In regard to simplifying motion analysis and estimating center of mass (COM) in ski skating, this study addressed 3 main questions concerning the use of inertial measurement units (IMU): (1) How accurately can a single IMU estimate displacement of os sacrum (S1) on a person during ski skating? (2) Does incorporating gyroscope and accelerometer data increase accuracy and precision? (3) Moreover, how accurately does S1 determine COM displacement? Six world-class skiers roller-ski skated on a treadmill using 2 different subtechniques. An IMU including accelerometers alone (IMU-A) or in combination with gyroscopes (IMU-G) were mounted on the S1. A reflective marker at S1, and COM calculated from 3D full-body optical analysis, were used to provide reference values. IMU-A provided an accurate and precise estimate of vertical S1 displacement, but IMU-G was required to attain accuracy and precision of < 8 mm (root-mean-squared error and range of displacement deviation) in all directions and with both subtechniques. Further, arm and torso movements affected COM, but not the S1. Hence, S1 displacement was valid for estimating sideways COM displacement, but the systematic amplitude and timing difference between S1 and COM displacement in the anteroposterior and vertical directions inhibits exact calculation of energy fluctuations.
Håvard Myklebust, Øyvind Gløersen and Jostein Hallén
Thomas Losnegard, Håvard Myklebust, Øyvind Skattebo, Hans Kristian Stadheim, Øyvind Sandbakk and Jostein Hallén
In the double-poling (DP) cross-country-skiing technique, propulsive forces are transferred solely through the poles. The aim of the current study was to investigate how pole length influences DP performance, O2 cost, and kinematics during treadmill roller skiing.
Nine male competitive cross-country skiers (24 ± 3 y, 180 ± 5 cm, 72 ± 5 kg, VO2max running 76 ± 6 mL · kg–1 · min–1) completed 2 identical test protocols using self-selected (84% ± 1% of body height) and long poles (self-selected + 7.5 cm; 88% ± 1% of body height) in a counterbalanced fashion. Each test protocol included a 5-min warm-up (2.5 m/s; 2.5°) and three 5-min submaximal sessions (3.0, 3.5, and 4.0 m/s; 2.5°) for assessment of O2 cost, followed by a selfpaced 1000-m time trial (~3 min, >5.0 m/s; 2.5°). Temporal patterns and kinematics were assessed using accelerometers and 2D video.
Long poles reduced 1000-m time (mean ± 90% confidence interval; –1.0% ± 0.7%, P = .054) and submaximal O2 cost (–2.7% ± 1.0%, P = .002) compared with self-selected poles. The center-of-mass (CoM) vertical range of displacement tended to be smaller for long than for self-selected poles (23.3 ± 3.0 vs 24.3 ± 3.0 cm, P = .07). Cycle and reposition time did not differ between pole lengths at any speeds tested, whereas poling time tended to be shorter for self-selected than for long poles at the lower speeds (≤3.5 m/s, P ≤ .10) but not at the higher speeds (≥4.0 m/s, P ≥ .23).
DP 1000-m time, submaximal O2 cost, and CoM vertical range of displacement were reduced in competitive cross-country skiers using poles 7.5 cm longer than self-selected ones.