This chapter contains sections titled: Introduction, SSCs reviewed, Evidence of use of SSC in cross-country skiing, Summary and discussion, References. High-performance cross-country ski racers give the impression of very smooth movements of the legs and arms throughout the stride pattern regardless of the technique in use—classical diagonal stride, some version of ski skating, or double poling. To the naked eye, some skiers seem to bob slightly at the torso or bounce a little during each step during diagonal stride but because the pattern is repeated on every cycle, the movement still looks smooth. In fact, "smoothness" in movement patterns is often seen as a characteristic of highly skilled performance in many sports and coaches can frequently be heard to instruct their athletes to "smooth out the movement." However, viewing of video, parricularly in slow motion, will sometimes show a small flexion of the shoulder (angle of upper arm relative to the torso) and elbow upon pole plant just before the pole thrust begins, and at the hip, knee, and ankle just before the onset of the strong leg push. Some skiers can also be seen to use a slight trunk flexion that precedes extension of the arms and legs and becomes an obvious trunk extension or even a hyperextension (angle of the trunk relative to the thigh of the pushing leg). When observed in this detail, the movement pattern for some skiers is not as smooth as it seems in real time rather than in slow motion. The authors believe that there are neuromuscular and biomechanical benefits to skiers who show these brief and rapid flexion/extension patterns at some joints. It maybe possible for these skiers to produce higher muscle and GRFs, or the same forces as other competitors—but at lower metabolic cost. Are these simply idiosyncratic movement patterns that started at an early age for some skiers that gradually became "grooved" as they evolved from young successful racers into world-class competitors over the years? Are these uses of body joints purposely adopted by some skiers by trial and error learning as they are developing because they find that they ski faster or more efficiently when they do this? Are some competitors actually taught by some coaches who are very familiar with neuromuscular physiology and biomechanics to ski this way? These patterns are not used by all skiers, not even by all world-class skiers. Is there now sufficient evidence for the feasibility and benefits of using muscles and joints in this way in cross-country skiing to recommend that all young competitors be taught to use this technique? These repetirive extension/flexion patterns are typical of muscle SSC and are commonly observed in sports such as running, jumping, and "windups" in throwing. For example, when children throw balls, even at very young ages, they seem to naturally resort to a kind of "windup" that precedes the throw—an extension/flexion pattern at the shoulder joint. This pattern usually does not have to be taught for throwing although it can be refined by coaching.
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