Cross country Skiing

Fitness Testing Assignment: Cross country Skiing - by Hilde Gjellesvik

Contents

• Introduction
• Characteristics of Cross-Country Skiers
• Aerobic Tests
• Anaerobic Tests
• Muscular Strength and Endurance Tests
• Other Tests
• References

Introduction

In Nordic countries, cross-country skiing has for long been a popular sport. Worldwide, over 16 million people regularly participate in the sport. Cross-country skiing is a classic example of an endurance activity (Karlsson 1984). It requires mostly a stong and efficient aerobic system (Morrissey et al 1987). Cross-country skiers use of upper arm musculature in the poling action leads to some of the highest levels of maximum oxygen consumption recorded in athletes (Smith et al 1996).

Characteristics of Cross-Country Skiers

The cross-country skier is 95-100% dependent on aerobic energy output. Maximal oxygen uptake is widely accepted as a crucial determinant for aerobic endurance capacity, and is commonly used in assessing performance capability in cross country-skiers (Bunc et al 1987, Niinimaa et al 1978). Rusko et al (1978) reported lower maximal oxygen uptake in U.S. cross-country skiers compared to Nordic skiers. Despite that, the U.S. got a silver medal in 1976 Olympics. It is therefore of importance to remember that maximal oxygen uptake is only one of several factors affecting the performance in cross-country skiing. Although normative data is available in the literature, only a few examples will be presented in this assignment.

Assessment Methods Utilised

The efficacy of transfer of biochemical energy to mechanical work is another important aspect for the athlete but no satisfactionary method exists for estimating this variable (Karlsson 1984). Environmental factors such as snow and ski equipment must also be taken into consideration. For these reasons, results from single races may not reflect real difference in endurance performance. Anaerobic threshold and coefficient of energy cost of moving should also be assessed in addition to maximal oxygen uptake. Moving economy has been shown to influence a cross-country skier's performance. This can be described as a coefficient of energy cost of moving, which indicates how much energy the athlete uses to transfer 1 kg of body mass over a horizontal path of 1 m (Bunc et al 1987).

Haymes and Dickinson (1980) also noted that the aerobic capacity of skiers is an aspect which has been studied several times, whereas less attention has been paid to other aspects of fitness. Factors like strength, power, muscle endurance, agility, balance and neuromuscular performance are of importance as well. The aim of their study was to assess aerobic capacity, strength and power in male and female skiers in relation to lean body weight. Maximal oxygen uptake was determined using a procedure similar to Costill and Fox (1969), isometric strength was measured using cable tensiometry, isokinetic strength was measured using a Cybex dynamometer, maximal power was estimated using Magaria-Kalamen stair run, the vertical jump was used as a test of explosive power, agility was assesed by the Barrow zigzag run, response time was tested in accordance to a visual light, and balance measured by use of a balanciometer. Results showed significant differences in maximal oxygen uptake between male and female skiers. Both had more leg strength than cross-country skiers. In both the Magaria-Kalamen stair test and the vertical jump, male alpine skiers had the best score. Female-cross country skiers performed poorly on the response time test. A 19% lower maximal oxygen uptake was found in cross-country female skiers compared to male skiers when VO₂ was divided by weight. This finding could be explained by the fact that female skiers have more body fat and that fat uses little oxygen. The researchers looked at VO₂ per kg LBW and this also showed that the male cross-country skiers had a 9% larger VO_{2 max}/kg/ LBW compared to female cross-country skiers. The authors suggest that this result can be due to differences in hemoglobin concentration and oxygen capacity of the blood, which is 12% lower in females. Finally alpine skiers performed better on the agility test whereas cross-county skiers achieved the best results on the balance test.

Cross-country skiing consists of repeated dynamic contractions for upper and lower extremity muscles. The force output in upper extremity muscles can reach tensions levels close to maximal. Force output is thought to be dependent on the skiing technique used, but according to Karlsson (1984) there is no evidence to this effect. Cross-country skiing involves muscle activity that exceeds the amount of activity compared to walking or running, and compared to alpine skiing, cross-country skiing requires less intense concentric muscle contractions (Morrissey et al 1987).

Physiological Characteristics

The neuromotor system in a skier learns specific sets of movements. Differences in motor skills between elite and recreational skiers have been demonstrated through EMG measurements. The results show more regular patterns of muscle contractions within elite skiers, indicating better motor control, good coordination and higher mechanical efficiency compared to recreational skiers (Karlsson 1984). Morrssey et al (1987) referred to a study by Hixson (1980) that looked at the importance of different muscle groups and their major functions in cross-country skiing, and found that the extensor muscles are the most important muscle group necessary for propelling the skier. The literature provides few EMG studies on cross-country skiing.

Karlsson et al (1975) found a high percentage of slow twitch muscle fibers in vastus lateralis and the gastrocnemius in cross-country skiers. This can be due to either adaptation or genetic predisposition. The slow twitch fibers contain more mitrocondria and myoglobin compared to fast twitch muscle fibers more commonly found in sprinters and strength sports. Komi et al (1977) reported that the percentage of fast twitch muscle fibers were the same for alpine, Nordic combined and cross-country skiers. This suggests that differences in strength and power between the groups are due to training. They also reported a significant correlation between% FT fibers and the Margaria-Kalamen stair test.

Demment et al (1988) looked at citizen skiers to see how physiological characteristics related to cross-country ski performance. They found in accordance with other authors that VO_{2 max} relative to body mass is an important determinant for succsess. VO_{2 max} was evaluated using a treadmill protocol so that results could be compared to other studies, although a skiers' true VO_{2 max} may be underestimated by using this protocol. Adequate upper body strength was also reported to be of importance. Demment et al (1988) used a Cybex dynamometer to evaluate strength at three different speeds while the subject performed an arm motion similar to poling. It is interesting that their results showed no correlation between body fat and performance. An explanation for this can be that the effect of body mass per se is less of an issue in skiing than for other aerobic sports. Both when skiing downhill and on the flat, the heavier skier has an advantage while the lighter skier has an advantage on the uphill portions of the course.

Bunc et al (1987) looked at physiological standards for top Czech cross-country skiers. They found that cross-country skiers were similar in height when compared to other athletes. Their body mass was slightly lower than swimmers, but higher than cyclist's and long-distance runner's. VO_{2 max} was consistent with previous measurements of endurance athletes similar to Czech endurance athletes, and higher than elite top swimmers. All groups were evaluated by the same protocol. Several authors have found significant correlations between VO_{2 max} + kg 1 and distance running. Bunc et al (1987) suggests that this is also the case for cross-country skiers to become world class skiers, but to be able to asses aerobic capacity one must also evaluate physical performance/speed at the same time. Their study showed that at high levels of adaption , the lower is the amount of energy necessary to transfer 1 kg over 1 meter.

Physiological standards presented by Bunc et al (1987) which can be used in the selection of talents for young cross-country skiers:

	Boys	Girls
VO2 max.kg-1 (ml.kg-1.min-1)	>74	>65
Vmax (km.h-1) (5%)	>18	>16
Lamax (mmol.l-1) at VT level	>12	>11
%VO2 max (%)	>82.5	>82.5
v (km.h-1.m-1)	>15.5	>13.0
c (J.kg-1.m-1)	>3.75	>3.73

Preparation for Laboratory Testing

According to Gilliam and Ellis (1991) the temperature within the laboratory should be 18-23 °C with the humidity less than 60%, no training on the day of testing and preferably no race in the two previuos days. Food should not be consumed 2 hours prior to testing. Familiarisation and warm up on treadmill should occur prior to testing. The test Methods Manual (Draper et al 1991) also provide a equipment checklist concerning what should be checked before testing.

Aerobic Tests

Based on evaluation of cardiopulmonary function, the elite cross-country skier utilizes extreme high maximal oxygen uptake, 7.5 to 8.0 liters per minute or 67-85 ml/kg/min (Demment et al 1988). The major portion of their training is focused upon a high central and peripheral circulatory capacity and oxygen utilizing system, which is demonstrated by their high maximal oxygen uptake. Aerobic power (VO_{2 max}) and determination of secondary ventilatory threshold (VT2) are tests preformed to evaluate the cross-country skiers aerobic power (Gilliam and Ellis 1991).

The protocol provided by Gilliam and Ellis (1991) for arm-leg VO_{2 max} and VT2

• following warm up, explain the purpose of test
• start test at 50 W
• RER should be below 0.9, stay at 50W until this is satisfied
• increase workload to 100W after one minute, for men increase to 150 W
• 25W increasements each minute until subject unable to hold the load for 30 seconds
• peak workloads for elite cross-country skiers expected to be 400 and 500W
• VO₂ should not increase more than 0.2 l.min^-1 over the last two collection periods, RER should exceed 1.10, and heart rate must not increase over the last 30 seconds if a true VO_{2 max} is obtained.
• the second ventilatory threshold is determined by using specific procedures, values generally between 75-85% VO_{2 max}. If variation in VT2 is more than 0.5 l.min^-1 a plot of VCO₂ vs VE should be used in the determination
• VO_{2 max} and VT2 and power data should be recorded in l.min^-1 and watts, and standarised for body weight (kg^-1 and kg^-2/3).

Table 1. Cardiorespiratory data for arm-leg ergometer presented by Gilliam and Ellis (1991)

	VO2 max (in ml.kg-1.min-1)	HR max (in bpm)	RER	VO2@VT2 (in ml.kg-1.min-1)	HR @VT2 (in bpm)	%VO2 max	% MHR
Males	72.7	192	1.11	57.7	180	79.4	93.5
Females	57.8	189	1.10	49.8	176	86.3	93.1

VT2 is the Ventilatory threshold (2)

Protocol provided by Gilliam and Ellis (1991) for upper body poling maximum oxygen uptake

• familiarisation before testing
• stand two meter in front of the kayak ergometer
• movements similar to that in diagonal technique is encouraged, trunc trotation is not allowed
• skier attatched to ECG and mouthpiece
• 25W starting load for two minutes, increased by 25W every minute until the subject is unable to hold the load for 30 seconds
• peak workloads of 100-200 W can be expected
• arm fatigue may limit end point due to fatigue, in skiers with less strength in upper bodies the peak VO₂ is taken as the VO_{2 max}

Table 2. Normative data for arm-leg VO₂ max and upper body VO_{2 max} is presented by Gilliam and Ellis (1991)

	Arm-leg ergometer VO2 max (in ml.kg-1.min-1)	Upper-body ergometer VO2 max (in ml.kg-1.min-1)
Males	76.6	68.7
Females	65.5	43.1

Specificity of Cross-Country Skiing Assessments

Bilodeau et al (1995) suggest that testing of maximal aerobic power in cross-country skiers should include a combination of arm and leg work. This is based on the thought that testing should be specific to the sport evaluated (Mygrind et al 1991) and that the involvement of large muscle groups is necessary to achieve the highest possible oxygen uptake (Bergh et al 1976, Stømme et al 1977). Leg and combined arm-leg tests have been shown to be quite similar (Bilodeau et al 1995). A number of studies show that testing of upper body work in cross-country skiers have shown a good association between tests and performance (Bilodeau et al 1995). Few studies have looked at the peak aerobic power of upper-body muscles in a fashion similar to cross-country skiing. It is shown that peak VO₂ arm values up to 66% and 86% of the maximal value can be reached ( Mygind et al 1991 ). Arm VO_{2 peak} is dependent on ski technique used, according to Bilodeau et al ( 1995 ) several authors have reported that double-poling attain higher arm VO₂ values than diagonal striding.

Protocol provided by Bilodeau et al (1995) for maximal aerobic power (VO_{2 max})

• automated open-circuit system with a O₂ and CO₂ analyzer
• ventilatory volumes determined by a pneumotachometer with a 5.3 litre mixing chamber and a microcomputer during a progressive test on a motor driven treadmill
• subject ran until exhaustion and two minutes stages were used with no rest between stages
• first 8 stages only the speed was increased by 0.3 m.s^-1 for each stage
• on the 9 stage the speed was kept constant while the slope was increased by 1.8 degrees for each subsequent stage
• heart rate was monitored by ECG

Protocol provided by Bilodeau et al (1995) for upper-body VO_{2 peak} test

• automated open-circuit system with a O₂ and a CO₂ analyzer and a computerized arm ergometer simulating double-poling action of cross-country skiing
• the height of the ergometer adjusted as the height of the ski poles
• subject asked to pull on the ropes as he/she would normally do while double-poling on skis
• warm up session of 5-10 minutes with a work-load of 1.2 kg
• the work load was increased by 0.2 kg every 2 minutes, starting with a workload of 1.4 kg
• test ended when the subject cannot increase power output or increase his/her oxygen consumption

Protocol provided by Bilodeau et al (1995) for upper-body test

• performed on an upper body ergometer with workload increased every 20 seconds
• initial workload for men set at 1.4kg and for women at 1.0kg
• increment of 0.2kg for both groups
• the subject was asked to keep a cadence over 400rpm
• the test was terminated when the subject no longer can maintain a minimum cedence of 375rpm over three consecutive 20 second periods
• heart rate was measured every 5 minutes with a polar Vantage XL Monitor

Bilodeau et al (1995) state that maximal aerobic power test for cross-country skiers should be an arm test, a leg test or a combination of both arm and leg exercise.

Protocol provided by Wisløff and Helgerud (1998) for ski-ergometer

Helgerud and Rasmussen (1988) developed an upper body ski ergometer to evaluate aerobic endurance and force development in the upper body in cross-country skiers. The ski ergometer is both reliable and valid. The device enables the testing to be very similar to the actual movements preformed when double poling in cross-country skiing. Classic technique with unilateral arm action can also be evaluated by using this device . The ski ergometer consists of a freely moving platform, two specially adapted ski poles, with wheels running on rails connected by wires to an electric motor. The inclination can be varied from 1 to 20 degrees, and resistance is variable and controlled by a computer.

Power output (W) is calculated using the equatation: Power output (W) = Sin(alpha) × Mb × v. Mb is the body mass in Newton, v is the motor speed in m.s^-1, and alpha is the inclination of the ski ergometer. A load cell in the front of the ergometer registers power output. Before testing, calibration of the load cell and motor speed is performed.

Protocol provided by Wisløff and Helgerud (1998) for measuring VO_{2 peak} in Series I

• VO_{2 max} was measured by a preliminary test on a treadmill
• then 8 separate VO_{2 peak} determinations were performed on the ski ergometer
• continuous and discontinuous protocols were compared at 3, 5, 6, 7 degree inclinations
• order of tesing was randomized
• continuous testing there was increased in testing each minute
• discontinuous protocol, 5 minutes exercise bouts were separated by 30 minutes rest intervals
• increment that an increase in power output of 20W required an increase in VO₂ of 5 ml.kg^-1.min^-1 was employed for both continuous and discontinuous testing

No statistically significant differences in VO_{2 peak} using continuous and discontinuous protocols were found. At 5 degree inclination, the highest values for VO₂ were recorded. A 7 degree inclination proved excessive for determination for peak oxygen consumption. The explanation for this was hypothezised, that working at 7 degrees or higher results in limitations of peripheral oxygen transport. Another reason that the 5 degree inclination was chosen as a standard incline for subsequent testing, was that the subjects' reported the poling speed at this inclination to be most realistic. Wisløff and Helgerud (1998) conclude that further research is needed for choosing an appropriate inclination for less trained subjects. While running on the treadmill VO_{2 peak} and Th_an were significantly higher than the ski VO_{2 peak} and Th_an.

Anaerobic Tests

The contributions of aerobic and anaerobic energy output are dependent on the performance time of the activity. High intensy training will result in high lactate levels in muscle and blood shortly after onset of exercise. This has a negative influence on the contracting muscle and the central nervous system, resulting in reduction of central drive for maintaining power output. Secondly performance time concides with the transient phase when the circulatory system is in the acceleration phase. The remaining portion of total energy output is dependent on the anaerobic energy flux system. The faster oxygen is transported the less lactate is formed and the lower the total energy output, the lower amount of lactate is produced (Karlsson 1984).

The OBLA (onset of blood lactate accumulation) test is recommended as a testing procedure of the aerobic energy delivery component in elite cross-country skiers (Karlsson 1984). He further reports that the lactate level is not entirely dependent on oxygen debt and delivery and utilization, but is also dependent on the athletes' muscle fiber composition. The identification of Th_an has in many laboratories been based on a specific blood lactate concentration between 2-4 mmol.l^-1. Other investigators use noninvasive techniques for determining Th, that are based on nonlinear increase of minute ventilation, carbon dioxide output, or respiratory exchange ratio during incremental exercise (Wisløff and Helgerud 1998).

In a study by Wisløff and Helgerud (1998) the aim was to develop a standard protocol for determining VO₂ peak and Th_an (anaerobic threshold) during upper body testing of cross-coutry skiers. They used a specially developed ski ergometer and incorporated the double poling technique in the evaluation. At different inclinations of the ski ergometer VO_{2 peak} was measured, both with continuous and discontinous protocols. In accordance with other investigators, using arm cranking, they found that continuous and discontinuos protocols are equivalent in determination of VO_{2 peak} during upper body ski ergometry.

Continous protocol provided by Wisløff and Helgerud (1998) for evaluating anaerobic threshold in Series II

• measured Th_an and VO_{2 max} while running on a treadmill
• power output for warm-up was estimated
• measured peak VO₂ on the ski ergometer, using a 5 degree inclination
• warm-up period that lasted for 10 minutes at 50-60% of VO_{2 peak}
• subjects performed several 20 minutes double poling bouts, constant power outputs and on separate days
• blood samples where taken every 5 minutes via the earlobe during 20 minutes exercise bouts

Wisløff and Helgerud (1998) defined Th_an by highest exercise intensity measured as power output, VO₂, heart frequency where blood lactate concentrations increased less than 1 mmol.l^-1 the last 15 minutes of exercise.

The graded protocol by Wisløff and Helgerud (1998) in Series II

• 10 minute warm-up at 50-60% of VO₂ peak followed by blood lactate measurements to get a baseline
• 5 minute exercise stages ranging between 60-95% of VO_{2 peak}

Comparison between power output, VO₂ and heart frequency between protocols to identify the average lactate concentration. Their reasons for choosing 3-5 minutes at each exercise stage were, that it was sufficient to reach steady state of both VO₂ and lactate concentration. Using a graded protocol the Th_an was reached at a power output, VO 2, or heart rate frequency which gave an average lactate of 1.8 mmol.l^-1 independent of using 3 or 5 minutes execise stages. This lactate value is higher than reported by others while running on a treadmill, but can be explained by greater participation of fast twitch muscle fibers during upper body exercise or due to earlier recruitment of fast twitch fibers at higher percentage of maximum strength in the upper body, during work on the ski ergometer. The values for average lactate in this study are only valid for cross country skiers with VO_{2 peak} between 50-70 ml.kg^-1.min^-1. Lower values for average lactate could be expected in subjects with higher values of VO_{2 peak}, since average lactate at Th_an decreases for subjects who increase their VO_{2 peak}. The authors suggest that an exercise intensity of 50-60% can be used as a valid and reliable baseline before carrying out a test on Th_an ski. Both protocols used short range radio telemetry to measure heart rate. VO₂, VE and breathing frequency was measured using an Ergo Oxyscreen. Blood lactate was determined using a YSI model 1500 Sport Lactate Analyser.

Procedure provided by Wisløff and Helgerud (1998) for VO_{2 max} and maximal heart rate

• oxygen consumption measured constantly, increase in speed of treadmill every minute to a level that brought the subject close to exhaustion after 5 minutes
• subject then ran for 2 minutes immediately after measuring VO₂ at an intensity of 60% of VO_{2 max} followed by a supra-maximal intensity for 3 minutes
• the highest heart rate during the last minute was taken as maximal heart rate

According to Wisløff and Helgerud (1998) several authors have indicated that Th_an is a better indicator of endurance capacity than VO_{2 max}. The identification of Th_an however is not well defined, and depends on the method used. A noninvasive index of physical work capacity is affected by the controversey concerning the ventilatory threshold. Some laboratories use a reference lactate concentration of 4 mmol.l^-1 to determine Th_an even though a physiological explanation for using a fixed lactate value is lacking.

Other anaerobic tests reported in the literature on ski athletes are, Thorstensson test assessing peak torque and average torque during 50 repititions and Wingate muscle power test assesing peak and average performance (Haymes and Dickinson 1980, Karlsson 1984). They are both isokinetic expressions for fatiguing phenomena. Peak power performance, average performance and fatigue are thought to be relevant measures in skiers. According to Karlsson (1984) these two tests have no practical value since the cross country athlete is trained not to maximally utilize the anaerobic energy output system, because of blood lactate adverse effects on performance late in the race, in the form of impaired neuromuscular control. Gilliam and Ellis (1991) presents the vertical jump test, leg alactacid power and capacity test, and arm crancking alactacid power and capacity test under the heading anaerobic power and capacity test.

Muscular Strength and Endurance Tests

Gilliam and Ellis (1991) describe the sit-up test, dip tast and pull-up test as evaluation of muscular strength and endurance in cross-country skiers. In the sit up test the subject peforms as many repetions as possible in 60 seconds, elbows touch the knees. The dip test is different for males and females. Males attempt to do as many dips as possible without touching the ground. Females are allowed to touch the floor while they try to achieve a maximum number of repetions. Also the pull up test is different between sexes. Females skier hang beneath the bar with heels resting on the floor, the chest has to touch the bar, and maximum number of repetitions is taken as the score. The males hang from the bar and are not allowed to be in contact with the ground. They must touch the bar with the chin and the maximum number of completed repititions is taken as the score.

Data presented by Gilliam and Ellis (1991) for vertical jump, muscular strength/endurance and flexibility

	Vertical Jump (in cm)	Sit-Ups (number in 60s)	Dips (number)	Chins (number)	Sit & Reach (in cm)
Males	49	59	26	14	15
Females	39	58	10	19	22

Other Tests

Regular blood sampling is recommended for cross-country skiers, particularly to evaluate iron status. Iron deficiencies were found in 50% of female cross-country skiers (Gilliam and Ellis, 1991).

Haematological data presented by Gilliam and Ellis (1991)

	WBC	RBC	Hb	Hct	MVC	Ferritin
Men / Junior Men	5.5	4.9	15.5	45.3	91	51
Women / Junior Women	5.2	4.75	13.8	41.4		69
Reference Males	4 - 10	4.5 - 6.3	14 - 18	39 - 52	77 - 81	320
Reference Females	4 - 10	4.2 - 5.4	12 - 16	36 - 46	77 - 81	200

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Exercise Physiology Educational Resources 1998