To identify and evaluate current scientific literature concerning the effect of strength, power and speed training on relevant physiological and biomechanical characteristics and performance of competitive cross-country skiers (XCS), the databases Scopus and PubMed were searched systematically for original articles in peer-reviewed journals. Of the 599 studies retrieved, 12 met the inclusion criteria (i.e., assessment of outcome measures with relevance for XCS performance; involvement of traditional resistance training; application of external resistance to the body; intervention longer than 4 weeks; randomized controlled trial). The methodological rigor of each study was assessed using the PEDro scale, which were mostly poor-to-fair, with good methodological quality in only two articles. All of the strength/power/speed interventions improved 1RM (0.8-6.8 ES), but findings with respect to jump performance, ability to generate force rapidly and body composition were mixed. Interventions demonstrated moderate-to-high ES on XCS specific performance compared with control (mean ES = 0.56), but the pattern observed was not consistent. None of the interventions changed anaerobic capacity, while in most studies VO_2max was either unchanged or increased. Work economy or efficiency was enhanced by most of the interventions. In conclusion, present research indicates that strength training improves general strength, with moderate effects on XCS performance, and inconclusive effects on work economy and VO_2max/VO_2peak. Strength training with high loads, explosive strength training, or sprint interval training seem to be promising tools for modern XCS training. Future investigations should include long-term (e.g., >6 months) strength training to allow sufficient time for increased strength and speed to influence actual XCS performance. Moreover, they should include both sexes, as well as upper- and lower-body muscles (trained separately and together) and employ free weights and core training. Methodological differences and limitations highlighted here may explain discrepancies in findings and should be taken into consideration in future research in this area.

• Although available evidence indicates that XC skiers are stronger than many other endurance athletes and have become even stronger in recent decades, most of the investigations reviewed here found only moderate positive effects of strength training on XCS performance.

• The great variety of strength training described here has generally led to improvements in strength (e.g., 1RM), with inconsistent positive effects with respect to work economy/efficiency, VO_2max/peak, jump performance and body composition.

• Strength training (2-3 times/week) focusing on high loads (oriented towards intramuscular coordination and/or hypertrophy), explosive strength (power) and/or specific sprint endurance training are recommended for inclusion in XCS training, with special consideration of the individual athlete’s needs.

• The methodological quality of the articles examined was poor-to-fair, being good in only two cases. Future investigations should involve more prolonged interventions (including also the competition phase and long-term follow-up); include both men and women, as well as upper-, core and lower-body muscles; and place special emphasize on the transfer of increased strength to changes in the biomechanics and, consequently, on the performance of XCS.

• Although free weight training is a promising concept, studies of the effects of such training on XCS are sparse. If free weight training (e.g., Olympic lifts) which is technically complex, is included in the training regimen, early development of proper lifting technique, with special guidance and gradual increases in load, are recommended.