The women in this study have been previously reported on in a larger cohort study examining women football players after ACL reconstruction and their uninjured peers . Study methodology is presented here in brief as the methods are similar to the previous study . All players received written and verbal information about the study, and gave written informed consent prior to testing. The study was approved by the Regional Ethical Review Board (Dnr 2012/24-31, 2013/75-32, 2017/324-32).
Inclusion criteria for this study were football players, between the ages of 16–25, with no history of ACL injury. The forty-six women included in this study were recruited as previously described , and selected from the larger female cohort because appropriately matched male football players were identified. The forty-six male football players were recruited from local football teams via word of mouth, coaches, and short presentations by the researchers to the team and matched to the women based on age, training frequency, and playing position (Fig. 1).
Players performed a standardized warm-up, followed by one trial of the tuck jump and three trials of the DVJ. Both the tuck jump and DVJ were preceded by a few familiarization practice jumps. The warm-up involved running exercises typical for football players, ten squats, ten toe raises, and 1 min of jumping rope . All activities were performed in the players’ own athletic shoes and clothing.
Players were instructed to stand with their feet shoulder width apart, bringing their thighs parallel to the floor, and landing in the same place. Players were instructed to immediately start the next jump upon landing and were filmed (two Panasonic HC-V500M video cameras, one in the frontal and one in the sagittal plane) as they jumped continuously for 10 s. Video was recorded at 50 Hz with advanced video coding high definition at 1080/50p. The tuck jump was analyzed according to a clinician friendly screening tool [6, 8]. The screening tool consists of ten criteria, each scored as either 0 indicating no flaw or 1 indicating flawed technique, for a total score ranging from 0 to 10. Players were classified as having an abnormal tuck jump score if their total tuck jump score was ≥ 6. This cutoff was previously proposed to indicate players who might be at a higher risk for ACL injury and might benefit from injury prevention programs. Grading was performed, by one researcher (AA), according to Herrington et al.  at normal video speed, reviewing each plane three times.
Drop vertical jump
Players were given standardized instructions to drop down off the box (31 cm) and immediately jump as high as possible reaching with both hands towards a ball suspended above them. The first landing of the DVJ was assessed with two different methods, quantitatively  and qualitatively [25, 34]. To simplify measurements and increase visibility, the athlete’s greater trochanter, the lateral knee joint line, fibular head, lateral malleolus, patella tendon, and center of the patella were identified by palpation and marked with a marker pen. Three DVJ trials were filmed using the same one camera in the frontal and one in the sagittal plane .
DVJ quantitative analysis
As previously reported, the worst assessed jump of the three trials, summarized from all criteria, was used in the quantitative analysis . The worst jump was chosen to represent the player’s potentially highest risk movement pattern, which could be overlooked if calculating the average of the three attempts. As described previously , each jump was given one point on the following criteria: if the feet left the box at different times, if the feet landed at the different time, if the feet were not parallel on landing, if the feet were rotated on landing, if there was knee valgus on landing, if the feet were not approximately shoulder distance apart, and if there was any weight displacement. The jump which had the most points, was deemed the worst jump and used in the analysis. In accordance with the previous study , the valgus knee motion was measured using Dartfish ProSuite (Dartfish Ltd, Fribourg, Switzerland) and calculated in centimeters as the medial displacement of the knee in the frontal plane from initial contact to peak knee flexion/the end of the deceleration phase of the DVJ. Valgus knee motion was inputted as zero if the athlete’s knee displacement was lateral from initial contact to peak knee flexion. A categorical variable (knee displacement) was also created based on the athlete’s frontal plane knee motion, grouping players based on if their knee displaced medially (valgus), laterally (varus), or no displacement (neutral), from initial contact to peak knee flexion. A nomogram was used to predict the probability of high knee abduction moment (pKAM) [23, 24], ranging from 0 (lowest) to 100% (highest). The nomogram is based on the player’s weight, tibia length, knee motion in the frontal plane, and knee flexion range of motion, and a surrogate value for hamstring–quadriceps ratio (multiplying the player’s mass by 0.01 and adding the resultant value to 1.10) [21, 23, 24].
The sagittal plane hip, knee, and ankle angles were measured at peak knee flexion (measurements were performed on the left leg only, as this side faced the frontal plane camera) using Dartfish ProSuite. The knee:ankle separation ratio was also calculated by dividing the distance between the center of the patella at peak knee flexion by the distance between the great toes (point estimated in Dartfish). One researcher quantitatively analyzed the women’s DVJs (IM), another analyzed the men’s (AA) and performed the qualitative analysis on all players. Inter-rater reliability testing was performed on three jumps of three women not included in this study (nine jumps total) with ICCs ranging from 0.82 to 0.99.
DVJ qualitative analysis
The qualitative analysis of the DVJ used a visual assessment first presented by Stensrud et al. [25, 34] Using the frontal plane view the athlete’s ability to control their knees during DVJ landing was subjectively graded on a 0–2 scale (0 = good control, 1 = reduced control, 2 = poor control). As previously described, good control was assigned when there was no obvious valgus motion of either knee, no mediolateral motion of the knee, and the knees were in line with the toes. Reduced control was indicated when there was slight mediolateral movement and/or slight valgus position of either or both knees. Poor control was assigned when players landed with knee valgus on at least one knee, alignment of the knees and toes was poor, and there was a substantial amount of mediolateral movement of the knee during landing . Per the previous studies methodology [25, 34], each trial was viewed once and the trial with the highest score was used in the analysis. One researcher (AA) assessed all players, 2 months after the quantitative assessment.
Mean ± standard deviation or absolute values with percent were calculated for descriptive statistics. One-way ANOVAs were used to compare men and women with regards to demographics and anthropometrics. Chi squared tests were used to compare the sexes with regards to limb dominance (based on preferred kicking leg), position, and training frequency.
A one-way ANOVA was used to compare men and women with regards to total tuck jump scores and Chi-squared tests were used to compare the frequency of each technique flaw during the tuck jump. A Fischer’s exact test was also used to determine if there was a difference in the number of men and women categorized as having an abnormal tuck jump score (total score ≥ 6).
DVJ quantitative analysis
Two-way repeated measures ANOVAs with planned least squares comparisons were used to compare the difference between limbs in men and women with regards to valgus knee motion and pKAM. Planned comparisons were the interaction effects. Fischer’s exact test was used to compare knee displacement (varus, neutral, valgus) between men and women. One-way ANOVAs were used to compare hip, knee and ankle angle at peak knee flexion as well as knee:ankle separation ratio.
DVJ qualitative analysis
A Fisher’s exact test was used to examine if there was a difference in the number of men and women assessed as having good, reduced, or poor control.
A sensitivity power analysis indicated that using a 2 × 2 repeated measures ANOVA with alpha set at P ≤ 0.05, power = 0.80, with 92 players, and effect size of np2 = 0.08 could be detected. Effect sizes were considered small (np2 = 0.01), medium (np2 = 0.06), and large (np2 = 0.14) .