be similar for both classic and skate techniques (7).

One study of World Cup cross-country skiers found an injury rate of 0.2 injuries per 1,000 km skied for men and 0.1 injuries per 1,000 km skied for women (13). A more recent study investigated injury rates among World Cup skiers of all disciplines over two seasons and found cross-country skiers to have the lowest injury rate (11.4 injuries per 100 athletes per season), lowest rate of injuries resulting in time lost from training or competition (6.3), and lowest rate of severe injuries requiring more than 28 d to return to sports (0.7) among all International Ski Federation (FIS) World Cup disciplines — alpine, snowboard, freestyle, jumping, Nordic combined, and cross-country. Just over 50% of injuries occurred during on-snow training, while about 30% of injuries happened during World Cup or World Championship races. Only 2% of injuries during the season occurred during dry land training. Of all on-snow injuries, 41% were slight, defined as not requiring any missed time from training or competition, and only 6% were severe. Muscle and tendon injuries (38%), joint and ligament injuries (31%), and contusions (15%) were the most common types of injury in cross-country skiers. Interestingly, cross-country skiers experienced no fractures during the two seasons of the study. A quarter of the cross-country ski injuries involved the lower back, sacrum, or pelvis, followed by the shoulder and clavicle, then the lower leg and Achilles tendon (14). For some of the disciplines, the authors used an assumption of 22 wk in the season and five skier days per week for World Cup athletes to generate injury rates per 1,000 skier days (14). Using that same assumption, cross-country skiing had an injury rate of 1.03 per 1,000 skier days for total injuries, 0.57 for all injuries causing any time loss, and 0.06 for severe injuries.

Overuse injuries are the majority of injuries for cross-country skiers. One small study of high-level Norwegian skiers (29 male skiers and 16 female skiers) looking at overuse injuries of four specific body sites found the average prevalence of overuse problems to be 8% for the knee, 5% for the lower back, 1% for the shoulder, and 12% for the anterior thigh. Notably, substantial overuse problems, those resulting in moderate-to-severe decreases in performance or training volume, had a prevalence of 1% at the first three sites but had a prevalence of 7% in the anterior thigh (11). The study did not attempt to determine specific diagnoses or causes of the thigh pain; however, it is a possible question for future research. In addition to usual predisposing risk factors for overuse injuries, one study used a combined group of endurance athletes (cross-country skiers, long-distance runners, and swimmers) aged 15 to 35 years to identify two other specific risk factors for overuse injury. Training more than 700 h·year−1 was associated with twice the increased risk for sustaining an overuse injury. Also, having less than two rest days per week increased the risk of overuse injury by five times (28).

Long-Term Health Effects Of Cross-Country Skiing

Low Back Pain

Low back is a concern for cross-country skiers due to the repetitive forward flexion and lumbar loading of the back. Anecdotally, low back pain is a fairly common complaint among cross-country skiers. One small study of adolescents (n = 22) showed a higher incidence of back pain in competitive male cross-country skiers training 10 h·wk−1 than that shown by noncompetitive skiers training 6 h·wk−1 (6). Another study found no difference in the incidence of back pain between adolescent male and female cross-country skiers (1). This pain was associated with different styles of skiing — double pole, double pole kick, diagonal stride, and/or skating — depending on the individual (1,61,6).

Using self-reported questionnaires to investigate low back pain in elite adult cross-country skiers, Bahr et al. (5) found low back pain in the previous 12 months to be more common in cross-country skiers than in age- and gender-matched controls from the general population (odds ratio (OR), 1.94; 95% confidence interval (CI), 1.29–2.92). Skiers also reported significantly more back pain in the previous 12 months compared with non-back loading athletic controls. The authors also noted that for both genders, twice as many skiers experienced low back pain with the classic ski techniques than when using the skating techniques (5).

Foss et al. (15) then sent a similar self-report questionnaire 10 years later to the same subjects from the same cohorts from the previous study of Bahr et al. (5) to investigate possible long-term issues with low back pain. They found that there was no difference between the cross-country skiers and the control group both for low back pain in the past 12 months and for frequent low back pain in the past year. They did find that there was higher risk of low back pain in subjects who trained more than 550 h·yr−1 in the past 12 months compared with those who trained less than 200 h·yr−1. They also found that low back pain was associated with previous episodes of low back pain. They concluded that low back pain was not more common in former elite cross-country skiers than in the general population (15). It seems that while back pain may be common during training and racing, it may not have long-term consequences for back health.

Cross-Country Skiing And The Heart

In the recent past, there has been increased discussion of the occurrence of atrial fibrillation and other cardiac arrhythmias in cross-country skiers. One study looking at a 10-year follow up of Swedish finishers of both genders for 10 years of a large Swedish marathon Nordic ski race without previously diagnosed cardiovascular disease (n = 52,755) found that among these skiers, there was a higher incidence of arrhythmias requiring hospitalization with increased number of races completed and with faster finishing times (2). There also was a higher risk of clinically significant bradyarrhythmias, primarily type 2 and 3 atrioventricular (AV) block and sick sinus syndrome, and atrial fibrillation with increased number of races completed and a tendency toward higher risk for both of these arrhythmias with faster finishing times (2). There was no association between supraventricular tachycardia, ventricular tachycardia, ventricular fibrillation, or cardiac arrest with either number of races finished or finishing time (2).

Arrhythmias were captured by hospitalization, so this study may underrepresent the total number of skiers developing arrhythmias, as many could be diagnosed and treated on an outpatient basis. Also, the cohort in this study was quite young (mean age at inclusion was 38.5 years old), and so, this study may not capture the overall risk for some arrhythmias, as the incidence of some arrhythmias increases significantly with age. There also was no control group in this study, so comparing the results with the incidence of arrhythmia in general populations or in other countries may be problematic.

Another study using a cohort of skiers (Norwegian men 40 years or older who completed the Norwegian Birkebeiner, a 54-km classic technique ski marathon, in 1999) and a cohort from the general population found that there was an increased adjusted OR per 10 years of routine endurance exercise of 1.16 (95% CI, 1.06–1.29) for atrial fibrillation and 1.42 (95% CI, 1.20–1.69) for atrial flutter. This study was not a direct comparison between skiers and the general population, as participants from both groups were included in those engaged in regular endurance exercise. However, as a reference point dividing the participants into three categories (no years of regular endurance exercise, less than 30 years, and 30 years or more of regular endurance exercise), a much larger percentage of skiers fell into the latter two categories, making up 65% and 88% of those categories, respectively (27). A 2013 study of Norwegian men over the age of 65 comparing finishers of the Norwegian Birkebeiner in 2009 or 2010 with the general Norwegian population found a small increased risk for atrial fibrillation (AF) in the skiers versus that in the general population (adjusted OR, 1.81; 95% CI, 1.04–3.14)(26). Whether the driver of the increased risk is specifically cross-country skiing or participation generally in endurance sports is not explored.

It is not all concerning news regarding heart health in cross-country skiers. A comparison of Swedish finishers of the Vasaloppet ski marathon with the general Swedish population shows that overall mortality rates of cross-country skiers of both genders appear to be lower than those of the general population by more than 50%. This large study of all Swedish finishers during 10 years of the Vasaloppet also showed that cardiovascular and cancer mortality rates are also significantly lower in cross-country skiers of both genders compared with those in the general population (12). Other studies also have shown decreased overall mortality in male cross-country skiers compared with that in the general population (17,3717,37). A 30-year study showed that continued unchanged training and competing in male cross-country skiers significantly attenuated the normal age-related decline in V˙O2max (16). The attenuated decline could be due to a number of factors — increased muscle mass, increased stroke volume, higher training time, higher training intensity — but the finding does provide encouragement for skiers of all ages to continue training and racing.

Developments In Training Methods

Cross-country skiing has historically been thought of as a sport where performance is determined primarily by aerobic and anaerobic capacities. However, recently, strength and force generation in adult cross-country ski race performance have gained importance in training, as the ability to generate maximal propulsive forces to increase and maintain cycle length in a short period has been shown to be critical in achieving maximum speed and improved race performance. Athletes need to train to develop specific explosive strength and maximum power to produce these necessary forces in both the upper and lower body.

Studies have shown that the ability to rapidly produce force is critical in double poling and V2 skate technique, where maximum speed is accomplished by increasing cycle rate while maintaining cycle length (22,23,3622,23,3622,23,36). There have been similar findings in herringbone classic technique and diagonal stride. One study showed that in herringbone classic technique, over three quarters of the propulsive force comes from the legs but a higher percentage of the total force of poling is translated into force moving the skier forward. Herringbone cycle time gradually shortened as skiers’ velocity increased. Cycle length during herringbone increased from moderate to higher velocities but then stayed static with an increased cycle rate eventually contributing more to reach the maximum velocity. Absolute durations of poling and leg thrust become shorter as the velocity increases (4). In diagonal stride, cycle length and rate increased as skiing speed increased from moderate to high speeds. However, as speeds progressed from high to maximal, the cycle rate continued to increase but the cycle length decreased. These results show the importance of cycle rate in obtaining maximum speed in diagonal stride. The rate of peak force development in the legs and poles also increased by 26% and 79%, respectively, as speed increased (3). Using diagonal stride, sprinters were 10% to 14% faster than distance specialists at higher speeds and demonstrated a 22% faster cycle rate and a 5% longer cycle length to achieve that higher speed (3). Being able to rapidly produce forces is thus important in both herringbone and diagonal stride.

The correlation between sprinting ski speed and general strength measures has been investigated. In one study of elite male skiers from three countries’ national teams, during double poling, cycle length at maximal and submaximal speeds correlated with bench pull performance, while inverted sit-up performance was associated with double pole cycle length only at submaximal speeds. Pole and leg forces during diagonal stride were not correlated with any general strength measures. The rate of force development during squat jump was correlated with the rate of leg force development in diagonal stride. For V2 skating, peak leg force was correlated with maximum force during single-leg isometric squat and jump height during vertical jump (35). These results demonstrate that the relationship between general strength and maximum ski speed may be technique dependent and that general strength may not be the primary determinant of ski speed. This study also indicated that the timing and coordination of these force applications may be the crucial factor in determining maximum skiing speed (35). Ski-specific motions, such as lateral bounding and spenst training, should be pursued in addition to general strength activities. The importance of continued work on technical skiing skills also should be emphasized to aid correct application of the explosive forces.

Generation of poling forces also is important to improve skiing performance. Over the past two decades, the double pole technique has changed to a more dynamic form, with higher force production and more dynamic movement patterns of the upper and lower body (20,2320,23). More classic races are being raced using double pole only as poling has become more efficient and skiers became stronger. Faster double poling velocities have been associated with increased range of motion of the knee, hips, and elbows (23). Free use of flexion and extension of the knees and ankles has been shown to increase peak V˙O2 (7.7%), maximum speed (9.4%), and time to exhaustion (11.7%) compared with if the knee and ankle joints are locked in position. Locking the knees and ankles also results in higher heart rate and blood lactate levels at submaximal speeds and higher poling frequencies with lower relative poling force (19). These results show that the movement of the lower body is an important aspect of double poling, and efforts should be made to train with integrated full-body movements to maximize coordination of the upper and lower body.

The cycle length and frequency of double poling also impact the metabolic efficiency of the activity. A study investigating the effect of three different poling rates (40, 60, and 80 cycles per minute) at various skiing velocities found that double poling cycle length and impact force increase as speed increases while peak pole force and time to peak force decrease. Oxygen uptake, heart rate, and pulmonary ventilation also increase with higher poling frequency at the same skiing velocity. Thus, at submaximal speeds, a slower rate of poling may be more metabolically efficient (22). In a situation where skiers dictated their own double poling frequency as velocity was increased, it was shown that increased double pole velocity was accomplished by increasing cycle frequency and length at submaximal speeds and then increased frequency alone to achieve maximum velocity. The cycle length increase was associated with decreased poling time and time to peak pole force while peak pole force and rate of force development increased (23). It may be useful during training to employ various frequencies of double poling to improve efficiency at different frequencies and specific strength needed to increase cycle length when necessary in order to respond easily to terrain changes and pacing or poling frequency changes in leading skiers when skiing in a pack.

Given this information on the role of power and rate of force production on ski speed and performance, the effect of strength training on ski performance and these parameters is less well-known. One recent study investigated the effect of heavy strength training on skiing performance using elite-level male and female Norwegian skiers. They found that a periodic 12-wk strength training program, involving two sessions per week — including half squat, seated pulldown, standing double pole and triceps press — improved V˙O2max during skate roller skiing significantly more than that in the control group (7% ± 1% vs 2% ± 2%, respectively), while running V˙O2max did not significantly change in either group. The strength group also improved significantly more in their double pole power output. Both groups, however, experienced a similar significant improvement in their skate roller ski time trials. The control group also saw a significant improvement in the double pole time trials, while the strength group experienced a slight, but not statistically significant, improvement. At submaximal efforts, there was no change in oxygen consumption during roller skiing on a treadmill in either group, although the strength group had significantly lower average heart rate and respiratory exchange ratios at the submaximal efforts while the control group had almost no changes (24). Another study has shown that 9-wk high-intensity strength training in female skiers can lead to improved performance in double poling as measured on a ski ergometer. The strength group also had increased time to exhaustion compared with that of the control group (18).

These studies give support for the value of strength training. While it certainly seems advantageous to improve V˙O2max while skate roller skiing and decreasing heart rate and respiratory exchange ratio, in an actual race setting, i.e., time trial, there is conflicting evidence of improved performance. Certainly, the increased double pole power, and improved physiologic variables during skate roller skiing, may allow skiers to adapt more to sudden changes in tempo or respond to attacks that occur in the more common mass-start races.

Strength training also can aid in the development of lean body mass, which has been shown to be correlated with higher classic technique peak speeds and performance in sprint and distance races (10,3210,32). Stoggl et al. reported that body mass, BMI, lean body mass, and lean trunk mass were positively correlated with peak double pole speed (32). Relative lean body mass, lean trunk mass, and relative lean arm mass were positively correlated with peak velocity using diagonal stride. Total arm length relative to height and upper arm relative length were the only body length dimensions to correlate with speeds, and interestingly, there was a negative correlation with double poling peak speed. Pole length relative to body height was correlated with peak speed using double pole and diagonal stride in the classic technique; however, absolute pole length and height were not correlated to peak speed (32). It should be pointed out that these results were focused on peak speed in a very short (<90 s) effort and not correlated to actual race performance or performance over longer distances. A different study found that absolute whole body, upper body, and lower body lean masses were significantly correlated with performance during a sprint prologue for both men and women and results in a distance race for women (10). Increasing lean body mass may result in higher speeds and improved race results.

Monitoring training and deciding what tests are most appropriate and most likely to predict performance remains an area of investigation. One study in elite male skiers found that lactate threshold, V˙O2max, and 60-s double pole tests best correlated with FIS points and performance in a 15-km classic race. There was no correlation with performance and knee extension peak torque, vertical jump, or 20- or 360-s double pole tests (9). In the same study, however, only three measures correlated with performance at the Swedish National Championship 30-km skiathlon race: time to exhaustion during a roller ski V˙O2max trial, time before blood lactate rose 1 mmol·L−1 above baseline, and mean V˙O2 from the 60-s double pole trial (9). This study suggests that different tests may be more applicable to different events. For example, considering double pole sprint race performance, 50-m double pole sprint time, maximum velocity during double pole, and maximum velocity in the 50-m double pole sprint correlated very well with 1-km double pole roller ski time trial performance (33). Even over a sprint format event with multiple heats, maximum double pole velocity and diagonal stride velocity were highly correlated with roller ski sprint performance (34). So, a short double pole or diagonal stride sprint could be used as a marker for monitoring training and potential performance in sprint distance classic races, such as the flat city sprints that are often completed using double pole technique alone. This result also suggests value in including ski-specific speed training into general training plans to improve sprint racing results. It may be prudent to choose tests to monitor training and predict performance success based on the duration, ski techniques, and movement patterns involved (9).

One question persists in how those training concepts transfer to youth skiers who may be physiologically different from adults. In a mixed gender group of junior skiers (17.4 ± 0.5 years old), V˙O2max and V˙O2 at ventilatory threshold were closely correlated with 1.5-km roller ski skate sprint performance (30). One recent study evaluated 16 tests of general motor ability and anthropometric measures for their correlation with youth cross-country ski performance (ages, 13.8 ± 0.6 years old for boys and 13.4 ± 0.9 years old for girls). For girls, they found that age and anthropometric measures had no correlation with cross-country ski performance while their performance of a push-up test, 20-m sprint, and particularly a 3-km run correlated well with performance. For boys, degree of maturation had a large effect on the results, but age, height, weight, maturity offset (time from peak height velocity), and upper body dimensions correlated well with cross-country ski performance. Push-ups, overhead medicine ball throw, pull-ups, drop jump test, and 20-m sprint also were correlated with boys’ ski performance (31). This does not negate possible improvement by girls from engaging in strength training. It may be just at that age and development, the aerobic capacity is more predictive of ski performance. Diagonal stride time trials by junior skiers have shown higher peak V˙O2 than that obtained in a traditional incremental ramp test (25). Diagonal stride and double pole time trial results in juniors also has been shown to correlate well with FIS sprint and distance rankings (8). Monitoring these field tests may be simpler and less expensive for youth ski programs than more complex and expensive laboratory-based tests.

The emphasis on strength training and power production in recent studies does not preclude the necessity of developing aerobic and anaerobic metabolic pathways in skiers. Some recent research has been done to identify the best way to develop those pathways. Traditional aerobic conditioning has been developed through long, low-intensity sessions combined with high-intensity intervals. One recent study investigated the duration and intensity of interval work most effective in junior skiers. Comparing 8-wk training plans using long-duration intervals (5 to 10 min each; total duration, 40 to 45 min) with shorter higher-intensity intervals (2 to 4 min each; total duration, 15 to 20 min) and a control group (no intervals, low-intensity workouts only), the authors found the long interval group improved significantly in V˙O2max, V˙O2 at ventilatory threshold, 12-km skate roller ski time, and 7-km hill run time. The long interval group’s improvements in both time trials and V˙O2 at ventilatory threshold were significantly better than the changes in both the short interval group and the control group. The short interval group improved V˙O2max significantly only compared with preinterval levels and the control group. The control group did not improve significantly in any of the outcome measures (29). The intervals in this study were performed at maximum sustainable intensity, which was generally 85% to 92% of maximum heart rate in the long interval group and >92% maximum heart rate in the short interval group (29). Another study found a similar result regarding sprint ski performance in junior skiers (average age, 17.4 ± 0.5 years). An 8-wk program involving 2.5 times as many high-intensity interval (5 to 10 min each at 85% to 92% of maximum heart rate) as baseline training was compared with a control group that increased low-intensity exercise by only 30% but made no change in the high-intensity workload. The high intensity group improved significantly on a 1.5 skate roller ski time trial, V˙O2max, and V˙O2 at ventilatory threshold. Compared with the control group, the high-intensity group had significantly better sprint performance and V˙O2 at ventilatory threshold. In fact, almost half of the control group had slower time trial times after 8 wk (30). In junior skiers, longer intervals appear to be effective and necessary for improving performance and physiologic variables related to both sprint and distance skiing performance. These studies do not address exactly how much of the high-intensity work is needed or if too much could be detrimental. The improved performance after high-intensity training could be due to both improved aerobic conditioning as indicated by the increased O2 consumption at ventilatory threshold and by improved neuromuscular efficiency, as more time was spent training at higher speeds.

Conclusions

Cross-country skiing is a low-risk sport that has many health benefits, in the short and long term, for recreational and competitive athletes. Cross-country skiing, in both classic and skate techniques, strengthens both the upper and lower body and provides a cardiovascular workout. Recent research shows that while cross-country skiers should continue to emphasize aerobic and anaerobic training, strength training is also beneficial to increase maximum force and the rate of force development. It also appears that this strength training should utilize ski-specific motions from a variety of ski techniques in order to aid the coordination and timing of the muscle forces. The overall goals of training should be to develop the ability to produce quick forceful motions and to develop the muscular endurance to sustain the quick forceful motions during long races. A variety of tests are available to monitor progress toward these goals, and which tests would be most appropriate depends on the specific events being emphasized. In addition to laboratory-based tests, there are also a number of simpler, more cost-effective tests that can be used, particularly at the junior skier level where access and cost may be more prohibitive.

The author declares no conflicts of interest and does not have any financial disclosures.

References

1. Alricsson M, Werner S. Self-reported health, physical activity and prevalence of complaints in elite cross-country skiers and matched controls. J. Sports Med. Phys. Fitness. 2005; 45: 547–52.
2. Andersen K, Farahmand B, Ahlbom A, et al. Risk of arrhythmias in 52 755 long-distance cross-country skiers: a cohort study. Eur. Heart J. 2013; 34: 3624–31.
3. Andersson E, Pellegrini B, Sandbakk O, et al. The effects of skiing velocity on mechanical aspects of diagonal cross-country skiing. Sports Biomech. 2014; 13: 267–84.
4. Andersson E, Stoggl T, Pellegrini B, et al. Biomechanical analysis of the herringbone technique as employed by elite cross-country skiers. Scand. J. Med. Sci. Sports. 2014; 24: 542–52.
5. Bahr R, Andersen SO, Løken S, et al. Low back pain among endurance athletes with and without specific back loading — a cross-sectional survey of cross-country skiers, rowers, orienteerers, and nonathletic controls. Spine. 2004; 29: 449–54.
6. Bergstrom KA, Brandseth K, Fretheim S, et al. Back injuries and pain in adolescents attending a ski high school. Knee Surg. Sports Traumatol. Arthrosc. 2004; 12: 80–5.
7. Butcher JD, Brannen SJ. Comparison of injuries in classic and skating Nordic ski techniques. Clin. J. Sport Med. 1998; 8: 88–91.
8. Carlsson M, Carlsson T, Hammarström D, et al. Validation of physiological tests in relation to competitive performances in elite male distance cross-country skiing. J. Strength. Cond. Res. 2012; 26: 1496–504.
9. Carlsson M, Carlsson T, Hammarström D, et al. Prediction of race performance of elite cross-country skiers by lean mass. Int. J. Sports Physiol. Perform. 2014; 9: 1040–5.
10. Carlsson M, Carlsson T, Hammarström D, et al. Time trials predict the competitive performance capacity of junior cross-country skiers. Int. J. Sports Physiol. Perform. 2014; 9: 12–8.
11. Clarsen B, Bahr R, Heymans MW, et al. The prevalence and impact of overuse injuries in five Norwegian sports: application of a new surveillance method. Scand. J. Med. Sci. Sports. 2015; 25: 323–30.
12. Farahmand BY, Ahlbom A, Ekblom O, et al. Mortality amongst participants in Vasaloppet: a classical long-distance ski race in Sweden. J. Intern. Med. 2003; 253: 276–83.
13. Florenes T, Nordsletten L, Heir S, Bahr R. Injuries to World Cup Nordic skiers and telemarkers — data from two seasons. Br. J. Sports Med. 2011; 45: 310.
14. Florenes TW, Nordsletten L, Heir S, Bahr R. Injuries among World Cup ski and snowboard athletes. Scand. J. Med. Sci. Sports. 2012; 22: 58–66.
15. Foss IS, Holme I, Bahr R. The prevalence of low back pain among former elite cross-country skiers, rowers, orienteerers, and nonathletes: a 10-year cohort study. Am. J. Sports Med. 2012; 40: 2610–6.
16. Grimsmo J, Arnesen H, Maehlum S. Changes in cardiorespiratory function in different groups of former and still active male cross-country skiers: a 28–30-year follow-up study. Scand. J. Med. Sci. Sports. 2010; 20: e151–61.
17. Grimsmo J, Maehlum S, Moelstad P, Arnesen H. Mortality and cardiovascular morbidity among long-term endurance male cross country skiers followed for 28–30 years. Scand. J. Med. Sci. Sports. 2011; 21: e351–8.
18. Hoff J, Helgerud J, Wisloff U. Maximal strength training improves work economy in trained female cross-country skiers. Med. Sci. Sports Exerc. 1999; 31: 870–7.
19. Holmberg HC, Lindinger S, Stoggl T, et al. Biomechanical analysis of double poling in elite cross-country skiers. Med. Sci. Sports Exerc. 2005; 37: 807–18.
20. Holmberg HC, Lindinger S, Stoggl T, et al. Contribution of the legs to double-poling performance in elite cross-country skiers. Med. Sci. Sports Exerc. 2006; 38: 1853–60.
21. Ketterl R. Recreational or professional participants in Nordic skiing. Differences in injury patterns and severity of injuries [in German]. Unfallchirurg. 2014; 117: 33–40.
22. Lindinger SJ, Holmberg HC. How do elite cross-country skiers adapt to different double poling frequencies at low to high speeds? Eur. J. Appl. Physiol. 2011; 111: 1103–19.
23. Lindinger SJ, Stoggl T, Muller E, Holmberg HC. Control of speed during the double poling technique performed by elite cross-country skiers. Med. Sci. Sports Exerc. 2009; 41: 210–20.
24. Losnegard T, Mikkelsen K, Ronnestad BR, et al. The effect of heavy strength training on muscle mass and physical performance in elite cross country skiers. Scand. J. Med. Sci. Sports. 2011; 21: 389–401.
25. McGawley K, Holmberg HC. Aerobic and anaerobic contributions to energy production among junior male and female cross-country skiers during diagonal skiing. Int. J. Sports Physiol. Perform. 2014; 9: 32–40.
26. Myrstad M, Lochen ML, Graff-Iversen S, et al. Increased risk of atrial fibrillation among elderly Norwegian men with a history of long-term endurance sport practice. Scand. J. Med. Sci. Sports. 2014; 24: e238–44.
27. Myrstad M, Nystad W, Graff-Iversen S, et al. Effect of years of endurance exercise on risk of atrial fibrillation and atrial flutter. Am. J. Cardiol. 2014; 114: 1229–33.
28. Ristolainen L, Kettunen JA, Waller B, et al. Training-related risk factors in the etiology of overuse injuries in endurance sports. J. Sports Med. Phys. Fitness. 2014; 54: 78–87.
29. Sandbakk O, Sandbakk SB, Ettema G, Welde B. Effects of intensity and duration in aerobic high-intensity interval training in highly trained junior cross-country skiers. J. Strength Cond. Res. 2013; 27: 1974–80.
30. Sandbakk O, Welde B, Holmberg HC. Endurance training and sprint performance in elite junior cross-country skiers. J. Strength Cond. Res. 2011; 25: 1299–305.
31. Stoggl R, Muller E, Stoggl T. Motor abilities and anthropometrics in youth cross-country skiing. Scand. J. Med. Sci. Sports. 2015; 25: e70–81.
32. Stoggl T, Enqvist J, Muller E, Holmberg HC. Relationships between body composition, body dimensions, and peak speed in cross-country sprint skiing. J. Sports Sci. 2010; 28: 161–9.
33. Stoggl T, Lindinger S, Muller E. Reliability and validity of test concepts for the cross-country skiing sprint. Med. Sci. Sports Exerc. 2006; 38: 586–91.
34. Stoggl T, Lindinger S, Muller E. Analysis of a simulated sprint competition in classical cross country skiing. Scand. J. Med. Sci. Sports. 2007; 17: 362–72.
35. Stoggl T, Muller E, Ainegren M, Holmberg HC. General strength and kinetics: fundamental to sprinting faster in cross country skiing? Scand. J. Med. Sci. Sports. 2011; 21: 791–803.
36. Stoggl TL, Muller E. Kinematic determinants and physiological response of cross-country skiing at maximal speed. Med. Sci. Sports Exerc. 2009; 41: 1476–87.
37. Teramoto M, Bungum TJ. Mortality and longevity of elite athletes. J. Sci. Med. Sport. 2010; 13: 410–6.
Copyright © 2015 by the American College of Sports Medicine.