Table 1. Characteristics of the participants involved and study design employed in the articles analyzed, together with PEDro scores.
Study Subjects Study design
N
(I/C)		 Sex Age (years) Height (cm) Weight (kg) VO2max and/or (VO2peak DP) (L·min-1 and/or mL·min-1·kg-1) Level of performance PEDro-SCORE Randomized?
Paavolainen et al. 1991 15 (7/8) M I: 19.8 ± 1.8
C: 20.0 ± 2.0		 I: 179.5 ± 7.3
C: 178.5 ± 6.0		 I: 71.4 ± 6.1
C: 69.2 ± 6.2		 I: 4.9 ± 0.4/70.0 ± 5.3
C: 5.2 ± 0.4/ 75.0 ± 4.3		 National-level XC skiers 5 No
Hoff et al. 1999 15 (8/7) W I: 17.9 ± 0.3
C: 18.0 ± 0.4		 I: 166.7 ± 1.3
C: 168.7 ± 1.4		 I: 166.7 ± 1.3
C: 168.7 ± 1.4		 I: 3.1 ± 0.1/55.3 ± 2.2
C: 3.5 ± 0.4/ 55.3 ± 1.6		 Regional XC skiers 7 Yes
Hoff et al. 2002 19 (9/10) M I: 20.4 ± 4.3
C: 19.2 ± 3.9		 I: 175.7 ± 3.1
C: 183.0 ± 4.1		 I: 70.7 ± 4.5
C: 75.6± 6.4		 I: 4.9 ± 0.3/ 69.7 ± 2.3
(4.1 ± 0.4/ 58.7 ± 3.9);
C: 5.2 ± 0.4/ 69.1 ± 2.3
(4.7 ± 0.4/ 61.1 ± 3.1)		 Well-trained XC skiers 6 Yes
Østerås et al. 2002 19 (10/9) M I: 21.0 ± 1.6
C: 24.4 ± 5.0		 I: 182 ± 2.8
C: 180 ± 3.8		 I: 77.8 ± 4.9
C: 74 ± 3.5		 I: (4.89 ± 0.6/ 63.1 ± 5.8);
C: (4.39 ± 0.35/ 59.2 ± 2.8)		 Highly trained XC skiers 5 Yes
Nilsson et al. 2004 20
(IT20-s = 6,
IT180-s = 7
C=7)		 M=12
W=8		 IT20-s: 21 (19-24)
IT180-s: 26 (20-33)
C: 25 (20-29)		 IT20-s: 178
(173-184)
IT180-s: 176
(173-179)
C: 180 (169-190)		 IT20-s: 70
(62-72)
IT180-s: 69
(62-74)
C: 74 (61-92)		 IT20-s: 63.8 ± 9.9 (54.2 ± 10.5)
IT180-s: 61.6 ± 7.1 (53 ± 7.3)
C: 63.4 ± 6.2 (53.7 ± 6.0)		 Well-trained XC skiers 5 Yes
(stratified for sex and level of performance)
Nesser et al. 2004 58 (20 drop-outs)
(Cir=16, RB=21, SS=13, W=8, C=16)		 M=29
W=29		 Cir: 15.9 ± 1.1
RB: 16.2 ± 1.1
SS: 16.0 ± 1.3
W: 16.1 ± 1.2
C: 15.8 ± 1.7		 Cir: 177.2 ± 6.6
RB: 174.6 ± 5.2
SS: 173.8 ± 4.6
W: 174.4 ± 5.7
C: 174.3 ± 6.8		 Cir: 62.3 ± 6.1
RB: 59.7 ± 5.0
SS: 60.8 ± 4.7
W: 62.8 ± 5.6
C: 56.4 ± 5.9		 NS Adolescent XC skiers 3 No
(geographical assignment)
Mikkola et al. 2007 19 (8/11) M I: 23.1 ± 3.9
C: 23.1 ± 4.5		 I: 179.3 ± 6.1
C: 178.7 ± 4.8		 I: 179.3 ± 6.1
C: 178.7 ± 4.8		 I: 65 ± 6
C: 66 ± 3		 National-level XC skiers 5 No
Losnegard et al. 2011 19 (9/10) M=11
W=8		 I: 21.2 ± 3.2
C: 21.7 ± 2.5		 I: 176.7 ± 8.9
C: 173.3 ± 7.0		 I: 71.4 ± 10.2
C: 67.8 ± 10.6		 I: 64.7 ± 4.9 (G3/2: 61.6 ± 5.5)
C: 64.6 ± 7.1 (G3/2: 62.0 ± 9.2		 Competitive XC skiers 4 No
(self-selected group)
Rønnestad et al. 2012 17 (8/9) M I: 19 ± 2
C: 20 ± 3		 I: 180 ± 4
C: 180 ± 4		 I: 69 ± 4
C: 69 ± 4		 I: 66.4 ± 1.8
C: 66.0 ± 1.6		 Well-trained Nordic Combined skiers 5 No
(self-selected group)
Skattebo et al. 2016 16 (9/7) W I: 18 ± 1
C: 17 ± 1		 I: 171 ± 5
C: 166 ± 6		 I: 61 ± 4
C: 60 ± 9		 Total group: 60 ± 5 Well-trained junior XC skiers 3 No
(self-selected groups)
Vandbakk et al. 2017 17 (8/9) W I + C: 18 ± 1 I + C: 166 ± 5 I + C: 60 ± 7 I: 56.1 ± 2.9
C: 53.8 ± 4.7		 Highly-trained XC skiers 3 No (participants attending two different schools). Group allocation based on school attended
Øfsteng et al.
2018		 29
STR VIB = 11, STR TRAD = 10,
C = 8		 M STR VIB: 24 ± 6
STR TRAD: 23 ± 2
C: 27 ± 7		 STR VIB: 182 ± 8
STR TRAD: 184 ± 8
C: 183 ± 7		 STR VIB: 78.1 ± 8
STR TRAD: 76.8 ± 5.8
C: 77.0 ± 7.8		 STR VIB: 67.3 ± 4.4
STR TRAD: 69.5 ± 6.0
C: 66.3 ± 7.9		 Well-trained XC skiers 4 No (self-selected)
I, Intervention group; C, control group; M; men; W, women; Cir, circuit training; DP, double poling; ERG; ergometer; ET, endurance running interval training; G2, gear 2 skating technique; G3, gear 3 skating technique; IT20-s: 20-s sprint interval training group; IT180-s 180-s sprint interval training group; MET, muscular endurance training; NS, not stated; RB, roller board training; SS, ski-specific; STR, strength; STR_TRAD, Strength training traditional; STR_VIB, Strength training vibration; WT, weight training; XC, cross-country.