In Nordic combined (NC), an athlete's competition ranking is determined by their performance in ski jumping (SJ) and cross-country skiing (XC). SJ requires high vertical jumping power, light body weight, and precise posture control during take-off (i.e., control of angular momentum) (1). Training for SJ focuses on enhancing lower limb power and the timing of power exertion during high-speed descents. Conversely, XC demands both high aerobic and anaerobic capacities (2) as well as efficient gliding (3). Improving these factors necessitates substantial endurance training. However, NC athletes have limited time available for each discipline compared with athletes specializing in SJ or XC (4), which requires them to train efficiently in both areas.

Maximal oxygen consumption (V˙O2max) is commonly used as an indicator of aerobic capacity in long-distance events, including XC. Numerous studies have shown a positive correlation between higher V˙O2max and performance in XC athletes (5–7). Some studies investigating XC athletes using running have observed seasonal variations in V˙O2 max (5, 8, 9). Meanwhile well-trained athletes may improve V˙O2max with high volume intensive training, improvements in V˙O2max may reach a plateau when training volumes are low (9). As mentioned earlier, NC athletes have less time for endurance training than XC athletes. These suggest that NC athletes may have limited opportunities to enhance their V˙O2max compared with XC athletes.

Moreover, XC races entail high intensities, particularly during uphill sections on undulating terrain (2), necessitating a highly developed anaerobic capacity. Sandbakk et al. (7) highlighted the importance of anaerobic energy mechanisms in differentiating competition levels. Therefore, supplying adequate aerobic and anaerobic energy is crucial for XC racing. Additionally, efficient gliding with minimal energy expenditure is vital. Oxygen uptake at a given velocity (O₂-cost) (6) and gross efficiency (3, 7) have been used as methods to measure gliding efficiency. Studies have found that XC athletes with lower energy expenditure exhibit higher performance levels (3, 6, 7). Furthermore, Losnegard et al. (2) demonstrated an improvement in O₂-cost leading up to the competition season, indicating that XC athletes ski more efficiently during off snow season preceding the competition.

Another measure of efficiency is exercise economy, which quantifies energy consumption per unit distance. Kyröläinen et al. (10) and Tanji et al. (11) calculated running economy by combining aerobic energy expenditure, calculated from oxygen intake and respiratory quotient, with anaerobic energy, as indicated by blood lactate concentration, which is then converted to aerobic energy expenditure. The calculation also has been used to compare roller skiing economy of XC athletes among skiing techniques (12). This index is considered possible to examine the contribution of aerobic and anaerobic energy systems for skiing economy. Previous studies have primarily focused on the efficiency of XC athletes, raising questions about whether similar findings can be applied to NC athletes. Understanding the characteristics of changes in roller skiing economy among NC athletes for each energy mechanism could potentially lead to efficient XC training in the context of SJ training.

The purpose of this study was to compare roller skiing economy between different phases in NC athletes and identify the aerobic and anaerobic factors responsible for changes in roller skiing economy. XC and NC athletes often use roller skis in their XC training during the off-snow season. The sub-technique examined in this study revealed slight differences in hip angle (8°) and pole push time (0.04 s) between roller skis and on snow. However, due to the similarity observed in ground-contact, hip acceleration, and hip displacement patterns, it is suggested that roller skiing can simulate movements on snow (13). Previous studies (14, 15) have reported performance improvements in XC athletes leading up to the competitive season. Losnegard et al. (14) highlighted the enhancement of O₂-cost and O₂-deficit, indicators of skiing economy during submaximal intensity exercise, as factors contributing to this improvement. NC athletes also aim to improve their performance for the competitive season, and endurance training is expected to influence these factors. Therefore, we hypothesize that the roller skiing economy of NC athletes will improve in the next competition season, driven by changes in both aerobic and anaerobic factors.

No significant differences were observed in incremental load test performance (Table 1).

A significant main effect of the season [F (1,6) = 6.97, p = 0.04, ES = 0.54] and stage [F (4,24) = 20.34, p < 0.001, ES = 0.77] was found for E_ALL (Figure 1), with no significant interaction between the two factors [F (4,24) = 0.74, p = 0.58, ES = 0.11]. E_ALL in autumn was consistently lower than in spring across all stages. Additionally, E_ALL significantly increased in stage 7 and 8 compared with stage 4 and 5 (p < 0.05).

A significant main effect of stage [F (1.6,9.6) = 10.53, p < 0.001, ES = 0.64] was observed for C_AE, with no significant interaction between the two factors [F (4,24) = 0.04, p = 0.99, ES = 0.01]. However, no significant main effect of season was found for C_AE (Figure 2).

Significant main effects of season [F (1,6) = 26.91, p < 0.001, ES = 0.82] and stage [F (1.5,8.8) = 55.81, p < 0.001, ES = 0.90] were observed for C_AN, along with a significant interaction between the two factors [F (2.1,12.8) = 4.29, p = 0.04, ES = 0.42] (Figure 3). C_AN in autumn was consistently lower than in spring across all stages. In spring, C_AN did not differ between stage 4 and 5 but significantly increased in later stages (p < 0.05). In contrast, in autumn, C_AN significantly increased from stage 4 to stage 7 but showed no difference between stage 7 and 8 (p < 0.05).

The aim of this study was to examine changes in roller skiing economy among high-level NC athletes during different seasons leading up to the competition. The results revealed that E_ALL decreased as the competition season approached, and this was attributed to an improvement in anaerobic energy system. These findings support the hypothesis that roller skiing economy improves as competition approaches, but we must partially reject the hypothesis that both aerobic and anaerobic factors contribute to the improvement.

No significant changes in physical characteristics were observed between each measurement. NC athletes must also perform SJ, and extreme changes in body composition, such as increases in body weight and fat-free mass (FFM), can negatively impact SJ performance. Additionally, there were no statistically significant changes in roller skiing performance (Table 1), but moderate ES was observed for La_pe, TTE, and V_peak. Some studies have reported improvements in performance measures, including TTE, over the competitive season, whereas aerobic capacity measures, such as V˙O2max, remained unchanged (14, 15). Although statistically significant changes were not observed in the present study, the trends were similar to those reported in previous studies. The small sample size may have contributed to the lack of significant changes.

E_ALL decreased at submaximal speeds leading up to the competition season (Figure 1), suggesting that NC athletes were able to glide more efficiently. This is consistent with findings from a previous study, in which XC athletes reported improved efficiency after three months of daily training (14). Considered in the context of decreasing E_ALL, significant changes were observed in C_AN at submaximal speeds, whereas C_AE did not change at such speeds between each test. These results indicate that the change in E_ALL was driven by changes in C_AN rather than C_AE. A previous study (8, 9) suggested improved anaerobic energy system as a factor contributing to enhanced endurance performance for well-trained athletes during training periods rather than aerobic energy system, which is consistent with the present findings. It is possible that the adaptation of anaerobic energy system occurs more rapidly than that of aerobic energy system.

C_AN was calculated from blood La. Blood La is also the value resulting from its production by the glycolytic (anaerobic) system and its removal by oxidative (aerobic) system (20). When considering factors affecting improved skiing economy, these include changes in kinematics (e.g., cycle time and length) (21), muscle fiber–type alterations, increased buffering capacity within the muscles (22), enhanced oxidative capacity through mitochondria (23), and increased capillary density (24). Considering that C_AE and V˙O2max did not improve, it is likely that the ability of mitochondria and capillaries to remove blood La through relevant oxidative system was not altered. It is also difficult to assume that NC athletes undergo changes in muscle fiber-type, given that they perform endurance training for XC and power training for SJ. Importantly, as the subjects in this study were well-trained, adaptations in physiological areas may occur at a slower pace. Therefore, changes in blood La concentrations are not likely to be due to oxidative system, but to depend on the generation of the glycolytic system. The changes in kinematics are considered the most likely reason for the reduced blood La production and improved contribution of anaerobic energy system. Moreover, kinematics can be modified through short-term training, suggesting that kinematics may be the primary reason for the reduced roller skiing economy associated with C_AN. Indeed, it has been reported that changing kinematics can alter the blood La (25, 26). Longer and more extensive training may be necessary to improve factors other than kinematics in the well-trained NC athletes in this study (14). However, NC athletes have limited time for endurance training owing to their involvement in SJ. For instance, compared to the training time and sessions of XC athletes, NC athletes only devote about 70% of that of XC athletes to cross-country ski training (72% of training hours and 77% of sessions) (4). Hence, the lack of improvement in aerobic system was an expected outcome.

In NC athletes, overall roller skiing economy decreased consistently at submaximal speeds leading up to the competition season. This decline was primarily attributed to an improvement in anaerobic energy system.