Vertical and Horizontal Impact Force Comparison During Jump Landings With and Without Rotation in NCAA Division I Male Soccer Players

Abstract

Harry, JR, Barker, LA, Mercer, JA, and Dufek, JS. Vertical and horizontal impact force comparison during jump landings with and without rotation in NCAA Division I male soccer players. J Strength Cond Res 31(7): 1780–1786, 2017—There is a wealth of research on impact force characteristics when landing from a jump. However, there are no data on impact forces during landing from a jump with an airborne rotation about the vertical axis. We examined impact force parameters in the vertical and horizontal axes during vertical jump (VJ) landings and VJ landings with a 180° rotation (VJR). Twenty-four Division I male soccer players performed 3 VJ and VJR landings on a dual-force platform system. Paired-samples t-tests (α = 0.05) compared differences in the first (F1) and second (F2) peak vertical ground reaction forces, times to F1 (tF1), F2 (tF2), and the end of the impact phase, vertical impulse, and anterior-posterior and medial-lateral force couples. Effect sizes (ES; large >0.8) were computed to determine the magnitude of the differences. Lower jump height (41.60 ± 4.03 cm, VJ landings; 39.40 ± 4.05 cm, VJR landings; p = 0.002; ES = 0.39), greater F2 (55.71 ± 11.95 N·kg⁻¹, VJ; 68.16 ± 14.82 N·kg⁻¹; p < 0.001; ES = 0.94), faster tF2 (0.057 ± 0.012 seconds, VJ; 0.047 ± 0.011 seconds, VJR; p = 0.001; ES = 0.89), greater anterior-posterior (0.06 ± 0.03 N·s·kg⁻¹, VJ; 0.56 ± 0.15 N·s·kg⁻¹, VJR; p < 0.001; ES = 1.83) and medial-lateral force couples (0.29 ± 0.11 N·s·kg⁻¹, VJ; 0.56 ± 0.14 N·s·kg⁻¹, VJR; p < 0.001; ES = 1.46) occurred during VJR landings. No other differences were identified. This kinetic analysis determined that landing from a jump with 180° airborne rotation is different than landing from a jump without an airborne rotation. Male Division I soccer players could benefit from increasing the volume of VJR landings during training to address the differences in jump height and force parameters compared with VJ landings.

Introduction

Soccer participation has increased steadily in the United States at the National Collegiate Athletic Association (NCAA) collegiate level (²). Soccer is a physically demanding sport that requires a number of explosive, maximal intensity movements during both practice and competition (²⁶), and the vertical jump (VJ) is one of the more essential explosive movements performed (^24,28). For instance, players are often required to jump vertically to play the ball in the air during corner kicks, which occur approximately 10.8 times per competitive match (²⁷). When combined with similar situations such as free kicks and 50/50 challenges to win the ball in the air during open play, it is evident that soccer requires repeated performance of vertical jumping movements (¹⁵). Vertical jumping movements in soccer are often accompanied by head (¹⁵) rotations when a player attempts to play the ball to a specific area or teammate. Head rotations subsequently produce twisting motions at the trunk because of the transfer of angular momentum (¹⁷). Additionally, a recent study concluded that VJ performance is sacrificed when an airborne rotation about the vertical axis is required, and that airborne rotation is initiated at the ground through an anterior-posterior force coupling of the left and right limbs (⁴).

During landing from a jump, vertical impact forces must be attenuated before a subsequent movement can be performed. As such, vertical impact force parameters are examined during landing (¹⁴). However, the horizontal force characteristics of VJ landings with and without rotation are not well examined, although horizontal forces can threaten both stability (²⁵) and the integrity of the knee joint (¹¹). Mechanically, the angular momentum of the body during a rotational jump must be conserved while airborne in the absence of an opposing force (^13,17). Coordinated movements of the limbs can increase or decrease the angular momentum causing rotation (¹⁷), but to stop rotation the remaining angular momentum must be decreased to zero during impact by coupling forces applied to the ground. Because training programs predominantly emphasize performance and impact force parameters during vertical landings without aerial rotations (^1,7,9,24), examinations of the vertical and horizontal impact force characteristics during VJ landings with an aerial rotation are warranted. In turn, an understanding of impact force characteristics during the performance of rotational jump landings can be provided. Additionally, it can be determined whether rotational jump landings should be included, or increased in volume, during training. However, there are no published data on ground reaction force characteristics (in any dimension) when landing from a jump that includes an aerial rotation about the vertical axis.

The purpose of this investigation was to examine vertical and horizontal impact force parameters in Division I male soccer players during 2 jump-landing tasks. The specific tasks examined were bilateral landings from a VJ and from a VJ with a 180° airborne rotation about the vertical axis (VJR), both of which were performed for maximum effort. The 180° rotation was selected to acquire bilateral impact force parameters during the jumping and landing phases. It was hypothesized that impact forces in each dimension would be different when landing from a VJ that did and did not include a 180° rotation about the vertical axis.

Methods

Experimental Approach to the Problem

Vertical jumping movements in soccer are often accompanied by airborne rotations about the vertical axis. These airborne rotations are initiated at the ground during the jumping phase through an anterior-posterior force coupling of the left and right limbs (⁴). However, the characteristics of both horizontal and vertical ground reaction forces have yet to be examined in VJR landings compared with VJ landings. As such, we performed a within-subjects comparison of VJ landings and VJR landings to determine the manner in which vertical and horizontal impact force parameters change during VJR landings in NCAA Division I male soccer players.

Subjects

A convenience sample of 24 male Division I soccer players (179.6 ± 8.4 cm, 75.7 ± 10.1 kg, 19.9 ± 1.5 years) volunteered to participate in this investigation. The participants and data examined in this study were a subset of data from a recent investigation (⁴). Before completing any laboratory tasks, participants provided written consent as approved by the local institutional review board. Demographic characteristics of the sample are provided in Table 1. At the time of testing, all participants were free of injury to the lower extremities, and were active members of the university's soccer team at the start of the current competitive season.

Table 1.: Participant demographics.*†

Procedures

Participants completed a single laboratory session. Anthropometric measures of height and mass were recorded, and demographic characteristics of age and playing position were provided. Participants completed a self-selected warm-up (≤10 minutes) consisting of both static and dynamic stretching. We required the combination of static and dynamic stretching because the combination of these stretching types does not produce adverse effects during subsequent jumping, sprinting, and agility tasks (^10,30,31) compared with the sole performance of either static or dynamic stretching. Next, participants performed up to 6 practice attempts to become familiarized with the experimental tasks. Up to 6 practice attempts were provided to allow the participants to adequately familiarize themselves with the laboratory environment and the experimental tasks while accounting for potential fatigue effects that could result from excessive practice attempts. Three trials were recorded for each task, and the tasks were presented consecutively such that the VJ landings were performed before the VJR landings. We did not anticipate order effects because of the skilled participant sample, the low complexity of the jump-landing tasks, and the freedom to perform up to 6 practice attempts (⁴). A dual-force platform system (Kistler Instruments Corp., Amherst, NY; 1000 Hz) was used to obtain 3-dimensional kinetic data bilaterally. The force platform system was interfaced to a PC running Bioware (version 4.0.1.2). During VJ landings, the participants began standing still with one foot on a respective force platform. Participants were instructed to jump vertically as high as possible, using a self-selected countermovement depth and arm swing to ensure that each attempt best represented both maximum effort and maximum jump height (²³). Once airborne, the participants were asked to land with each foot contacting a force platform before returning to a motionless standing position. During the VJR landings, participants were instructed to jump vertically as high as possible, using a self-selected countermovement depth and arm swing. Once airborne, the participants were asked to complete a 180° rotation about the vertical axis in their preferred direction and land with each foot contacting the opposite force platform from takeoff. Participants were instructed to return to a motionless standing position on landing. The preferred direction of rotation was determined during the practice trials, and all rotations across recorded trials during VJR landings were performed in that direction. Trials were discarded and repeated if a participant was judged to have targeted the force platforms, landed without each foot contacting an individual force platform, was unable to return to a motionless standing position, or rotated less than or greater than 180° during VJR landings (determined by visually monitoring foot placement on landing). A total of 6 attempts were allowed for each task to complete 3 successful trials. All 24 participants were able to complete 6 successful trials in fewer than 6 attempts for each task. Rest was provided between trials (≤1 minute) and tasks (≤2 minutes). These rest intervals were selected based on the combination of the small number of trials recorded and the highly trained status of the participants.

Data Reduction

Data were processed and analyzed in MATLAB (R2014a; The MathWorks, Inc., Natick, MA, USA). Raw ground reaction force signals were smoothed using a fourth-order low-pass Butterworth digital filter with a cutoff frequency of 50 Hz (^19,22). For both tasks, the start of the airborne phase (takeoff) was defined as the event at which the total vertical ground reaction force (vGRF) profile (left leg + right leg) decreased below 20 N. The start of the landing phase was identified as the event at which the vGRF profile exceeded 20 N after takeoff. The end of the landing phase was identified as the time when the vertical velocity profile crossed zero, representative of the time at which the center of mass changed direction from downward to upward. To obtain vertical velocity, vertical acceleration was first computed from the vGRF profile using Newton law of acceleration accounting for gravity. Velocity was then computed as the time integral of the vertical acceleration profile. The vertical and horizontal impulse values were computed as the areas under the vGRF (²¹) and horizontal force-time histories from both force platforms, respectively. The anterior-posterior and medial-lateral force couples were calculated as the absolute values of the impulse magnitudes summed from the 2 force platforms, creating a force couple index as described by Barker et al. (⁴). Dependent variables of interest included jump height (estimated using the time in the air equation), the first (F1) and second (F2) peak vGRF magnitudes, time to F1 (tF1), time to F2 (tF2), vertical impulse, the duration of the impact phase (the time between the start and end of the landing phase), and the anterior-posterior and medial-lateral force couples. All kinetic variables were computed relative to body mass. To present a representation of the ground reaction force-time histories used to compute the force couples, exemplar vertical and horizontal ground reaction force profiles during VJR landings are documented in Figure 1.

Figure 1.: Vertical and horizontal GRF profiles during VJR landings. AP, anterior-posterior; GRF = ground reaction force; ML = medial-lateral.

Statistical Analyses

To test for task differences, mean values for each variable of interest were computed across the 3 trials recorded per task, per participant. Paired-samples t-tests were used to compare each dependent variable between tasks. Statistical significance was set a priori at α = 0.05. Additionally, effect size (ES) values were computed to further examine differences between tasks (²⁹) using the magnitude interpretation of Cohen (¹²). The ES values of 0.2, 0.5, and 0.8 were identified as the lower thresholds for small, moderate, and large mean differences, respectively.

Results

A significantly greater jump height was observed during VJ landings compared with VJR landings (41.60 ± 4.03 cm, VJ landings; 39.40 ± 4.05 cm, VJR landings; p = 0.002). A significantly greater F2 magnitude (p < 0.001; Figure 2) and a significantly more rapid tF2 (p = 0.001; Table 2) occurred during VJR landings. Additionally, both the anterior-posterior and medial-lateral force couples were significantly greater during VJR landings compared with VJ landings (p < 0.001; Figure 3). No significant differences were identified between tasks for F1 (p = 0.945; Figure 2), tF1 (p = 0.642; Table 2), vertical impulse (5.16 ± 0.75 N·kg⁻¹·s⁻¹, VJ; 5.10 ± 0.93 N·kg⁻¹·s⁻¹, VJR, respectively; p = 0.762), or time to the end of the impact phase (p = 0.945; Table 2). Large ES values were identified between mean values for the VJ and VJR landing tasks for F2 (ES = 0.94), tF2 (ES = 0.89), and the anterior-posterior and medial-lateral force couples (ES = 1.83; ES = 1.46), whereas a small ES was identified for jump height (ES = 0.39). The magnitudes of the mean differences between tasks for F1 (ES = 0.10), tF1 (ES = 0.19), vertical impulse (ES = 0.07), and the time to the end of the impact phase (ES = 0.01) were trivial.

Figure 2.: First and second vGRF magnitudes during vertical jump and vertical jump + rotation landings. Data are presented as mean ± one standard deviation; VJ = vertical jump landings; VJR = vertical jump with a 180° rotation; vGRF = vertical ground reaction force. *Significantly greater than VJ for the respective variable.

Table 2.: Temporal characteristics of the VJ and VJR landings.*

Figure 3.: Force couple Indices during VJ and VJR landings. VJ = vertical jump; VJR = vertical jump with a 180° rotation; anterior-posterior = ground reaction force couple in the anterior-posterior axis; medial-lateral = ground reaction force couple in the medial-lateral axis. *Significantly greater than VJ for the respective axis (p < 0.001).

Discussion

The current findings indicate that impact forces in each dimension are different when landing from a VJ and landing with from a VJ with a 180° rotation about the vertical axis. We observed a significantly greater F2 magnitude and a significantly more rapid tF2 during VJR landings. Surprisingly, the greater F2 and the more rapid tF2 identified during VJR landings coincided with a significantly lower jump height. Theoretically, the lower jump during VJR landings should have been associated with a decreased F2 magnitude because of a lesser velocity at impact. The greater anterior-posterior and medial-lateral force couples exhibited during VJR landings might have induced greater shear joint forces that required more attention than the vertical force to control both the vertical and angular velocity of the center of mass throughout impact. However, as we did obtain kinematic data, it was not possible to quantify forces at the joints.

It could be expected that the increased horizontal force couples observed during VJR landings would increase the time required to complete the impact phase. However, these data suggest otherwise. We instructed these participants to perform the landings and return to a controlled standing position. This simple instruction likely influenced the lack of differences in impact completion times, as these participants might not have attempted to land as quickly as possible during one or both tasks. Not performing a landing as quickly as possible opposes what would be expected during competitive situations, where these athletes must land quickly in anticipation for a subsequent task.

A noteworthy result was the combination of greater medial-lateral and anterior-posterior force couples during VJR landings. This result might not be expected because the forces applied at the ground to generate angular velocity of the center of mass during the jumping phase are the result of a force coupling in anterior-posterior axis and not the medial-lateral axis (⁴). However, the presence of the medial-lateral force couple during VJR landings could indicate that these participants did not jump completely vertical while rotating about the vertical axis. Theoretically, VJ landings should be associated with horizontal force couples equal to zero because of the intention to jump vertically without rotation or translation. However, the noticeable force couple observed in the medial-lateral axis during both tasks suggests that lateral translation occurs while airborne during both VJ and VJR landings, requiring a medial-lateral force couple to stop the translation. Consequently, the anterior-posterior force couple appears to both generate (⁴) and terminate rotation about the vertical axis during VJR landings.

The cumulative effect of the increased forces and more rapid loading times could lead to injury (⁸) because of musculoskeletal overload (¹⁸) when performing a high volume of VJR landings over time. However, with appropriate training and skill development, these unfavorable impact characteristics can be addressed. Specifically, increasing the number of VJR landings performed during training could allow athletes to better attenuate vertical and horizontal forces when presented with a VJR landing during practice and competition. Furthermore, increasing the number of VJR landings in training could allow athletes to improve their explosiveness such that jump performance is not sacrificed when compared with VJ landings. Together, athletes can improve jump-landing performance and reduce injury potential during jump-landing tasks. In addition to the proposed benefits of a greater volume of VJR landings in training, increasing an athlete's eccentric strength could improve the stability of the knee joint when horizontal forces are experienced during impact (¹⁶). Furthermore, increasing eccentric strength could improve the utilization of the stretch-shortening cycle during both VJ and VJR landings such that elastic energy absorption and storage improve in agonist muscles groups (³). In turn, energy transfer can be enhanced when a jumping, sprinting, or change of direction movement is required immediately after landing.

A possible limitation of this study was the use of 3 trials per participant, per condition for analysis (^5,6). Although computing the average of 3 trials is common in research studies (^4,16), these participants might not have achieved performance stability (²⁰) after only 3 trials. A lack of performance stability could have produced skewed mean values that influenced the large standard deviations observed in the vGRF and force couple index results. Conversely, the variability around the mean values could reflect different skill sets and abilities among participants related to the distinct demands of each playing position. As such, we cannot rule out the possibility that the limited number of trials recorded and/or the distinct characteristics of the participants in this sample influenced the observed variability. Another possible limitation of this study was the absence of kinematic and electromyographic data. Including kinematic and electromyographic analyses could assist in revealing mechanical and neuromuscular parameters and/or strategies that help explain the differences between VJ and VJR landings. Such analyses could provide the precise contributions of the body segments when stopping the angular velocity of the center of mass during VJR landings.

Practical Applications

The results of this study identified differences relative to vertical and horizontal impact force parameters between VJ landings and VJ landings with a 180° rotation in Division I male soccer players. The added rotation during the VJR landings yielded a significantly lower jump height, a significantly greater F2 magnitude, a significantly more rapid tF2, and significantly greater anterior-posterior and medial-lateral force couples. These findings provide additional evidence that can improve the efficacy of training programs designed for competitive soccer players. Because ground reaction force characteristics are different during landing from jumps with and without rotation about the vertical axis, it seems reasonable to include, or increase the volume of, VJR landings in training programs. Added VJR landings in training could improve force attenuation and better prepare an athlete to perform a subsequent movement following a VJR landing.