The present study results revealed that the shoulder extension and elbow flexion strength were the performance determining factors in DP. In detail, the muscle strength of shoulder extension at 60 °/second accounted for 34% of the variance in the 30-s test, while the muscle strength of shoulder extension at 60 °/second accounted for about 40% of the variance in the 3-min test. These findings add to an emerging body of literature explaining how upper limb muscle strength significantly contributes to cross-country sit-ski output power variations.
The correlation analysis showed that output power was significantly and positively correlated with upper limb isokinetic muscle strength. Correlation analysis of shoulder muscle and output power showed that isokinetic shoulder muscle strength at all angular velocities and modes were positively correlated with 30-s and 3-min output power. This is consistent with previous reports suggesting that the largest muscles of the shoulder, i.e., trapezius, deltoids, pectoralis major, and serratus anterior, are responsible for the majority of the upper-extremity tasks[17]. The muscle strength of elbow extension was not significantly correlated with output power count. One explanation for this discrepancy was that the subjects in this work were all primary skiers who, during the DP mainly swing the upper arm, and elbow extending strength contributed less. And, our results supported this argument, i.e., our ∆s in a 3-min test was about 0.8 m, which is much smaller than in the able-bodied elite skiers, whose ∆s = 1.8-2.2 m[4]). Further, Figure 1A-B and Figure 1E-F showed that larger ∆s corresponding to larger power output, these results showed the importance of elbow extension at the later stage of the poling. And this is consistent with the previous study reporting that arm swings contribute considerably to the overall force generation and propulsion[19], as the arm swings more backward, the pole force sustains more efficiently and a longer time.
The above correlation results confirmed the importance of maximal strength training in cross-country skiers training programs[17]. Moreover, the same trend of lasting-mode (3-min) and short-mode (30-s) output power with the muscle strength conformed to results in abled-body cross-country skiing UBP tests, both in the aerobic energy system[7] and shorter sprint-type[8], suggesting the upper body muscle strength was import determinants in the racing.
Stepwise multiple regression analysis of muscle strength showed that shoulder extension at 60 °/second accounted for 34% of the variances in 30-s output power, while shoulder extension at 60 °/second accounted for about 40% of the variance in 3-min output power (Table 3). The muscle strength of shoulder extension velocity at 60 °/second is consist with our kinematic analysis, that shoulder extend at about 55 °/second during poling phase (calculated from markers sticking on the upper limb, data not shown). In addition to a general increase in lean upper-body mass and maximal upper-body strength [13], our results pointed out the contribution of upper limbs isokinetic muscle strength to the DP technique in cross-country sit-skiing.
Table 3
Unstandardized coefficient, standard error, partial correlation, and adjusted R2 of stepwise multiple regression analysis on the upper limb extend muscle strength and output power of 30-s and 3-min
Predictor variables | B | SE | P-value* | r | Adjusted R2 |
30-s output power | | | | | |
Step1 | | | | | 0.341 |
Constant | 110.417 | 29.554 | 0.002 | | |
Shoulder Extend at 60 °/second | 3.053 | 0.951 | 0.005 | 0.614 | |
3-min output power | | | | | |
Step1 | | | | | 0.397 |
Constant | 348.706 | 212.651 | 0.119 | | |
Shoulder Extend at 60 °/second | 24.527 | 6.844 | 0.002 | 0.656 | |
Above results indicated that isokinetic muscle strength and muscle coordination were important for the power output generation in sit-skiing DP. In detail, at the initial stage of the poling phase, the isokinetic extension strength at 60 °/second of the shoulder muscle group dominated the poling action, and at the later stage of the poling phase, the elbow swing action was the key to enhance the performance. These results may provide guidance for designing strength-training programs of cross-country sit-skiing athletes.
This study has a few limitations. Influenced by COVID-19, the researchers did not recruit as many subjects as expected, let alone the professional athletes they could not contact. The researchers also did not consider longer testing of DP on the ergometer, which could make the testing closer to the real situation of a cross-country sit-skiing race.