Introduction
Vertical jumping is commonly used as a measure of both physical fitness and motor skill (4,27). Although research has studied the effects of physical fitness training on jump performance, less attention has been placed on training of jumping as a motor skill (2,7). However, a growing body of research reports that discrete biomechanical variables such as peak force and power, parameters associated with muscular fitness, are not sufficient to discriminate good from poor jumpers (11,12,18). Rather, the shape of time-series kinematic and kinetic data, which reflect the skill component, is different for jumpers of varying ability. Garhammer and Gregor (18) found that better jumpers generated lower propulsive ground reaction force than poor jumpers; however, their propulsive ground reaction impulse was greater. Vanezis and Lees (30) found no statistical differences in the peak moments generated at the hip, knee, and ankle between good and poor performers, yet qualitatively, differences were observed in moment-time series data. As skill is a critical component of vertical jump performance, an understanding of the biomechanics of skilled vs. unskilled jumping is required to determine how to best train to improve performance.
Two features of the skill component of jumping that have been identified are the sequencing of joint and segment actions (29), and the influence of arm swing (21). A proximal-to-distal sequence has been described for jumping, where the hip extends first, followed by the knee and finally the ankle (5,29). The proximal-to-distal sequence has 2 rationales: (a) optimized timing of segment motion to maximize vertical velocity of the body's center of mass and (b) efficient energy transfer from proximal to distal segments (5,29). However, there is dispute as to whether this strategy or one involving simultaneous sequencing of joint extensions is optimal for vertical jumping (23). Aragón-Vargas and Gross (1) reported that a time delay between initiation of hip and knee joint extensions was not associated with higher jump height. However, this research has 2 potential limitations. First, rotation of the hip is mechanically constrained to rotation of the knee as both joints involve the thigh segment. In a proximal-to-distal sequence, it is possible for the pelvis to extend while the thigh continues to flex, as depicted in Figure 1 in Haguenauer et al. (20). Thus, rotation of the pelvis segment may be a better indicator of proximal extension than rotation of the hip joint. Second, only the absolute time delay between the hip and knee joints extending was studied. As individuals may use varying time to perform a vertical jump, the time delay between these joints extending relative to total jump time may be more informative.
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Figure 1: Correlations between vertical jump height and relative time delay between pelvis and knee extensions. A) Countermovement vertical jump with arms. B) Countermovement vertical jump without arms.
Vertical jumping may be performed with and without arm swing. Generally, arm swing increases vertical jump height (21), however, this effect of arm swing is not uniform and some individuals have greater enhancement than others (31). Arm swing influences the kinematics of the pelvis segment and hip joint at the transition from flexion to extension (16,17), suggesting that proper use of arm swing may enhance the mechanical effects of the proximal-to-distal sequence. Therefore, the purpose of this research was to investigate: (a) the relation between sequencing strategy and vertical jump height, (b) the relation between sequencing strategy and mechanics of vertical jumping, and (c) whether arm swing influences the sequencing strategy and the mechanics of vertical jumping. For purpose (a), it was hypothesized that higher jumpers would have a longer relative time delay between pelvis segment and knee joint extensions. For purpose (b), it was hypothesized that sequencing strategy would be associated with lower extremity kinetics and kinematics. For purpose (c), it was hypothesized that lower extremity kinetics and kinematics associated with a proximal-to-distal strategy would be improved when jumping with vs. without arm swing.
Methods
Experimental Approach to the Problem
This investigation used a cross-sectional design to investigate whether sequencing strategy influences vertical jumping performance and mechanics. Sequencing strategy was quantified as the relative time delay between initiation of pelvis segment and knee joint extensions. Jumping mechanics investigated were lower extremity net joint moments (NJM) and segmental accelerations. Computer simulation studies indicate that changes in coordination are reflected in the timing of muscle activations (4). As NJM reflect the sum effect of muscles acting at a joint, lower extremity NJM are a kinetic variable representing motor skill. Acceleration is the kinematic consequence of the muscle forces exerted, angular acceleration the consequence of NJM exerted, therefore, segmental accelerations are a kinematic variable representing motor skill. The relations between jump height, sequencing strategy, and jumping mechanics were determined using correlation analysis. Furthermore, to examine if the effect of arm swing on jump height was influenced by sequencing strategy, sequencing strategy and jumping mechanics were compared between vertical jumps performed with and without arm swing.
Subjects
A convenience sample of women volleyball players (n = 16; age range: 18–23 years) participated in this investigation. Before participation, study protocol was explained to participants who provided written informed consent as approved by a Research Ethics Board at the author's institution. All participants were competitive varsity athletes and performed jumping regularly in practice and competition. Participants were 20.4 ± 1.5 years of age, and they had a stature of 1.78 ± 0.06 m and a body mass of 70.1 ± 7.1 kg.
Procedures
Participants performed a brief warm-up consisting of unloaded squats and submaximal vertical jumps. Data were collected with participants performing countermovement vertical jumps with and without arm swing. Four maximum effort jumps of each type were performed with an overhead target used. All jumps were performed standing with each foot on a separate force platform (AMTI OR6-6, Advanced Mechanical Technology, Inc., Watertown, MA, USA). Retroreflective markers were placed on participants using a 6 degree-of-freedom configuration (8). Calibration markers were placed on important bony landmarks to locate the proximal and distal ends of the pelvis, thigh, leg, and feet segments. Tracking markers consisting of 3 (feet) or 4 (legs and thighs) markers affixed to molded thermoplastic plates were used to track motion of these segments. The proximal calibration markers on the pelvis (left and right iliac crests) and a marker placed at L5/S1 were used to track the pelvis. Marker data were captured using 6 optoelectronic cameras (MCU240; Qualisys ProReflex, Gothenburg, Sweden) simultaneously with force platform data using Qualisys Track Manager software (version 2.3.510). Sampling frequencies were 120 and 1200 Hz for camera and force platform data, respectively. Participants were allowed to take as much time as they desired between each jump.
Data Processing
Motion analysis data were processed using Visual 3D (version 4.91; C-Motion, Germantown, MD, USA) for rigid-body link-segment modeling. Marker and force data were filtered using a fourth order low-pass recursive Butterworth with a cutoff frequency of 10 Hz. Segments were recreated using marker data, with segment mass from Dempster's data (33) and location of the segment's center of mass and inertial parameters based on each segment having the shape of a conical frustum. Sagittal plane angular acceleration about the center of mass of the leg and thigh, and vertical acceleration of the pelvis center of mass were calculated. Movement began at initiation of the countermovement to take-off. The time when the pelvis segment and knee joints switched from flexion to extension was determined from the zero crossings of pelvis and knee angular velocity. Time delay between pelvis segment and knee joint extension was determined by subtracting time of pelvis extension from time of knee extension, which was expressed as a percentage of total movement time. A positive time delay indicates earlier pelvis extension, whereas a negative time delay indicates earlier knee extension. This variable will be referred to as the “relative time delay between extensions.”
Force platform data were combined with modeled segments for inverse dynamics calculations to determine NJM. Hip, knee, and ankle NJM were calculated as internal moments, which were expressed in the coordinate system of the distal segment. The average concentric and ratio of average concentric to average eccentric NJM were determined. These variables were selected as they reflect the shape of the NJM-time series data. Previous research has not found differences in peak NJM between good and poor jumpers; however, the shape of NJM-time data seems to be qualitatively different (30). The demarcation of eccentric and concentric phases was joint specific (i.e., time of hip extension, knee extension, and ankle plantar flexion). All kinetic data were normalized to body mass. Jump height was determined using the pelvic kinematic method, which determines the difference between pelvis center of mass height in standing vs. the apex of the jump (9). Temporal and kinematic data were averaged between limbs and kinetic data were summed across limbs. Data were averaged across the 4 jumps of each type. The reliability of these data collection and processing methods has previously been established (10).
Statistical Analyses
The first purpose of this research was to investigate the relation between sequencing strategy and vertical jump performance. To accomplish this, the relation between jump height and relative time delay between extensions was assessed using Pearson Product Moment Correlations for each jump type. The second purpose of this research was to investigate the relation between sequencing strategy and mechanics of vertical jumping. For this purpose, relations were determined between lower extremity mechanics (NJM and accelerations) with (a) jump height and (b) relative time delay between extensions using Pearson's correlations for each jump type. A mechanical variable was deemed to be influenced by relative time delay between extensions and have an effect on jump performance if a significant correlation was observed between the variable with both relative time delay between extensions and jump height.
The third purpose of this research was to investigate whether arm swing influences the effect of sequencing strategy. Paired t-tests were used to determine if differences existed for jump height and relative time delay between extensions between jumps with and without arm swing. The mechanical variables identified in purpose (b) were also compared using paired t-tests. All statistical analyses were performed in SPSS (version 11.0; SPSS Inc., Chicago, IL, USA) with alpha set a priori (α = 0.05). Magnitudes of correlations were interpreted according to Hopkins (22).
Results
Correlations between jump height and relative time delay between extensions indicate higher jumpers use a proximal-to-distal strategy. A moderate correlation was found between jump height and relative time delay between extensions in the countermovement vertical jump without arm swing (r = 0.58, p = 0.02; Figure 1). A strong correlation between jump height and relative time delay between extensions was found in the countermovement vertical jump with arm swing (r = 0.82, p < 0.001; Figure 1).
Jump height and relative time delay between extensions were significantly correlated with concentric hip extensor and ankle plantar flexor NJM, in both jumps with and without arm swing (Table 1). Jump height and relative time delay between extensions were significantly correlated with concentric knee extensor NJM in jumps with arm swing but not in jumps without arm swing (Table 1). The ratio of concentric to eccentric knee extensor NJM was correlated with jump height and relative time delay between extensions for both jumps with and without arm swing (Table 1). Jump height and relative time delay between extensions were significantly correlated with vertical acceleration of the pelvis, and angular accelerations of the thigh and leg (Table 1) in both jumps with and without arm swing. Time series comparisons of lower extremity NJM (Figure 2) and accelerations (Figure 3) between the highest and lowest jumpers are provided.
Table 1: Correlation coefficients for biomechanical variables with jump height and pelvis-to-knee time for countermovement vertical jumps with and without arm swing.*
Figure 2: Hip, knee, and ankle net joint moment (NJM) time-series from countermovement vertical jumps with arm swing for the highest (solid black) and lowest (dashed gray) jumpers. Data are normalized to 101 data points and averaged across trials. Vertical lines indicate initiation of pelvis extension.
Figure 3: Pelvis vertical acceleration and thigh and leg angular acceleration time-series from countermovement vertical jumps with arm swing for the highest (solid black) and lowest (dashed gray) jumpers. Data are normalized to 101 data points and averaged across trials. Vertical lines indicate initiation of pelvis extension.
Vertical jump height in countermovement vertical jumps with arm swing was greater than without arm swing (p < 0.001; Table 2). Relative time delay between extensions was longer in vertical jumps with vs. without arm swing (p ≤ 0.05). Ankle plantar flexor NJM (p < 0.001) and the ratio of concentric to eccentric knee extensor NJM (p = 0.02) were greater in vertical jumps with arm swing. Pelvis vertical acceleration (p = 0.02) and thigh (p = 0.005) and leg (p = 0.05) angular accelerations were higher in jumps with arm swing than without.
Table 2: Biomechanical variables determined to be influenced by jump height and RTDExt in countermovement vertical jumps with and without arm swing.*
Discussion
A longer relative time between initiations of pelvis extension vs. knee extension was associated with higher vertical jump heights. This relation was stronger when vertical jumps were performed with arm swing than without. Furthermore, when arm swing was used, the relative time between extension of the pelvis and extension of the knee was longer. These findings indicate that 1 difference between higher vs. lower vertical jumpers is the use of sequencing strategy. Higher jumpers use a proximal-to-distal strategy, where the pelvis extends before the knee, whereas lower jumpers use simultaneously extend their pelvis and knee. Moreover, arm swing promotes the use of a proximal-to-distal strategy.
Bobbert and coworkers (3,6) found the proximal-to-distal strategy generated NJM-time curves, where the hip extensor NJM reached its maximum earlier in the jump movement, followed by knee extensor NJM, and ankle plantar flexor NJM reached its maximum last, nearer to take-off. Qualitatively, graphical NJM-time data presented by Bobbert and coworkers (3,6) are similar to those in the highest jumper from our study (Figure 2). However, the lowest jumper from our study had markedly different NJM-time data. The highest jumper had a long and the lowest jumper a short relative time delay between extensions, thus demonstrating proximal-to-distal and sequential strategies, respectively. The lowest jumper had a later time of maximum hip extensor NJM, and lower hip extensor and ankle plantar flexor NJM. These differences are reflected in the correlations between average concentric hip extensor and ankle plantar flexor NJM with relative time delay between extensions and jump height. Average concentric knee extensor NJM was only correlated with jump height and relative time delay between extensions when arm swing was used, however, the ratio of concentric to eccentric knee extensor NJM was related with jump height and relative time delay between extensions for both types of jumps. This is reflected in the comparison of the highest and lowest jumpers. The highest jumper's knee extensor NJM has an inverted “U” shape, similar to that reported by Bobbert and coworkers (3,6). In contrast, the lowest jumper has multiple peaks in their knee extensor NJM.
The multiple peaks in the lowest jumper's knee extensor NJM graph may be explained by differences in muscle activation patterns. Bobbert and coworkers (3,6) found that peak hamstrings electromyographic (EMG) activity occurs at the same time as peak hip extensor NJM. Similarly, peak vasti EMG activity occurs at the time of peak knee extensor NJM. Using a proximal-to-distal strategy, the vasti EMG and knee extensor NJM peaks occur after the hamstrings EMG and hip extensor NJM peaks. While this NJM pattern was observed in the highest jumper, the lowest jumper had simultaneous hip and knee extensor NJM peaks, followed by a decrease in knee extensor NJM. In addition to a hip extensor NJM, the hamstrings exert a knee flexor moment and cocontraction of the hamstrings with the vasti would decrease the knee extensor NJM (24). Thus, it is hypothesized that the proximal-to-distal strategy allows maximum effort of the hamstrings before maximum effort of the vasti, minimizing the antagonism between these muscle groups. Furthermore, it is hypothesized that the simultaneous strategy has maximum effort of these muscle groups at the same time, resulting in a brief decrease in knee extensor NJM.
The effects of these differences in knee extensor NJM are also observed in the thigh and leg angular acceleration patterns. The knee extensor NJM is exerted on both the thigh and leg segments, which according to Newton's second law of motion would elicit angular acceleration of these segments. The highest jumper has greater angular accelerations in the direction of extension for both the thigh and leg. Moreover, thigh and leg angular accelerations were correlated with relative time delay between extensions and jump height. Qualitatively, the lowest jumper has thigh and leg angular accelerations near zero throughout most of the movement. Furthermore, thigh angular acceleration crosses zero in the lowest jumper, indicating acceleration in the direction of flexion. These zero crossings are also observed in the pelvis linear acceleration, where the pelvis accelerates upwards at the time the pelvis extends, downwards after the pelvis starts to extend and returning to upwards. As a corollary to the hypothesis of greater hamstrings and vasti cocontraction in the simultaneous strategy, it is hypothesized that the briefly reduced knee extensor NJM would result in lower thigh and leg angular accelerations.
Therefore, the proximal-to-distal strategy may be ideal for vertical jumping by minimizing the deleterious effects of antagonist cocontraction. In the case of vertical jumping, hamstrings and vasti cocontraction would occur with a simultaneous strategy and affect the jump height reached. Hamstrings cocontraction during activities where high knee extensor efforts are required may also result in excessive loading of joint tissues. Computer modeling studies have found cocontraction of these muscles to increase tibiofemoral (28) and patellofemoral compressive forces (13,32). Increased tibiofemoral compressive forces are associated with development of osteoarthritis (26) and high patellofemoral compressive forces may lead to patellofemoral pain syndrome (13,32). If antagonist cocontraction can be reduced by using a proximal-to-distal strategy, an appropriate intervention to influence how the skill component of vertical jumping, and possibly other tasks, may be effective to reduce these type of orthopedic injuries.
The current results suggest not all participants may use a proximal-to-distal strategy. Similarly, Walsh et al. (31) reported a sex bias in the effectiveness of arm swing on enhancing jump performance, where men improved jump height when using arm swing more than women. As our data suggest that the use of arm swing may promote the proximal-to-distal strategy, this may suggest that men are more adept at using this strategy than women. Haguenauer et al. (20) found elderly individuals use a simultaneous strategy when performing vertical jumps. Despite the research conducted on proximal-to-distal strategy, the mechanisms, such as muscle activations, eliciting this strategy are not known. Greater knowledge regarding the muscular mechanisms responsible for this sequencing strategy would contribute to development of interventions, such as training programs, that would improve coordination for vertical jumping. For example, this study found ankle plantar flexor NJM to be positively associated with jump height and relative time delay between pelvis and knee extensions. Greater anterior projection of the total body center of mass relative to the foot was observed in young, who used a proximal-to-distal strategy, vs. elderly, who used a simultaneous strategy, individuals (20). Greater anterior projection of the total body center of mass relative to the foot would increase the ankle plantar flexor NJM, which corresponds with the relation observed between ankle plantar flexor NJM and relative time delay between pelvis and knee extensions in this study. Altogether, this suggests that strength or activation of the ankle plantar flexor muscles may play a role in the ability to use a proximal-to-distal strategy.
Sequencing strategy should also be considered in tasks other than vertical jumping. A proximal-to-distal strategy has been observed in long jumping (25) and speed skating (29). When the proximal-to-distal strategy is used, the pelvis and trunk begin to extend, whereas the knees continue to flex. This description is similar to the second knee bend portion of the snatch and clean (14,15), suggesting that weightlifting exercises may also use this strategy. Therefore, the proximal-to-distal strategy is not unique to vertical jumping, but, instead is a movement pattern that may be shared across a variety of tasks. That this strategy is not unique to vertical jumping highlights the importance of identifying the muscular mechanisms responsible for causing movement to occur in this manner. An improved understanding of these mechanisms is required for practitioners to teach movement incorporating the proximal-to-distal strategy. Furthermore, if the muscles contributing to this sequencing strategy have insufficient fitness, which may include strength and flexibility, identifying the muscles involved is necessary to appropriately intervene through exercise.
In summary, this research confirms an association between motor skill, specifically the use of a proximal-to-distal strategy, and vertical jump performance. Furthermore, the use of arm swing promotes and enhances the effect of proximal-to-distal sequencing. The proximal-to-distal strategy allows large hip extensor, knee extensor, and ankle plantar flexor NJM to be generated successively, which results in large angular accelerations extending the lower extremity and maximizing vertical acceleration of the pelvis. A simultaneous strategy has large hip and knee extensor NJM concurrently, which may result in antagonist cocontraction at the knee. The resultant effect is a reduction in thigh and leg angular accelerations and lower vertical acceleration of the pelvis. These findings highlight the importance of multijoint coordination in vertical jump performance. As a proximal-to-distal strategy is not universally used, the mechanisms causing this sequencing strategy to occur should be determined, which would allow appropriate training intervention to be used to enhance motor coordination.
Practical Applications
Vertical jump performance is important across a range of sports and is a key indicator of athletic success. For these reasons, testing of vertical jump performance is common in athletic and laboratory situations. In practice, vertical jump height is typically assessed, such as during a jump and reach type test. In the laboratory, jump performance may also be assessed using technological means such as force platforms. A growing body of research, including this study, suggests that coordination should also be considered when evaluating vertical jump performance. Specifically, this research indicates better jumping is associated with a proximal-to-distal strategy, where the pelvis extends earlier than the knees. Thus, a qualitative analysis may provide greater insight into jumping performance than jump height alone. To facilitate qualitative kinematic analysis, coaches may take advantage of commonly available technology, such as digital video capturing on modern smartphones or tablets (19). Qualitative analyses may also be applicable to other tasks that use the proximal-to-distal strategy, such as horizontal jumps and the weightlifting pulling motion, as a means to improve movement quality. Although further research is required to confirm, qualitative analyses and consideration of proximal-to-distal strategies may also be relevant for orthopaedic situations, for example, where decreasing antagonist quadriceps and hamstrings cocontraction may reduce patellofemoral pressures.
Acknowledgments
This research was funded by a grant from the Sport Science Association of Alberta.
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