The concept of attentional focus has garnered extensive consideration in recent years. The benefits of adopting an external focus of attention (FOA) strategy and its effects on motor behavior have been thoroughly examined (for a review, see Wulf (16)). An external FOA is when a subject focuses on the result of the movement rather than the movement itself. In contrast, an internal FOA involves focusing on the movement production characteristics such as body segments or musculature involved (19). Providing internal FOA instructions to athletes is a common practice by coaches, even at the elite level. In turn, these athletes often use internally focused cues (10). However, the literature on attentional focus has consistently reported that subjects yield enhanced performance outcomes when they use an external compared with internal FOA. For example, participants displayed significantly better performance while adopting an external FOA during a basketball free throw shot (23), tennis stroke (5), golf swing (20,22), dart throw (4,6), and vertical jump (18) compared with when attention was directed internally. Furthermore, several studies have shown an external FOA results in greater efficiency, as expressed by lower overall electromyography (EMG) measures (7,11,18,23). For example, Vance et al. (11) found that participants' biceps muscle activity, as measured by integrated EMG during flexion, was greater when they focused on their arms (internal FOA) than when focusing on the bar (external FOA) during a biceps curl exercise, although the weight of the bar and the velocity of movement were held constant.
The standing long jump (SLJ) is a test in which an individual stands with his or her feet parallel and attempts a single jump in the air as far horizontally as possible. The SLJ is a common assessment tool that is administered to athletes to test for power and explosiveness (9,13,15). The SLJ can also be considered a predictor of anaerobic performance (2). The popularity of this dynamic movement, and more importantly the predictions made about the athlete's potential based on his or her performance, has given greater importance to optimization of the task. Coaches have traditionally trained athletes to improve SLJ distances through strength and conditioning methods (e.g., weightlifting, plyometrics, sprint-training) for a number of years. However, recent empirical findings have demonstrated that the verbal instructions presented to athletes have considerable potential to benefit performance, as well. Specifically, the quality and category of verbal instructions presented to an athlete before performing an SLJ has been shown to influence the performance outcome. In a series of studies, researchers found that participants improved their SLJ performance when they directed their attention externally (jump to a cone in the distance) instead of internally (rapidly extend knees) (8,9,15).
Wulf et al. (21) proposed the constrained-action hypothesis to explain why an external FOA improves performance outcome measures compared with an internal FOA. Based on this hypothesis, complex coordinated movements are performed using automatic and organized motor patterns. When an individual focuses internally, or on the movement production, these automatic processes become disrupted. Conversely, focusing externally allows these natural processes to occur nonconsciously, which subsequently leads to more efficient and effective movement production.
Although the constrained-action hypothesis serves as a plausible theoretical explanation for the benefits of an external compared with internal FOA, the precise mechanisms that constitute “effectiveness” are still being explored. That is, what exactly is occurring during the movement production to improve performance? Wu et al. (15) found that, although SLJ outcome measures were enhanced using an external FOA, production data analysis revealed no differences in peak force. Therefore, additional variables must be assessed to elucidate these discrepancies.
One variable of potentially considerable importance is the performer's projection angle, or takeoff angle. The concept of an optimal projection angle (to optimize SLJ performance) has been previously explored (1,3,12). Too high of a projection angle will result in a reduction of forward projectile distance; too low of a projection angle will result in insufficient airtime. In either case, using an optimal projection angle is critical in maximizing the distance that can be achieved. Another variable for consideration is impulse, which refers to the time integration of force. Wulf et al. (18) noted that an external FOA allows performers to produce greater peak forces, as well as impulses when performing the vertical jump. Conversely, in the study conducted by Wu et al. (15), the authors revealed there were no differences in peak force measures when performing the SLJ. However, the authors did not examine force impulse; therefore, it is possible that changes in impulse resulted in the significant SLJ performance difference between the internal and external conditions. However, to date, the interaction between impulse and FOA has not been tested while performing the SLJ.
Hence, the purpose of this study was to examine kinetic and kinematic properties associated with the SLJ that may explain disparities in performance measures between an internal and external FOA. The hypotheses were that the external focus condition would yield greater jump distances, as well as exhibit greater impulse values and a more theoretically optimal projection angle (i.e., near 45°), than the baseline and internal conditions.
Experimental Approach to the Problem
Participants began standing on a Vernier force platform (Vernier Software & Technology, Beaverton, OR, USA) before each jump. The force platform was used to record vertical reaction forces at a sample rate of 120 Hz for each trial. The analog force data were converted into digital format using a LabPro interface. Directly in front of the force platform was a rubber broad jump mat, which was used to record jump distances. A Canon video camera (Canon USA, Inc., Melville, NY, USA) was positioned 3.66 m to the left side of the participants, perpendicular to the jumping direction, and in line with the start jump position. The camera was used to calculate projection angle. Reflective markers were placed on the left side of each participant at the fibular head and lateral malleolus. The line between these two markers represented the lower limb. The video data were collected at a sample rate of 60 Hz. Both video and force data were imported into LoggerPro 3.8.4 (Vernier Software & Technology, Beaverton, OR, USA) for analysis.
Twenty-one university students (10 males, 11 females) participated in the study. Subjects had a mean height of 169.1 ± 10.1 cm, mean mass of 67.4 ± 10.5 kg, and mean age of 21.3 ± 1.74 years (age range 19–24 years). The subjects were considered “untrained” in that they did not participate in SLJ training or competition. All participants were provided information regarding the potential benefits and risks in the investigation. Participants read and signed the informed consent form prior to participation. The consent form and experiment methodology were approved by the university institutional review board before data collection.
Participants performed a 5-minute warm-up on a stationary bike at a low self-selected intensity, followed by a 2-minute rest period in which participants sat in a chair. After the rest period, participants performed their first jump onto the jump mat, which served as the baseline measurement. The verbal instruction given during this trial was to “jump as far as you can.” After the baseline jump, participants performed 4 additional jumps in which either internal or external focus cues were introduced. During the trials in which internal cues were given, participants were instructed to “Jump as far as you can. While you are jumping, I want you to think about extending your knees as rapidly as possible.” During the trials in which external cues were given, participants were instructed to “Jump as far as you can. While you are jumping, I want you to think about jumping as close to the green target as possible.” The green target/cone was placed 4.5 m from the start line. The cone was not present during the baseline or internal focus condition trials.
After each jump, participants were given a 2-minute rest period. Participants performed 2 jumps consecutively with 1 focus condition, followed by the other, and the order was delivered in a counter-balanced manner across participants to control for possible order effects. This means that, when 1 participant performed 2 jumps using an internal focus, the following 2 jumps were conducted using an external focus. However, the next participant was first given external cues, followed by internal cues.
Jump distance was identified by measuring the distance from the starting line to the rear heel of the foot closest to the starting line. Peak force was identified as the highest recorded force value in Newtons (N) for each trial. Impulse (in Newton second) was identified as the force (in Newtons) multiplied by time (in seconds) during the positive impulse period. Hence, the formula for impulse can be described as impulse = force × time. Specifically, this period was identified as the time from the lowest force value recorded, just before the steepest force slope, to the peak force value. Projection angle was defined as the angle (from horizontal) of the lower limb immediately after toe-off.
For all analyses, the mean of the 2 trials within each condition was used. That is, the average value of the 2 internal FOA trials was obtained for each subject, as was for the 2 external FOA trials. Subjects performed only 1 trial in the baseline condition. The mean values and SDs of the averaged trials were used for subsequent data analyses. Data were analyzed using the Statistical Package for Social Sciences (SPSS, IBM Corp., Armonk, NY, USA) version 16. Separate analyses of variance (ANOVAs) with repeated measures on condition were used to identify differences in jump distance, peak force, impulse, and projection angle. An intraclass correlation coefficient reliability (ICCR) analysis determined reliability of the dependent variables. An alpha level of p = 0.05 defined significance. Effect size (ES) statistics were reported using partial η2. When main effects were significant, Tukey's honestly significant difference post hoc tests were performed with an alpha level of p = 0.05 to confirm significant pairwise comparisons.
Figure 1 displays the results of the outcome measures reflecting the average distance jumped in each experimental condition. The analyses revealed that there was an overall main effect for condition (F2,34 = 15.02, p < 0.001, ES = 0.469). The external condition jumped significantly farther than the internal and baseline conditions (pairwise comparisons, both p < 0.001). Specifically, the external FOA condition had a mean jump distance of 172.3 ± 48.4 cm (SEM = 11.10), whereas the internal and baseline FOA conditions had a mean jump distance of 156.5 ± 52.3 cm (SEM = 11.99) and 158.2 ± 48.9 cm (SEM = 11.22), respectively. There was no observed difference between the baseline and internal conditions (p = 0.667). The ICCR analysis revealed that this dependent variable was reliable for the external (r = 0.995) and internal (r = 0.991) conditions. The statistical power based on our sample size (n = 21) and ES (0.469) was 0.997.
Figure 2 and Figure 3 represent peak force and impulse values, respectively. Analysis of peak force revealed no significant differences between the baseline, internal, and external conditions (F2,36 = 0.864, p = 0.43). The mean peak force values for the baseline, internal, and external conditions were 1,471.2 ± 358.5 N (SEM = 82.25), 1,480.9 ± 305 N (SEM = 69.97), and 1,444.8 ± 347.1 N (SEM = 79.63), respectively. In addition, there were no observed differences between conditions for impulse (F2,36 = 1.28, p = 0.291). The mean impulse values for the baseline, internal, and external conditions were 607.9 ± 209.9 N (SEM = 48.15), 583.3 ± 159.6 N (SEM = 36.61), and 562.2 ± 182.6 N (SEM = 41.89), respectively.
Figure 4 represents the mean projection angles for each experimental condition. The ANOVA revealed a significant effect of condition with respect to projection angle (F2,36 = 5.63, p = 0.007, ES = 0.238). Pairwise tests revealed that the external group (45.7 ± 5.32, SEM = 1.22) exhibited a significantly lower projection angle than the baseline (49.0 ± 7.04, SEM = 1.62, p = 0.012) and internal conditions (49.5 ± 7.03, SEM = 1.61, p = 0.002). There was no observed difference in projection angle between the baseline and internal conditions (p = 0.71). The ICCR analysis revealed that this dependent variable was reliable for the external (r = 0.770) and internal (r = 0.893) conditions.
The results of this study revealed that when participants were in the external FOA condition they jumped significantly farther than trials completed in the internal FOA and baseline conditions. Additionally, when participants were in the external FOA condition, they used a smaller and more effective projection angle compared with attempts completed in the internal FOA and baseline conditions. These results support the predictions of the constrained-action hypothesis, in which the external FOA condition optimized movement coordination through use of automatic processes. The results of the kinetic analyses, however, are not in agreement with previous FOA literature (18) examining vertical jump performance in which the external FOA produced greater peak force and impulse values.
The theoretical optimum projection angle for a projectile is 45°. This angle is only optimal, however, if the speed of the projectile remains constant. During the SLJ, the speed of the jumper decreases as the projection angle increases. That is, there is an inverse relationship between projection speed and projection angle (12). Therefore, the optimum projection angle must be lower than 45°. An experiment by Wakai and Linthorne (12) revealed that the optimum takeoff angle, or the angle that resulted in the greatest jump distance, was 19°. In addition, they found that the preferred jump takeoff angle for their participants was 33°. Aguado et al. (1) measured kinematic and kinetic factors during the SLJ and found that participants jumped at an angle between 25 and 31°. Linthorne et al. (3) found that, during a standard long jump task, the optimum takeoff angle is around 21°. In each case, the angle was considerably lower than the observed jump angle in this study. These data suggest that, although the participants in our study used a more effective projection angle during the external FOA condition compared with the internal FOA condition, the angle used was still far from the optimal angle reported in previous studies.
There are numerous potential factors that may explain these takeoff angle discrepancies exhibited by the participants in the current experiment.
Although the previously mentioned long jump studies did not incorporate FOA instructions, they did influence the production of the participants' movements based on instructions given. During the intervention trials, Linthorne et al. (3) instructed participants to jump “low,” “high,” or “very high” compared with their preferred takeoff angle. Wakai and Linthorne (12) instructed participants to jump “vertical,” “much higher,” “higher,” “lower,” or “much lower” than their preferred takeoff angle. In the present experiment, participants were instructed to jump as far as possible, with additional FOA instructions, but no additional instructions related to takeoff angle. Subjects in the present experiment consisted of undergraduate students, and none of which were track and field athletes. Moreover, none of the subjects had received previous training about how to perform the SLJ. In a paradigm similar to the current experiment, Wu et al. (14) found that unskilled female college students jumped at takeoff angles between 48 and 50°. A potentially limiting factor for projection angle is fear of falling. That is, the lower the projection angle, the less time the subjects have to bring their feet in front of their torso, to land properly and safely. The study by Wakai and Linthorne (12) used physically active male participants, and the protocol allowed them to jump into a sand pit. Linthorne et al. (3) studied experienced long jumpers who also jumped into a sand pit. This softer landing area may have reduced fear of falling, compared with landing on a firm rubber jump mat, as was the case in this study. Subjects in the study by Aguado et al. (1) were considered healthy and physically active. In addition, they were given familiarization of the procedure by performing several submaximal and maximal jumps before data collection. Therefore, participants in this study may have potentially used a lower projection angle if (a) they were specifically instructed to do so, (b) they were more physically active, (c) they jumped into a softer landing area, thus reducing fear of falling, or (d) they had more experience with the SLJ task, either before data collection day, such as while training for a sport, or performing several submaximal and/or maximal jumps before the actual recorded jumps with different FOA instructions. Additional research is needed to fully examine these possibilities.
A potential limitation in this study was that the force plate used only measured vertical reaction forces. There may have been valuable information regarding differences in horizontal reaction forces among different FOA conditions that might help explain performance outcome disparities. Aguado et al. (1) suggested a potential technique to improve SLJ performance that would be to instruct participants such that horizontal force production increases concomitant with lower projection angles. In their study, however, kinetic analysis revealed no differences in vertical or horizontal force production among different projection angles. Future studies may benefit from incorporating multiaxis force plate measurements so that horizontal reaction forces can be analyzed. Moreover, the force plate's sampling frequency (120 Hz) may have been too low to discern potential differences in peak forces or impulse values. Wulf and Dufek (17) found that participants produced greater impulse values when using an external compared with an internal FOA during a vertical jump. In that study, however, the force plate's collection rate was 1,080 Hz.
In addition to evaluating projection angle, takeoff velocity may also provide valuable insights into the discrepancies in performance. As noted earlier, projection speed is inversely related to projection angle (3,12). An external FOA may yield superior outcome measures by minimizing projection angle to maximize takeoff velocities (12). Additionally, other kinetic measures such as EMG might be collected to discern between differences in muscle recruitment patterns and synergies. Wulf et al. (18) noted that an external FOA may optimize muscle onset coordination patterns, resulting in greater performance measures. Aguado et al. (1) also stated that the most important variables of SLJ performance are most likely technique and coordination. Detection and analysis of different muscle coordination patterns can only be attained with the collection of EMG data. Finally, it should be noted that this study involved participants with minimal or no experience in the SLJ. To verify that this phenomenon is not limited to novices, future studies should evaluate collegiate or professional long jumpers.
The external FOA condition was shown to exhibit greater SLJ distances, as well as smaller and more effective projection angles, compared with the internal FOA and baseline conditions. Practically, research in various FOA strategies may be of valuable use for coaches and practitioners when instructing athletes. There are numerous instructions that coaches may normally present to athletes (such as “swing your arms” or “bend your knees”) that promote an internal FOA. Although research consistently reports greater observable performance with an external FOA, coaches often continue to present verbal information based on traditional ideas. Porter et al. (10) surveyed elite U.S. track and field athletes and found that 84.6% reported being provided with cues that promoted an internal FOA by their coaches. The subject pool was comprised of experts in various events, indicating the results obtained are universal to the sport. Additionally, 69.2% of the athletes reported focusing their attention on the movement, while only 7.7% used external cues. Coaches and athletes have the opportunity to take advantage of the most current evidence available. To optimize practice and performance outcome measures while minimizing movement constraints, athletes should focus on aspects of the environment that promote an external FOA, while at the same time avoid focusing on features of the movement that may promote an internal FOA.
Note: Scott W. Ducharme is now with the Department of Kinesiology at University of Massachusetts, Amherst, MA 01003.
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Keywords:Copyright © 2016 by the National Strength & Conditioning Association.
attentional focus; projectile; takeoff angle; motor performance