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W. Rapp
Science and Skiing IV
630
Biomechanics in classical cross-country skiing – past,
present and future
W. Rapp1, S. Lindinger2,3, E. Müller2,3 and H.-C. Holmberg4
1 Medical Clinic - Dep. of Sports Medicine, University of Tübingen , Germany
2 Department of Sport Science and Kinesiology, University of Salzburg, Austria
3 Christian Doppler Laboratory “Biomechanics in skiing”, Salzburg, Austria
4 Swedish Winter Sports Research Centre, Mid Sweden University, Östersund
Past
From a biomechanical perspective cross-country skiing is complex and the two
main styles, freestyle and classical, are subdivided into nine different tech-
niques. These are used alternately during a race depending on the physical
capacity and technical competence of the skier, the track profile and the fric-
tion between the skis and snow. The classical style was the only racing style
until 1984, which explains why early biomechanical studies examined the clas-
sical techniques; double poling with a kick, double poling and diagonal stride
(for refs see Smith, 1990). A majority of these investigations focused on the
latter technique. Kinematical recording enabled researchers to study whole
body movement characteristics, such as mean cycle velocity, or more isolated
movements in specific joints, e.g. the elbow and knee joints, in a specific tech-
nique. It was also possible to derive temporal data such as cycle velocity, as
well as cycle and phase duration. Furthermore, kinetic evaluation of pole force
and forces between the ski boots and the skis was carried out in order to in-
crease understanding of the causes of the observed kinematical patterns.
Diagonal skiing
The diagonal technique is only used uphill. It has similarities with the basic
human movement pattern, using the arms and legs in opposition to each other
and is the closest, of all the various skiing techniques, to the fundamental mo-
vement pattern of walking and running. The movements are mainly performed
in the sagittal plane and the force produced by the arms increases with inclina-
tion (Komi & Norman, 1987; Smith, 2002). Velocity during diagonal skiing is
strongly related to stride length during competition (for refs see Smith, 1990)
Biomechanics in classical cross-country skiing – past, present and future
Science and Skiing IV 631
and decreases with increased inclination in order to maintain friction (“the
grip”) between the skis and the snow. The absolute and relative duration of the
stride when the skis are stationary increases with inclination, but at present
there is a lack of quantitative data that describes the force and movement
pattern changes related to the diagonal technique on varying terrain (Smith,
2002). The vertical forces during the kick phase in diagonal skiing have been
measured, and found to be up to 2-3 times body weight (BW). The largest part
of this force (~2/3) is directed vertically, in order to press the mid-section of the
ski with grip wax against the snow, while the real propulsive force is substan-
tially lower (~10-25% of BW) and is generated during a very short time (~0.1-
0.25 s) (Komi & Norman, 1987; Nilsson et al., 2004). Pierce et al. (1987) re-
ported pole force values of 13-17% of BW during diagonal skiing. One of the
few studies that have compared the movement patterns of elite skiers during
diagonal skiing was performed by Komi et al. (1982). They found that the an-
gular displacements of the hip and knee joints within each stride cycle were
quite similar between subjects, but that there were fairly large inter-individual
differences in the velocity of the centre of mass (COM). A recent study charac-
terized pole and ski forces in diagonal skiing at different speeds uphill (Va-
hasoyrinki et al., 2008). Ski vertical force during the gliding phase decreased
and the braking ski horizontal and vertical forces remained the same with
higher skiing speed. During the subsequent kick phase, both ski horizontal and
vertical forces increased as a function of the skiing speed. Consequently, the
horizontal force ratio between the ski and the pole plant increased with faster
skiing speed.
Double poling
In contrast to the diagonal technique, double poling (DP) is used on the flatter
parts of a course when the ski velocity is high; it is considered to be a pre-
dominantly upper body exercise mode (Mittelstadt et al., 1995; Hoff et al.,
1999; Staib et al., 2000). In contrast to the diagonal arm and leg motion in the
diagonal technique, DP involves both arms acting together in parallel at the
same time. According to the literature, considerable trunk flexion and only a
minimal leg motion occurs during DP (Smith, 2002). The resultant force is
delivered axially through the pole. At present, comparably few studies have
W. Rapp
Science and Skiing IV
632
been specifically devoted to DP. Smith et al. (1996) used a two dimensional
(2D) kinematic video-based system and analyzed female elite skiers´ DP tech-
nique during the 1994 Olympic Games in Lillehammer on a moderate downhill
section. They found that faster skiers had a greater range of elbow motion with
an initial flexion immediately followed by an extension, with higher angular
velocities. They also demonstrated that faster skiers began the poling phase
with the poles in a more elevated position with respect to the trunk, and angled
the poles closer to vertical compared to slower skiers.
Later, Millet et al. (1998b, a) performed two studies where they combined a
temporal evaluation with kinetic measurements of pole force. They analyzed
the effect of increased DP velocity (13-19 km h-1 and maximal) and inclination
(2.1 versus 5.1%). This provided useful information about the magnitude of
force production and a number of time-related variables. At 2.1% inclination
they reported peak pole force values between 26 to 33% of BW. They also
found that the poling duty cycle (poling time/total cycle time) increased with
velocity; thus the decrease in recovery time occurred to a greater extent than a
decrease in poling time as the velocity increased. The increase in DP velocity
was achieved through increased poling frequency while the cycle length was
maintained. The second study, comparing 2.1% and 5.1% inclination, revealed
that the pole force was higher at the steeper inclination. Moreover, the skiers
had a longer duty cycle (poling time/cycle time), a shorter recovery time, and a
higher poling frequency. Millet et al. (1998a) discussed how a longer duty cy-
cle may have a positive effect on DP economy by decreasing the magnitude of
velocity fluctuations during each DP cycle.
The interest in DP has increased noticeably during the last decade, as a result
of increased utilization of this technique due to a combination of improved
track preparation and the enhanced training status of the skiers´ upper body.
The latter has enabled them to use this technique more during races; DP is the
key technique in the classical sprint event, which is mainly performed on flatter
terrain. In a recent study (Holmberg et al., 2005) our group revealed that pole
force, in contrast to poling frequency, was related to DP velocity and influ-
enced by specific muscle activation patterns and a specific characteristic flex-
ion-extension pattern in the elbow, hip and ankle joints with the angle minima
occurring around the peak pole force. Moreover, we found that the muscles
Biomechanics in classical cross-country skiing – past, present and future
Science and Skiing IV 633
are engaged in a sequential order during DP, starting with trunk and hip flex-
ors, followed by shoulder and elbow extensors. Finally, we found that the best
skiers used a special technical strategy with specific characteristics related to
DP velocity. In another study (Holmberg et al., 2006) we compared free and
locked knee and ankle joints during DP and found that the locked situation
elicited 1) a higher blood lactate concentration and heart rate response at
submaximal intensities, with no differences in oxygen consumption; 2) a lower
pole force and a higher Pf at 85% vmax; 3) a lower VO2peak and vmax; and 4) a
shorter time to exhaustion. Altogether, these data demonstrated that move-
ments of the knee and ankle joints are an integrated part of the skillful use of
the DP technique and that restrictions of the motion in these joints during DP
markedly affect both biomechanical and physiological parameters and impair
DP performance, emphasizing the functional importance of involving the lower
body during DP to optimize DP performance. From these data we recom-
mended that cross-country skiers should consider the potential of involving
more muscle mass in DP with a minor technique modulation towards a more
dynamic use of the legs to improve DP performance, decreasing cardiovascu-
lar and metabolic response at submaximal velocities.
Present
The potential of integrating biomechanical methods
To achieve a better and deeper understanding of the influence and adaptation
of different parameters for motor performance, we suggest combining different
biomechanical methods. This will increase knowledge of the basic constraints
of the mechanical and physiological demands involved in XC skiing. As previ-
ously mentioned, the majority of studies investigating XC skiing have used
kinematic and kinetic methods (Hoffman et al., 1995; Smith et al., 1996; Millet
et al., 1998b, a; Nilsson et al., 2004). In contrast, the use of electromyography
(EMG) as a method for analyzing neuromuscular behavior in XC skiing is as-
tonishingly rare (Komi, 1985; Komi & Norman, 1987; Holmberg et al., 2005).
This is somewhat unexpected as, in combination with kinematic and kinetic
W. Rapp
Science and Skiing IV
634
methods; EMG is a reliable method for obtaining deeper insight into neuro-
muscular coordination.
Studies of other forms of locomotion have shown that a well coordinated neu-
romuscular activation pattern will increase motor performance (Cavagna,
1977; Komi, 1984, 2000). As found by Komi (1984), the most natural muscular
contraction mode is a combination of stretching a muscle and a subsequent
contraction of the muscle. This so called stretch-shortening cycle (SSC) has
been shown to have great relevance, especially in fast and explosive move-
ments. In almost every functional movement a muscle is stretched due to high
impact loads during contact with the ground. In this eccentric loading phase
elastic energy can be stored in the tendo-muscular system and utilized in the
subsequent concentric phase of a movement (Cavagna, 1977; Bosco et al.,
1982; Komi, 1984). To utilize the stored energy and to achieve the optimal
mechanical benefit it is necessary that the time span between the eccentric
and the concentric phase is kept as short as possible (Komi, 1984; Gollhofer &
Kyrolainen, 1991; Komi, 2000). As a further prerequisite, in SSC it is neces-
sary to have an adequate pre-activation before loading. It was demonstrated
that the force development is better if the muscle is pre-activated (Cavagna et
al 1968) and a pre-activated muscle can decelerate a fast muscular stretching
in a more efficient way (Sale 1988).
In leg exercises, like running and jumping experiments, the benefit of SSC for
athletic motion has already been documented (Mero & Komi, 1987; Gollhofer
& Kyrolainen, 1991; Gollhofer et al., 1992; Avela et al., 2006). Studies on ex-
plosive movements and the use of SSC for upper extremity muscles are sur-
prisingly rare. Newton et al. (1997) simulated explosive throwing through the
use of a bench press. He showed a positive effect for throwing performance in
kinematic and kinetic variables, as well an increase in muscular activation.
This leads to the conclusion that SSC-like contraction takes part in upper ex-
tremity muscles. A similar conclusion was drawn by Bober et al (1980) using a
pendulum approach to elicit the loading situation. The authors demonstrated
that higher muscular loading results in an increase in pushing performance
and a decrease in movement time. In a study investigating fatigue, Gollhofer et
al. (1987) demonstrated an increase in both the eccentric and concentric con-
traction time for the elbow extensors. Whilst the concentric period was more
Biomechanics in classical cross-country skiing – past, present and future
Science and Skiing IV 635
affected, the author concluded that the elastic behavior is better in the eccen-
tric part.
From a recent study (Lindinger et al 2007) we know that the acting forces at
pole plant during DP will increase across higher velocities and, as a conse-
quence, the angular velocities of the upper extremities will also increase. From
these results it could be hypothesized that the neuromuscular patterns will
adapt in a similar way. Referring to the pre-requisite of an SSC type muscular
behavior, a pre-activation before pole plant should be apparent and, shortly
after an eccentric loading, a concentric action should take place.
In a pilot study, from our group, 6 subjects performed DP technique at 4 sub-
maximal and at one individual maximum velocity on a treadmill. Surface EMG
was recorded from triceps brachii (TRI) and from latissimus dorsi (LAT). A
force transducer, built into the pole, recorded pole force and a goniometer
were used to analyze elbow angle. Fig.1 shows an example of the activation
pattern of one subject, skiing at a velocity of 27km h-1.
Fig. 1: Mean EMG of one subject skiing on a treadmill. The signal represents an
average of ten pole cycles triggered at pole contact (vertical dashed line).
LAT=latissimus dorsi, TRI = triceps brachii. For further explanation, see text.
W. Rapp
Science and Skiing IV
636
In the elbow joint a flexion occurs after pole plant, which is reversed into an
extension of the elbow in order to produce forward propulsion. The reversion
of the elbow joint coincides rather well with the maximum pole force. Based on
the selected EMG recording of one subject (Fig. 1) it can be qualitatively
shown that both muscles are already pre-activated before pole plant. In the
pole contact phase a further increase in the activation of both muscles can be
observed. A quantitative analysis was performed, dividing the activation pat-
tern into the functional activation periods of pre-activity (PRE), reflex induced
activation (RIA) and late EMG response (LER) (Gollhofer et al., 1990). In pre-
activity the mean EMG amplitudes were calculated from the phase 90 ms be-
fore pole plant (pre90). RIA is defined as the time period from 30 to 120 ms
after pole plant and LER lasts until end of pole contact (Fig. 1). The results
shown in Fig. 2 indicate that the mean EMG in the RIA phase increases for
TRI and LAT across the running velocities. Of note is that at velocities of 21
km h-1 and higher, the activation of LAT exceeds the level of a previously re-
corded MVC contraction. For the pre-activity a similar increase in the mean
amplitude was observed but the MVC level was never achieved.
Fig. 2: Mean EMG amplitude (N=6) from the RIA phase across the velocities for
latissimus dorsi and triceps brachii.
Biomechanics in classical cross-country skiing – past, present and future
Science and Skiing IV 637
For methodological reasons neuromuscular studies that analyze the activation
pattern in upper extremity muscles are rare in comparison to the lower ex-
tremities. As shown in recent studies (Holmberg et al., 2005; Holmberg et al.,
2006), the action to produce powerful and effective forward propulsion using
DP technique requires highly developed muscular coordination. These studies
showed that with increasing running velocities movement times were short-
ened and the resulting forces increased. Therefore, the movement action be-
comes more of an explosive type of contraction and thus the contraction prin-
ciples of an SSC may play a role in DP. As has already been shown, the
benefit of SSC contraction is seen in an absorption of high impact loads, stor-
age of elastic energy and recoiling this energy in the concentric part of a
movement (Komi, 2000). Together with reflex mechanisms this will enhance
force and improve motor performance, especially in fast and dynamic move-
ments. For DP, it may be suggested that one focus should be to train at high
movement speeds to improve the capability for using SSC-like contraction for
forward propulsion.
Future
Biomechanical studies in cross-country skiing are performed less in compari-
son to their physiological counterpart. Among the biomechanical studies the
methods used are dominantly kinematics or kinetics, with a small number of
studies that use electromyography. In total, up to the present day, only ~10%
of the biomechanical studies have used a combined kinematic-kinetic-EMG
method design. It is obvious that several biomechanical studies over the last
decade have added important knowledge that can improve XC performance.
Moreover, it is our belief that better technological equipment and data process-
ing will enable future studies to choose to use a combined approach to a
greater extent; not only with multiple biomechanical measurement systems but
also more often integrating both physiology and biomechanics in the same
experiment. One important aspect that has the potential to be investigated
more fully from the data presented in this article is to more extensively analyze
the occurrence, and elucidate the possible importance, of stretch-shortening
during cross-country skiing.
W. Rapp
Science and Skiing IV
638
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