Kinetics, Kinematics, and Fixed Postures: An Exploration of How Attentional Focus Manipulation Enhances Movement

Abstract

There is debate in the literature regarding how manipulating focus of attention (FOA) influences ground reaction forces during the standing long jump (SLJ), and gaps in understanding as to which phases of the SLJ are effected (takeoff, flight, landing) and whether FOA manipulation benefits remain when tasks are performed in fixed body postures.

Purpose:

This study compared SLJ performance under external (EXT) and internal (INT) FOA conditions with free and fixed postures.

Methods:

Twenty participants performed SLJs under EXT and INT FOA conditions while being allowed to swing arms freely and having to keep hands on their hips. Kinematics and kinetics were recorded using 3D-motion capture and force plates. Jump distances, projection angles, and ground reaction forces and impulses were compared across conditions using a 2 x 3 repeated measures ANOVA.

Results:

Jump distances were significantly further with EXT FOA (p < .001). These differences were due to increases in the takeoff distance (p < .001) and landing distances (p < .001), with flight distances not being different between conditions (p = .061). Peak horizontal ground reaction forces (p < .001) and impulses (p < .001) were both greater while projection angles were lower (p = .002) in the EXT FOA condition.

Conclusions:

Improvements in SLJ distance with an EXT FOA are due to the takeoff and landing phases; manipulating FOA does change forces during the SLJ; and that benefits of an EXT FOA are realized even when movements are performed with constrained body postures.

INTRODUCTION

Acquiring, enhancing, and accelerating motor skill development can lead to increased functionality in activities of daily living, increased performance in sport and the performing arts, and greater participation in general physical activity (Wulf, 2013). As such, there is a large body of literature evaluating how to best help individuals control and learn movement skills. Numerous studies have evaluated how directing an individual’s focus of attention (FOA) influences performance and learning of motor skills. Through verbal instructions, an individual’s FOA can be directed internally or externally (Becker & Smith, 2015; Hebert & Williams, 2017). When using an internal (INT) FOA individuals are instructed to pay attention to the components of their body responsible for producing a given movement or action. In contrast, when using an external (EXT) FOA individuals are instructed to focus on the outcomes of the movement or the effects the movement have on the environment (McNevin, Shea & Wulf, 2003; Porter, Anton, & Wu, 2012; Wulf & Prinz, 2001; Wulf, Zachry, Granados, & Dufek, 2007). When interpreted in the context of the constrained action hypothesis, an EXT FOA allows the body to automatically control complex, multi-segment movements in the most efficient manner possible (Wulf, Shea, & Park, 2001; Wulf, McNevin, & Shea, 2001). In contrast, an INT FOA promotes conscious control over movement and disrupts the efficient automated processes, thus impairing movement outcomes movement (Wulf & Dufek, 2009). As a result, the positive effects of using an EXT FOA are well documented, with EXT FOA having a larger impact on kinematics, kinetics, and performance outcomes when compared to an INT FOA (An, Wulf, & Kim, 2013; Becker, Fairbrother, & Couvillion, 2020; Chiviacowsky, Wulf, & Ávila, 2013; Porter, Anton, Wikoff, & Ostrowski, 2013; Porter, Wu, Crosley, Knopp, & Campbell, 2015; Vidal, Wu, Nakajima, & Becker, 2018; Zachry, Wulf, Mercer, & Bezodis, 2005).

The standing long jump (SLJ), also known as the standing broad jump, is a test in which individuals stand with their feet parallel and perform a single jump with the goal of covering maximum horizontal distance. The SLJ is commonly used to measure lower body power, explosiveness, and all-around athleticism, and is often included in athlete performance combines for professional sports leagues. As such, there is a large body of literature examining strategies for maximizing SLJ performance (Ashby & Heegaard, 2002; Ducharme, Wu, Lim, Porter, & Geraldo, 2016; Wakai & Linthorne, 2005). Given the SLJ’s universality, simplicity, and replicability, it has also often served as a model for investigating the effects of manipulating FOA. To date, the literature has consistently demonstrated that individuals jump further when adopting an EXT FOA (i.e. focusing on jumping towards a target) compared to when using an INT FOA (i.e. focusing on extending one’s knees or other body part) (Becker & Smith, 2015; Ducharme et al., 2016; Makaruk, Starzak, & Porter, 2020; Porter et al., 2012; Porter et al., 2013; Vidal et al., 2018; Wu, Porter, & Brown, 2012).

While the SLJ literature clearly demonstrates the benefits of using an EXT FOA, the mechanistic basis behind this benefit remains unclear for several reasons. When distilled to its simplest form, the flight phase of the SLJ represents an example of parabolic motion, where the displacement is determined by the center of mass height at takeoff, resultant velocity, and projection angle (Wakai & Linthorne, 2005). These in turn are largely determined by the ground reaction forces and impulses generated during the takeoff. Two studies by Wu and colleagues reported no differences in peak forces or impulses when jumping with INT and EXT FOAs (Ducharme et al., 2016; Wu et al., 2012). However, these studies only measured the vertical forces. It may be that for an activity such as the SLJ, where the goal is horizontal displacement, differences would show in the horizontal forces, not the vertical forces. Indeed, Ducharme et al. reported that while there were no differences in vertical forces between INT and EXT FOA conditions, when using an EXT FOA individuals adopted a lower projection angle (Ducharme et al., 2016). A smaller projection angle, with similar vertical forces implies a larger horizontal force, however this has not been evaluated experimentally, and requires studies using multi-axis force plates.

There are also unanswered questions regarding how whether the benefits of using an FOA remain when movements must be performed in specific postures. Previous studies have shown that using an INT FOA results in worse movement outcomes, even if the body part being focused on is not involved in the production of the movement. For example, using a vertical jump paradigm, Wulf and colleagues (Wulf & Dufek, 2009; Wulf, Dufek, Lozano, & Pettigrew, 2010; Wulf et al., 2007) have shown that when participants are provided and INT FOA cue asking them to focus on the position of their index finger they produce smaller ground reaction forces and lower extremity joint moments and powers than if they are provided an EXT FOA cue asking them to focus on reaching as high as possible. An extension of this conscious focus on a body part not involved in the movement would be having to maintain a given posture during the movement, as is common in many athletic endeavors. Whether the benefits of using an EXT FOA would still be realized even with the requirements of using a fixed posture is unclear.

Given these gaps in the literature the purpose of this study was to evaluate differences in the components of SLJ distance and ground reaction forces when participants performed SLJ trials under INT and EXT FOA conditions. Secondly, this study also evaluated whether the benefits of an EXT FOA are still realized when participants are required to perform jumps while maintaining specific body postures. It was hypothesized that compared to an INT FOA, an EXT FOA would yield greater takeoff, flight, and landing distances, larger horizontal forces and impulses, and lower projection angles. Further, it was hypothesized that these benefits would be present when using an EXT FOA even if body posture was constrained by requiring participants to keep their hands on their hips during the jumps.

METHODS

Experimental Approach to the Problem

A within-participant experimental design was used to observe SLJ differences when performed under internal and external focus of attention conditions, with and without controlled arm swing. Participants performed blocks of three SLJ trials under two experimental conditions: internal (INT) and external (EXT) focus of attention, and while being allowed to swing arms freely (A) or while keeping their hands on their hips (NA). Prior to either experimental condition participants performed baseline (BASE) trials where they received no instruction under both A and NA conditions. This resulted in participants performing a total of 18 SLJ trials under 6 conditions (BASE-A, BASE-NA, INT-A, INT-NA, EXT-A, and EXT-NA). The order of the BASE-A and BASE-NA conditions were randomized across participants, as were the focus of attention and arm swing experimental conditions. Whole body kinematics were recorded using a 10-camera motion capture system (Motion Analysis Corp., Rohnert Park, CA) sampling at 200 Hz. All SLJs were performed with each foot on a strain gauge force plate (AMTI, Watertown, MA) sampling at 1000 Hz.

Participants

An a priori power analysis suggested a minimum of 15 participants would be required to sufficiently power this study to detect large effects (α = 0.5, β = 0.8, Cohen’s f = 0.45). The ability to detect such effects was deemed reasonable as previous studies using the same cues as the current study to compare SLJ jump distance under BASE, INT, and EXT FoA conditions have reported Cohen’s f values of 0.886 and 0.939 (Ducharme et al., 2016; Wu et al., 2012). Therefore, we recruited twenty individuals to participate in this study. Participants were all college aged (sex: 10 males, 10 females; age: 20.3 ± 1.31 years; height: 1.74 ± 9.7 cm; mass: 73.0 ± 8.2 kg) and were recruited via word of mouth. Participants were eligible to participate if they met weekly physical activity guidelines of at least 80 minutes per week of moderate intensity physical activity, had not sustained a physical activity related injury in the three months prior to participating in the study, had no previous instruction or training in how to perform the SLJ, and, based on self-reports when arriving at laboratory, had never participated in a focus of attention study before. Prior to enrollment, all participants provided written informed consent, and all study procedures were approved by the university’s Institutional Review Board. Participants were required to wear their own athletic clothing and shoes and were not provided any information about the hypothesis of the experiment prior to data collection.

Procedures

A total of 54 reflective markers were placed on bony landmarks in order to create a 13-segment biomechanical model. Markers for defining anatomic coordinate systems were placed bilaterally on the acromial clavicular joints, medial and lateral epicondyles of the humerus, radial and ulnar styloid processes, anterior superior and posterior superior iliac spines, lateral and medial femoral epicondyles and malleoli, posterior calcaneus, and head of second metatarsal. Additional tracking markers were placed on the head secured by an elastic headband, the 7^th cervical spinous process, manubrium, iliac crests, and base of first and fifth metatarsals. Lastly, tracking clusters of noncollinear markers were secured bilaterally on the lateral aspects of both thighs and shanks using elastic straps.

Following marker placement, participants completed a standardized warm-up consisting of walking on a treadmill for five minutes followed by a dynamic stretching routine consisting of four exercises which specifically targeted the quadriceps, hamstrings, glutes, and calves. Participants then performed SLJ under baseline and experimental conditions. Participants were provided verbal instructions for each condition as shown in Figure 1. Prior to the first recorded trial participants were shown a single demonstration of how to perform the standing long jump. Verbal instructions on how to perform the movement accompanied the demonstration. Participants were then allowed to perform up to three practice trials. The same researcher, who was highly skilled at performing SLJ, provided all demonstrations and instructions. Prior to the experimental conditions, participants were asked if they understood the cue and any questions were clarified. Three trials of each condition were performed, with three-minutes of active rest between trials. The order of both arm swing/no arm swing, and FOA conditions was counterbalanced across participants.

Figure 1.

Illustration of the experimental design including the instructions participants received in each experimental condition.

Two 10-cm-tall orange cones were placed in directly in front of the participant at a distance of two times the participants’ height (i.e. a subject had a height of 1.50 m, the cones were placed 3 m from the start line). To maintain the same visual information, the orange cones were present for all six conditions. However, they were not mentioned or emphasized in the INT or BASE conditions. The laboratory surface were jumps were performed was black rubber athletic flooring and thus, apart from the cones, contained no other visual cues for jump distance. Immediately following each jump, participants were asked to rate their effort to adopt the desired FOA using a10-point numerical rating scale ranging from 0 (completely uninvolved, not trying hard at all) to 10 (extremely involved, trying as hard as possible) (Becker & Smith, 2015). These methods for controlling visual field and confirming desired FOA were created utilizing protocols reported previous studies examining FOA effects on SLJ performance (Mcnevin et al., 2003; Porter et al., 2013).

Data Analysis

Raw marker trajectories were gap filled using a cubic spine and smoothed with 2^nd order, low pass, zero-lag Butterworth filters at a 6 Hz cutoff frequency using Cortex (v 7.0, Motion Analysis Corp, Santa Rosa, CA). Smoothed marker trajectories and ground reaction forces (GRFs) were then exported to Visual 3D (C-Motion, Inc., Rockville, MD). GRF data was then smoothed using low pass zero-lag Butterworth filters with a 50 Hz cutoff frequency. Takeoff was identified as the point where the combined left and right vertical GRF dropped below a 50 N threshold. Landing was identified using the largest positive peak in the vertical acceleration of the posterior heel marker following the takeoff. Total SLJ jump distance was calculated as the sum of three components: take-off distance, flight distance, and landing distance, as described by Wakai and Linthorne (Wakai & Linthorne, 2005) and shown in Figure 2.

Figure 2.

Illustration of the three phases of the SLJ analyzed in the current study.

Horizontal, vertical, and resultant takeoff velocities were calculated using impulse momentum relationships and the respective GRF components (Wakai & Linthorne, 2005). Peak horizontal and vertical GRFs during the takeoff phase of the jump were also identified and horizontal and vertical impulses were calculated by integrating the force-time curves. Peak forces and impulses were normalized by participant body weight. Projection angle was calculated as the angle of the whole-body center of mass velocity relative to horizontal at the instant of takeoff.

Statistical Analysis

The numeric ratings for degree to which participants maintained desired FOA were averaged within across trials within condition and differences between EXT and INT efforts were evaluated using a Wilcoxon signed-rank test. Values for kinematic and force variables from the three trials in each condition were also averaged and differences between conditions were evaluated using 2 x 3 (arm swing x focus of attention) repeated measures ANOVAs. Participant reported NRS scores were analyzed using an independent samples t-test. An omnibus alpha level of < .05 was used to indicate statistical significance and pairwise comparisons were conducted using a Bonferroni correction. Effect sizes (d, standardized difference in means) were calculated to aid in interpretation of results, with small, medium, and large effects being interpreted as < 0.2, 0.6, and >1.2, respectively (Hopkins, 2006). All statistical analyses were performed using Statistical Package for the Social Sciences v. 26 (IBM SPSS Statistics Inc., Chicago, IL, USA).

RESULTS

Using Desired Focus of Attention

A Wilcoxon signed-rank test showed that mean ratings for the degree to which participants were able to maintain the desired FOA was slightly greater for the EXT (8.9 ± 1.21) than INT (8.58 ± 1.26) FOA condition (Z = −2.47, p = .013).

Jump Distances

For overall jump distance there was a statistically significant main effect of FOA (F_2,38 = 9.40, p < .001, η² = 0.33), with jumps being performed with an EXT FOA being further than either BASE (p < .001, d = 0.26) or jumps performed with an INT FOA (p = .003, d = 0.23). There was also a main effect of arm swing (F_1,19 = 127.61, p < .001, η² = 0.87), with jumps being, on average, 18% further when arm swing was allowed, regardless of FOA condition (p < .001, d = 0.87).

Mean values for the three sub-components of jump distance are shown in Figure 3. There were main effects of FOA for the takeoff (F_2,38 = 9.95, p < .001, η² = 0.34) and landing phases (F_2,38 = 5.58, p < .007, η² = 0.23), but not for the flight phase (F_2,38 = 3.016, p =.061, η² = 0.14). Takeoff distance was greater when jumps were performed using an EXT FOA than in either BASE (p = .001, d = 0.56) or INT (p = .002, d = 0.39) conditions. However, landing distance was only greater in EXT condition than the INT (p = .013, d =0.33). In contrast, there were main effects of arm swing for the takeoff (F_1,19 = 93.76, p < .001, η² = 0.83), flight (F_1,19 = 82.91, p < .001, η² = 0.81), and landing (F_1,19 = 51.62, p < .001, η² = 0.71) distances, with the distances on jumps utilizing arm swing always being greater than on those without an arm swing (takeoff distance: p < .001, d = 0.93; flight distance: p < .001, d = 0.73; landing distance: p <.001, d = 1.49; Figure 3).

Figure 3.

Mean takeoff, flight, and landing distances for the three focus of attention conditions (A), and the two arm swing conditions (B). * indicated significantly different than internal focus of attention condition, # indicates significantly different than baseline, and % indicates significantly different than the no arms condition, all at the p < .05 level.

Ground Reaction Forces

For peak horizontal GRFs there were significant main effects for both FOA (F_2,38 = 14.47, p < .001, η² = 0.43) and arm swing (F_1,19 = 64.55, p < .001, η² = 0.77). Peak horizontal forces were greater when using an EXT FOA than in either BASE (p = .002, d = 0.31) or INT (p < .001, d = 0.39) conditions (Table 1). Peak horizontal GRFs were also greater for all jumps with arms than without (p < .001, d = 1.07). Horizontal impulses showed a similar pattern, having significant main effects for both FOA (F_2,38 = 13.98, p < .001, η² = 0.42) and arm swing (F_1,19 = 108.27, p < .001, η² = 0.85). Horizontal impulses were greater under EXT FOA conditions than either BASE (p < .001, d = 0.37) or INT FOA (p < .001, d = 0.34) conditions. Horizontal impulses were also greater when using arms than without arms swing (p < .001, d = 1.01).

Table 1.

Mean (± standard deviation) values for ground reaction forces, impulses, take off velocities, and projection angles.

	With arm swing			No arm swing			p values
Variable	Base	External	Internal	Base	External	Internal	FOA	Arms	Int.
Peak horizontal GRF (BW)	0.92 (± 0.14)b	0.97 (± 0.16)a,b	0.91 (± 0.14)b	0.77 (± 0.12)	0.81 (± 0.13)a	0.76 (± 0.12)	<.001	<.001	.668
Horizontal impulse (BW*s)	0.26 (± 0.04)b	0.28 (± 0.04)a,b	0.26 (± 0.04)b	0.23 (± 0.03)	0.24 (± 0.03)a	0.23 (± 0.03)	<.001	<.001	.919
Peak vertical GRF (BW)	2.18 (± 0.27)a,b	2.11 (± 0.25)b	2.07 (± 0.19)b	2.09 (± 0.19)a	2.04 (± 0.20)	1.98 (± 0.17)	.003	.005	.854
Vertical impulse (BW*s)	0.19 (± 0.06)b	0.17 (± 0.05)b	0.18 (± 0.05)b	0.17 (± 0.04)	0.17 (± 0.06)	0.17 (± 0.06)	.049	.009	.080
Vertical takeoff velocity (m/s)	1.73 (± 0.42)b	1.61 (± 0.42)a,b	1.71 (± 0.39)b	1.64 (± 0.41	1.59 (± 0.41)a	1.65 (± 0.46)	.022	.043	.440
Horizontal takeoff velocity (m/s)	2.62 (± 0.41)b	2.76 (± 0.42)a,b	2.61 (± 0.39)b	2.25 (± 0.32)	2.37 (± 0.31)a	2.26 (± 0.33)	<.001	<.001	.778
Projection angle (°)	33.35 (± 6.18)b	29.82 (± 5.87)a,b	33.17 (± 6.08)b	35.88 (± 7.53)	33.58 (± 6.71)a	34.77 (± 7.71)	.002	<.001	.352

^aindicates significantly different than other FOA conditions.

^bindicated significantly different with arm swing than without arm swing.

Peak vertical GRFs demonstrated significant main effects for both FOA (F_2,38 = 6.70, p = .003, η² = 0.261) and arm swing (F_1,19 = 10.18, p = .005, η² = 0.349). Vertical GRFs were greater in the BASE condition than either EXT (p = .002, d = 0.26) or INT (p = .023, d = 0.52) conditions (Table 1). Vertical GRFs were greater on jumps using arm swing than jumps without arm swing (p =.005, d = 0.38). Vertical impulses demonstrated a significant main effect for FOA (F_2,38 = 3.28, p = .049, η² = 0.147). However, when controlling for pair-wise comparisons, there were no statistically significant differences among conditions. Vertical impulses also displayed main effects of arm swing (F_1,19 = 8.56, p = .009, η² = 0.311), where jumps with arm swing resulted in larger impulses than jumps without arm swing (p = .009, d = 0.14).

Takeoff Velocities and Projection Angle

Vertical velocity at takeoff displayed significant main effects of both FOA (F_2,38 = 4.23, p = .022, η² = 0.182) and arm swing (F_1,19 = 4.72, p = .043, η² = 0.199). Jumps with an EXT FOA had lower vertical velocities at takeoff than jumps with either INT FOA (p = .025, d = 0.19) or BASE (p = .012, d = 0.21) conditions (Table 1). Jumps with arm swing had greater vertical velocities at takeoff than jumps without arm swing (p = .043, d = 0.13). Horizontal velocities at takeoff also displayed significant main effects both FOA (F_2,38 = 12.31, p < .001, η² = 0.393) and arm swing (F_1,19 = 157.88, p < .001, η² = 0.893). Horizontal velocities were greater when using an EXT FOA than in either BASE (p < .001, d = 0.36) or INT FOA (p = .001, d = 0.35), and were greater when using arms then without arm swing (p < .001, d = 1.01, Table 1). Lastly, projection angles also displayed significant main effects of both FOA (F_2,38 = 7.16, p = .002, η² = 0.274) and arm swing (F_1,19 = 19.11, p < .001, η² = 0.502). Projection angles were lower when using an EXT FOA compared to both BASE (p = .001, d = 0.44) and INT FOA (p = .046, d = 0.34) conditions. Projection angles were also lower on jumps which used arm swing than jumps which did not use arm swing (p < .001, d = 0.39, Table 1).

DISCUSSION

The purposes of this study were to evaluate how manipulating FOA influences the component distances of a SLJ and GRF production during the jump, and to determine whether the benefits of using an EXT FOA are still realized when movements have to be performed in constrained body postures. In support of our hypotheses, the main findings were that using an EXT FOA resulted in greater jump distance then when using an INT FOA or under baseline conditions. The improved jump distance was due to increased takeoff and landing distances, with no changes in the flight distance. Secondly, using an EXT FOA resulted in greater horizontal peak forces and impulses than using an INT FOA or BASE conditions. Finally, the results showed that the benefits of using an EXT FOA were still realized even when participants were required to keep their hands on their hips.

Results of the current study are consistent with multiple previous studies which have documented that SLJ distance is improved when using an EXT FOA compared to an INT FOA (Becker et al., 2020; Ducharme et al., 2016; Porter et al., 2012; Porter et al., 2013; Vidal et al., 2018; Wu et al., 2012). However, this is the first study to show that the improved total jump distance with an EXT FOA comes primarily from improvements in takeoff and landing distances, with no changes in flight distance. Greater takeoff distances indicate participants displaced their center of mass further horizontally relative to their toes before their foot leaves the ground. This resulted in their body leaning more anteriorly at takeoff and explains the lower projection angles with an EXT FOA. However, even though projection angles in the current study were in the ranges of those reported in previous SLJ studies (Aguado, Izquierdo, & Montesinos, 1997; Ducharme et al., 2016; Wakai & Linthorne, 2005), they were still greater than theoretically optimal takeoff angles reported by Wakai and Linthorne (Wakai & Linthorne, 2005). This suggests participants could further improve their SLJ performance by increasing horizontal displacement of the center of mass relative to the toes.

The landing distance was also greater when using an EXT FOA than an INT FOA. One potential explanation is that the cue used for the EXT FOA prompted participants to reach out for the cones or otherwise change their landing strategy while the cue used for the INT FOA condition did not elicit this response. The cones were in place for both the EXT and INT FOA conditions, so the visual information provided to participants was similar in both conditions. Porter and colleagues previously reported that using a short EXT FOA increased SLJ distance compared to a control condition, and that increasing the distance of an EXT FOA further increased SLJ distance (Porter et al., 2012). However, in that study they did not analyze takeoff, flight, and landing distances separately. It may be that, like the current study, the short EXT FOA results in improvements in takeoff distances whereas the far EXT FOA cue has greater influence on participant landing strategies.

The constrained action hypothesis postulates that an EXT FOA allows the body to automatically control complex, coordinated movements in the most efficient manner possible (Wulf, 2013). It has previously been shown that an EXT FOA results in better movement outcomes than an INT FOA even if the part of the body being focused on is not involved in the production of the movement (Vance, Wulf, Töllner, McNevin, & Mercer, 2004; Wulf & Dufek, 2009; Zachry et al., 2005). The results of the current study suggest that an automatic, unconscious control promoted by an EXT FOA overcomes the conscious requirement to maintain a specific posture or set of coordinated movements. The results of the takeoff and landing distances demonstrate the automatic, unconscious control within the two jump segments. The EXT condition did not provide any explicit information on mechanics of the take-off or landing. Regardless of the lack of explicit movement related information, participants in the EXT condition were able to self-organize their movement strategy to best fit the goal of the action and maximize their jump performance. The results of the takeoff distance, landing distance, and composition of the EXT cue provide evidence in support of the constrained action hypothesis. Specifically, participants in the EXT condition demonstrated kinematic features of the movement that were advantageous for both the takeoff and landing components of the jump without being aware of the mechanics of the action. This is particularly relevant because participant in the EXT condition exhibited biomechanical advantages in the only two segments of the jump distance that an individual can physically alter: the takeoff and the landing.

Consistent with previous SLJ research (Ashby & Delp, 2006; Ashby & Heegaard, 2002), the current study found that when participants were allowed to swing their arms all aspects of SLJ performance improved, regardless of FOA condition. However, the lack of any arm swing by FOA condition interactions suggest the enhanced performance effects of the EXT condition carryover when compared to the INT condition. This is likely because the constraining the posture by changing the task characteristics did not impact how participants organized the movement planning of the takeoff and landing. Lohse and colleagues have reported results from several FOA studies using dart throwing as a model, with throws performed in relatively fixed postures and planes of movement (Lohse, 2012; Lohse, Sherwood, & Healy, 2010, 2014). Similar to results of the current study, in all cases their results show better performance was achieved with an EXT FOA. However, one cannot differentiate the influence of the fixed posture as the same limited plane instructions were used for conditions. In contrast, in the current study, SLJ were performed under EXT and INT FOA in both fixed and free postures. This manipulation provides additional biomechanical support for the constrained action hypothesis as movement kinematics of the takeoff and landing were still optimized in the EXT condition, when compared to the INT condition, even though body posture was restricted. Moreover, the EXT condition provided no additional information about how to optimize the kinematics of the jump. Instead, participants in the EXT condition oriented their COM at a greater distance in front of their feet for the takeoff so that the application of force was oriented in a more horizontal direction than the INT condition. When viewing the landing phase, the EXT condition fostered COM placement at a greater distance than the INT condition behind the feet to optimize landing mechanics. These are all examples of how an EXT FOA attention allows the system to automatically control the movements of the action in the most efficient manner given the task constraints of the study and individual constraints of the participants.

There is currently discrepancy in the literature regarding the influence of FOA manipulation on ground reaction force production. Two studies using a SLJ paradigm reported that manipulating FOA did not influence vertical ground reaction forces (Ducharme et al., 2016; Wu et al., 2012). In contrast, several other studies have reported greater forces with an EXT FOA in vertical jump tasks (Wulf & Dufek, 2009; Wulf et al., 2010, 2007), isometric lifting assessments (Halperin, Williams, Martin, & Chapman, 2016), and impacts while punching in combat sports (Halperin, Chapman, Martin, & Abbiss, 2017). The current supports the hypothesis that manipulating FOA influences force production, as there were increases in horizontal forces and impulses when using an EXT FOA compared to an INT FOA. One possible reason for the discrepancy in the current study and previous SLJ studies is the equipment used. Ducharme et al. (Ducharme et al., 2016) and Wu et al. (Wu et al., 2012) both used a single axis force plate which only measured vertical ground reaction forces, and at relatively low sampling rates. However, while the individual ground reaction force components differed, the peak resultant force was not different among FOA conditions. Thus, rather than increasing or decreasing force production, the results of the current study suggest that manipulating FOA helps athletes improve the efficiency of force production in a task specific manner. Efficient application of vertical and horizontal forces is one critical element for sports skills such as sprinting and the changes ground force components may be one reason why studies have observed improved sprint times when athletes receive EXT FOA cues (Porter et al., 2015; Winkelman, Clark, & Ryan, 2017).

The results of this study also have implications for future research using the SLJ as a model, as to date, FOA studies using the SLJ have only evaluated total jump distance, not the individual components. However, FOA cues often target specific areas of the jump. For example, many SLJ FOA studies have used cues which start with “while jumping…”. Such cues inherently influence the movement planning and execution of the takeoff phase of the jump. Thus, one might expect improvements in this phase, but not necessarily flight or landing phases. Conversely, other studies have promoted greater emphasis on the landing phase by manipulating the distance of the external focus (Porter et al., 2012). In such situations one might expect improvements in the landing distance, but not in the takeoff or flight distances. Such a result might influence the interpretation of how the FOA cues influence movement planning and execution in the takeoff. Thus, we suggest that the relationship between instructions provided and performance in different phases of the jump is an important area of future research.

There are a few limitations which must be considered when interpreting the results of this study. First, the numeric rating scale indicated that participants found it slightly more difficult to focus on the INT FOA cue than the EXT FOA cue. While this was not a large difference, this does provide evidence for the ease of focusing on the EXT cue over the INT cue. However, we cannot rule out the possibility that this difference may have influenced the results. Future studies might alleviate this issue by using alternative methods for assessing individuals’ adherence to the cues or providing feedback to participants regarding their use of the cue. Secondly, the cue used for our no arms condition asked participants to “keep their hands on their hips”. By referring to the body parts this may have unintentionally hybridized the cue, making it more internal in nature than intended. We do not believe this impacted the results as the second part of the cues, where participants were instructed to focus “while jumping” did contain only externally or internally directed cues. However, we cannot fully rule this out as currently, there is no method in the literature for rating the extent to which different cues promote an internal verse external focus of attention. Secondly, while all the participants in this study were physically active, none had prior experience performing the standing long jump. The literature suggests that individuals experienced with a motor task might respond to FOA cues different than novices (Wulf, 2013). Thus, we cannot be certain our results would also be observed if experienced jumpers were used. Thirdly, this study was conducted with participants taking off from force plates and landing on rubber flooring. This surface might have resulted in less than maximal performance if participants had a fear of falling during the landing. Using a more compliant landing surface (i.e. a sand pit) might better elucidate the differences in landing distances and strategies. Lastly, while we observed differences in ground reaction forces, take off velocities, and projection angles, the current study is not able to determine what caused these from a mechanistic perspective. Future work requiring integrated measurements of electromyography (EMG), dynamic electroencephalography (EEG), and personalized musculoskeletal modeling simulations are required to fully identify the neurophysiologic and neuromechanical factors responsible for improvements with an EXT FOA.

In summary, this study showed the benefits of using an EXT FOA in the SLJ are primarily due to improvements in the takeoff and landing distances, and that these benefits are realized even when the available postures for one can adopt for completing a movement are reduced. These results provide support for the constrained action hypothesis as they show that with EXT FOA cues participants were able to optimize their movement strategies to produce better jump distances, even when doing so with reduced postures available. From an applied perspective, our results provide coaches, clinicians, or other practioners information on how to optimize SLJ performance, and suggest they should consider using EXT FOA cues for improving movement even if they are working with individuals who are required to perform movements with fixed postures.