Science and Skiing IV (eBook)

Vibration exposure in alpine skiing and consequences for muscle activation levels

P. Federolf1, V. von Tscharner1, D. Haeufle1, B. Nigg1, M. Gimpl2 and E. Müller2

1 Human Performance Laboratory, University of Calgary, Alberta, Canada

2 Christian Doppler Laboratory Biomechanics in Skiing, University of Salzburg, Austria

1     Introduction

Vibration exposure is known to affect muscle physiology and neuromuscular activity. The effect of whole body vibrations on muscle activation has been studied in the context of balance and postural control, passenger safety in vehicles, and vibration training in sports. Mester et al. (1999) have shown that strong ski vibrations are generated at the ski-snow interface that propagate through the whole body of the skier. The vibrations create resonance phenomena in soft tissue compartments. Vibrations are especially harmful for the brain, the eyes or ears and organs sensitive to vibrations (Griffin, 1975; Zou et al., 2001). They further hamper motion control (steering quality) and increase the risk of falls and injuries.

To prevent the damaging effects of vibrations different damping mechanisms are used by the human body. The main damping is believed to occur in the leg joints: passively by the cartilage and soft tissue attached to the bones or actively by stiffening the joints by muscle contraction (co-contraction). Vibration dependent muscle tuning has been proposed as damping mechanism (Nigg, 1997; Nigg and Liu, 1999). Muscle tuning has been studied for walking and running using accelerometers for measuring the vibrations (Wakeling et al., 2003; Boyer & Nigg, 2007). However, vibrations and possible muscle tuning has not been studied in skiing, an activity with high soft tissue vibrations. Thus, the purpose of this study was

1. to characterize intensity and frequency content of equipment vibrations for different skiing techniques and for different snow conditions using the wavelet analysis method,

2. to quantify simultaneously vibration intensities and muscle activation signals on four muscles of the lower extremities during alpine skiing,

3. to characterize vibration damping within the body,

4. to determine resonance frequencies of the soft tissue compartments of calf, thigh, and hamstrings and

5. to determine whether muscle activation levels change for conditions with different vibration exposure.

2     Methods

Ten experienced skiers completed 24 runs performing 5–7 short turns, 6 carving turns, and gliding in tuck position. The 24 trials of each subject were conducted between 9am, and 12.30pm. For eight of the ten subjects, snow conditions changed from hard frozen to soft snow during this time.

The skiers were equipped with 1-D acceleration sensors (Analog Devices™ (ADXL series), range: 35 to 120g) placed in axial direction on the shaft of the ski boot (parallel to the tibia), on the muscle compartments of triceps surae, quadriceps and hamstrings, and on the skin covering bones close to the ankle, knee hip and neck joints. Muscle activation was measured using bipolar surface EMG sensors on the gastrocnemius, vastus medialis, vastus lateralis, and semitendinosus. All sensor signals were recorded with a mobile EMG measurement device (Biovision™) carried in a backpack. EMG and acceleration signals were collected at a frequency of 2000 Hz.

For each run three specific movements were selected for further analysis: four consecutive short turns (2.8 ± 0.3 sec.), four consecutive carving turns (6.4 ± 0.6 sec.) and two seconds of gliding in tuck position. Short turns are highly dynamic movements in which the muscles act mainly concentric. Carving turns are executed at high speeds with little body motion. Due to centripetal forces the skiers’ muscles are loaded mainly eccentrically. Gliding was executed in the tuck position, characterized by relatively small hip and knee angles. In this position the muscles are mainly isometrically contracted.

The recorded acceleration signal was resolved with a wavelet transformation into intensities calculated for a set of 22 center frequencies between 0.6 and 80 Hz. EMG data was resolved using 13 wavelets with center frequencies between 6.9 and 542 Hz. In both cases, wavelet transformations (von Tscharner, 2000) were used, because the intensity calculated for each wavelet (each frequency range) is normalized with respect to the energy content of the analyzed signal. At a given time, the total intensity was calculated by adding the intensities of all wavelets. The square root of the total intensity is proportional to the signal amplitude and was called magnitude of the signal. The mean total intensity of a signal (EMG or acceleration) during a specific movement was calculated by averaging the total intensity over time. The mean total intensity characterized the vibration intensity or muscle activation level of the selected movement and can be compared between trials. The spectrum of a signal (EMG or acceleration) was calculated by integrating the intensity of each wavelet over time. Hence, the frequency range and frequency resolution of the spectrum derived from the wavelet analysis depended on the number wavelets and on their center frequencies.

Vibration damping within the skier’s body was characterized by dividing the vibration magnitudes determined at hip and neck by the vibration magnitudes measured at the ankle. Ankle vibrations were considered input vibrations for the body. This procedure provided only approximate values for the damping because all vibration signals were recorded with 1-D acceleration sensors.

Resonances of soft tissue compartments were determined by dividing the intensity spectrum measured at a soft tissue compartment by the intensity spectrum measured at the ankle (vibration input). The resonance frequency was determined as the frequency at which this quotient was maximal. Frequencies below 10 Hz were not considered, because the skiing movements are in this range.

To determine if muscle activation levels change if the intensity of the vibration exposure changes, Pearson’s correlation coefficient r between mean total intensity of the EMG signal and the mean total intensity of the acceleration signal was calculated for the 24 trials of each subject.

3     Results

Equipment vibrations measured at the ski boot showed peak accelerations between 20 to 30 g in the steering phase in short turns. Vibrations were small during turn initiation (~2 g) and during gliding (~5 g). In carving vibration amplitudes were in the range of 5 to 20 g. It seemed that vibrations were high when the ski skidded, and when the ground reaction forces were high. Frequency spectra were highly subject specific, but in all subjects the peak intensities were found in the range of 5 to 30 Hz. As the snow turned softer in the course of the day, frequencies above 15 to 20 Hz were increasingly damped.

All subjects showed strong vibration damping within the body. At 10 Hz, mean vibration magnitudes measured at the hip and neck decreased to 30% and 20%, respectively, compared to vibration amplitudes measured at the subject’s ankle. With increasing frequency these vibration amplitudes decreased further. Above 60 Hz the vibration amplitudes were less than 12% for the hip and less than 5% for the ankle.

Resonances frequencies of the muscle compartments were subject, muscle and movement specific. During short and during carving turns resonance frequencies occurred typically in the range of 10 to 30 Hz, in gliding the resonance frequencies were for most subjects and must muscles higher, typically in the 20 to 40 Hz range.

For eight subjects the snow turned significantly softer during the course of the day. In these cases the intensity of the vibrations the skiers were exposed to decreased significantly. The mean total intensity measured at the ankle decreased for short turns and carving by a factor between 2 and 3.5. In straight gliding the vibration intensities decreased by a factor of about 1.5. For short turns and carving, a concurrent decrease of muscle activation levels was observed. In short turns, the correlation coefficient r between the mean total intensity of the vibration exposure and the mean total intensity of the EMG signal was between 0.4 and 0.9. For the muscles the biceps femoris, gastrocnemius, vastus lateralis, and vastus medials the correlation was statistically significant for 6, 7, 6, and 5 of the subjects, respectively. In carving, statistically significant correlations were found for 7, 8, 6, and 5 subjects. Although the variability of

the vibration intensity in straight gliding was much smaller, significant correlations were found for 1, 2, 2, and 3 subjects.

4     Discussion

Vibration intensities observed at the equipment level (at the ski boot) differ for different skiing styles. This can be explained by different speeds, different skisnow interaction mechanisms (e.g. lateral skidding vs. cutting of the snow surface vs. gliding), or different equipment resonances (e.g. in case of edged skis torsional modes are excited, in case of flat skis mainly bending modes are excited). The results of this study also clearly indicate that the snow properties have substantial effect on the vibration intensity.

The vibrations present at the equipment level can be considered as the input vibrations the skier’s body is exposed to. Even at slow speeds and simple skiing styles used in this study, which are typical...