Abstract
Based on the assumption that the stretch-shortening cycle is a natural way of muscle
function, the occurrence of such a cycle was investigated in a diagonal technique
of cross-country skiing. Cinematographic, special force platform, and telemetered
EMG techniques were employed. The four subjects studied were all elite international
level cross-country skiers, and the measurements were taken either under world championship
conditions (cinematography) or during special test situations outdoors (EMG, force
platform, and cinematography). The skiing was performed on the fixed uphill course
(competition) and on the variable uphill tracks (2.5°-11°). The latter condition allowed
mounting of a special long force platform system under the track. The results indicated
that the leg kick phase is preceded by a typical unweighting phase, which is followed
by braking and propulsion phases. Angular velocity curves of the hip, knee, and ankle
joints revealed indirectly the segmental occurrence of the stretch-shortening cycle.
Similar phenomena could be identified for the elbow joint during the pole plant and
thrust phases. On a steep uphill track, the muscle activation pattern and the ground
reaction forces resembled in many instances those of slow level running. Based on
the results, a model was suggested to describe how the preloading of the leg thrust
phase takes place as a sequential flow from one joint to another.
Key words
cross-country skiing - stretch-shortening cycle - electromyography - muscle mechanics
