Effects of attentional focus strategy on drop jump for athletes with different experience levels: Performance,kinetics and kinematics

Abstract

The attentional focus strategy (AFS) affects drop jump (DJ) performance; however, its effects on DJ kinetics and kinematics are unclear. This study examines the effects of AFS on DJ performance, kinetics and kinematics. Forty collegiate males were divided into a DJ experienced group and a DJ inexperienced group, and they performed two trials of DJs after listening to instructions designed to elicit internal (INT), external (EXT) or neutral (NEUT) focus of attention. In the experienced group, the reactive strength index (RSI) was larger with the NEUT than with the INT. Hip joint torque and positive hip joint power in the concentric phase was smaller with the NEUT than with the INT, and the hip joint angle at the lowest centre of mass was larger with the NEUT than with the INT (p < .05). In the inexperienced group, RSI was larger with the EXT than with the INT. Further, the hip joint torque in the concentric phase was smaller with the EXT than with the INT; positive ankle joint power was larger with the EXT than with the INT; and the hip joint angles at the lowest centre of mass and take-off were larger with the EXT than with the INT (p < .05). These results showed that the AFS affected the DJ performance, kinetics and kinematics and suggested that effective DJ training may be achieved with the NEUT for DJ experienced and with the EXT for DJ inexperienced.

Keywords

Joint torque motion analysis plyometrics relative strength index stretch-shortening cycle

Introduction

The attentional focus strategy (AFS) is changing the direction of an athlete's attention during exercise, and AFS can promote effective various sports performance. Attentional focus is defined as the conscious effort of an individual to focus their attention on an effort to execute a motor skill with superior performance.¹ Coaches can manipulate the direction of an athlete's attention through instruction. The AFS can improve athletic performance by eliciting internal (INT), external (EXT) or neutral (NEUT) focus of attention.² The INT directs one's consciousness to specific body parts and their movements, EXT directs one's consciousness to the effects of the exercise environment, and NEUT does not aim to induce either INT or EXT but instead promotes non-awareness.^2–4 Previous studies on AFS have reported improved athletic performance with EXT compared with INT in many exercises, such as darts,⁵ sprinting,⁶ soccer,⁷ agility⁸ and jumping.⁹ Thus, the AFS can significantly affect athletic performance, and it is necessary to use an appropriate AFS for athletes in the training field.

One of the training exercises to improve the ability to execute stretching-shortening-cycle (SSC) movements of the lower limb during short contact times (CT),¹⁰ which is important for performance of exercise that includes running and jumping, is the drop jump (DJ).¹¹ The reactive strength index (RSI), which measures DJ performance, correlates with sprinting ability,¹² agility,¹³ and jumping ability.¹⁴ Therefore, improving the DJ performance of athletes is important to improve athletic performance. Viitasalo et al.¹⁵ compared the DJ performance, force exertion and movement between DJ experienced athletes and DJ inexperienced athletes. The DJ experienced athletes achieved higher jump height (JH), shorter CT, higher maximum vertical ground reaction force (vGRF) and smaller angular displacement of the ankle and knee joints than the DJ inexperienced athletes. Furthermore, Bobbert et al.¹¹ suggested that DJ techniques, such as ankle and knee joint force exertion, contribute to an increased training load in DJ training. Thus, it is important to consider the kinetic and kinematics data of the three lower extremity joints in jumping exercises to evaluate the SSC ability of the athlete's lower extremity in detail.¹⁰ In conclusion, DJ training is effective in enhancing athletic performance. Force exertion and movement should be considered in addition to performance when assessing DJ performance at each level of DJ experience.

Previous studies have reported that AFS effectively improves DJ performance.^3,16–19 Furthermore, using the EXT improves DJ performance variables, such as RSI, JH, CT, vGRF and leg stiffness (k_vert).^3,16,18,19 The constrained action hypothesis is postulated as a plausible explanation for the performance enhancing effects associated with EXT.²⁰ The hypothesis is that using INT inhibits the automatic motor processes that regulate movement, whereas EXT promotes automatic control processes, thus increasing the efficiency of movement.²¹ Thus, EXT should be used to enhance the DJ performance. However, Furuhashi et al.¹⁷ studied the effects of the AFS on DJs between DJs who were experienced athletes and inexperienced athletes. The results revealed that DJ experienced athletes had higher RSI with the NEUT and DJ inexperienced athletes had higher RSI with the EXT, suggesting that it is necessary to use different AFS depending on the DJ experience level of the athletes. Furthermore, Perkins-Ceccato et al.²² found that skilled golfers improved their golf performance with EXT and inexperienced golfers with INT, suggesting that the AFS effects depend on an individual's skill. Thus, as reflected in several previous studies, the AFS effect on sports performance depends on individual skills and should be investigated in both DJ experienced athletes and DJ inexperienced athletes. However, most studies investigating the AFS and DJ have focused primarily on the AFS effects on DJ performance; their effects on force exertion and movement during DJ have not been clarified to date. Therefore, the findings obtained to date are insufficient for adopting the AFS for DJ in the training field. Bezodis et al.²³ reported that in 10 m sprints, experiments were conducted under three conditions (control, INT and EXT) with different instructions. The results showed that skilled sprinters had faster sprint times in the control condition than EXT. They also found significant differences in force exertion and joint angles between conditions. Thus, AFS affects exercise force exertion and movement, and skilled athletes were shown to obtain higher athletic performance with NEUT (or control condition). As force exertion and movement are significantly related to high performance in the DJ,^11,15 AFS may affect not only DJ performance but also force exertion and movement. Additionally, the results of a previous studies^17,23 predict that the effects of the AFS on DJ may differ between DJ experienced athletes and DJ inexperienced athletes.

Therefore, this study aims to clarify the effects of AFS on DJ performance, force exertion and movement according to DJ experience level. Thus, coaches can adopt optimal AFS in DJ for athletes with various DJ experience levels, not only to enhance DJ performance but also to consider force exertion and movements. The AFS effects on DJ performance depends on individual DJ skills¹⁷ and it affects force exertion and movement in sport.²³ Therefore, we hypothesised that DJ experienced athletes can be improved with NEUT and DJ inexperienced athletes with EXT using rational force exertion and movement.

Methods

Research design

This study used a within-subject experimental design to investigate the effects of the INT, EXT and NEUT on DJ performance, force exertion and movement. Participants participated in a single experimental session and received verbal instructions that prompted one of the INT, EXT or NEUT, followed by two trials of DJ from a 0.3-m high box. Overall, six trials were conducted, and the order of the verbal instructions was randomised to eliminate the possibility of order effects. The dependent variables were the RSI, JH, CT, peak vGRF, k_vert, joint torque, joint power and joint angle.

Participants

Forty male collegiate participants were divided into the DJ experienced group (20 sprinters, Age: 21.6 ± 1.5 years, Height: 1.76 ± 0.03 m, Mass: 69.0 ± 5.1 kg) and the DJ inexperienced group (9 Kendo players, 5 Soccer players, 1 Rugby player, 1 Basketball player, 1 Baseball player and 3 non-players). An a priori power analysis²⁴ with an assumed type 1 error of 0.05 and a type 2 error rate of 0.20 (statistical power: 80%, effect size: 0.7) was conducted for RSI of DJ. The results showed that 20 participants in each group would be sufficient to find the statistically significant main effects of ‘focus of attention’. The exclusion criteria were as follows: the use of medications affecting exercise capacity or orthopaedic limitations. The experienced group was familiar with the DJ technique and regularly performed the DJ. Both the DJ experienced, and the DJ inexperienced groups practiced professionally for approximately 5 days a week on average. However, the inexperienced group was not familiar with the DJ technique and had intermediate experience with DJ. Informed consent was obtained from all the participants. The consent form and all experimental methods were approved by the Research Ethics Committee of the University of Tsukuba before the initiation of this study (tai 022-26).

Procedures

The participants were instructed to avoid resistance training the day before the experiment to prevent fatigue from affecting the experiment. Before the experiment, all participants underwent a 15-min warm-up, including jogging and dynamic stretching, followed by five DJs using a 0.3-m box.²⁵ The practice of the DJ in advance was to minimise the change in performance due to the learning effect that occurs during measurement and suppress variations in jumping techniques. All participants rested for 2 min after the warm-up. The participants were instructed to stand on the box, and the examiner read the DJ instructions for INT, EXT and NEUT. The verbal instructions were developed based on previous studies³ and are shown in Table 1. The verbal instructions for each condition were read before each trial. On the box, the posture of the pre-set phase was standardised for each trial by marking the eye height of each participant on the wall. All participants uniformly performed the test barefoot. The participants performed two DJs under each condition (INT, EXT and NEUT). If the participant explicitly failed the DJ, for example, when the participant landed off the force plate, the trial was repeated. The participants were not provided with any feedback on their performance. Participants were given a 1-min rest between trials under the same conditions and a 2-min rest between each condition.³ The rest times were used to perform the manipulation checks described below.

Table 1.

Drop jump verbal instructions designed to invoke internal, external and neutral focus of attention.

Conditions	Verbal instructions
INT	‘Step off the box, land on the balls of your feet, and after landing fully and explosively extend your ankles, knees, and hips as rapidly as possible to jump high’
EXT	‘Step off the box, jump fast, imagine the ground is a hot surface, get off the ground as quickly as possible, imagine you are like a stiff spring, and focus on jumping to the roof’
NEUT	‘Perform the jump to the best of your ability’

INT = internal focus of attention; EXT = external focus of attention; NEUT = neutral focus of attention.

Manipulation check

A questionnaire was used to obtain subjective ratings for manipulation checks. After performing the DJ in each trial, the participants were assessed for their understanding of the instructions and self-assessment of performance in that trial. A 7-point Likert scale and 5-point Likert scale was used for evaluation (7-point: 1 = did not understand the instructions at all, 4 = neutral, and 7 = understand the instructions completely; 5-point: 1 = poor, 3 = good, and 5 = excellent).^26,27 The participants were also requested to freely write on the questionnaire what they actually focused on when they performed DJ.

Data collection and measured variables

The three-dimensional (3D) coordinates of 47 retroreflective markers fixed on landmarks on the participant's body were collected using Vicon MX + cameras (Vicon Motion System, Ltd, Oxford, UK) with 10 cameras operating at 250 Hz. The GRF was measured using two force platforms (Kistler 9287C, 0.9 × 0.6 m, Kistler Instrumente AG, Winterthur, Switzerland) at 1000 Hz. We temporally synchronised the obtained 3D coordinates for each body part and GRF data using the Vicon Nexus software (Vicon Motion System, Ltd, Oxford, UK). The x-, y- and z-axes of the global coordination system were defined as the anterior–posterior, medial–lateral and vertical directions, respectively. The 3D coordinate data were smoothed with a fourth-order Butterworth low-pass digital filter without a phase shift at 20 Hz, based on Wells and Winter.²⁸

The JH, CT, RSI, relative peak vGRF and k_vert values were used to evaluate the DJ performance. Flight time, CT, peak vGRF and centre of mass values were obtained directly from the force platform data. Using the obtained flight time, the JH was calculated using the following formula:

JH = (9.81 × flight time²) / 8.²⁹

The RSI for each DJ trial was calculated by dividing the JH in meters by the CT in seconds.³⁰ The mass-spring model was used to calculate the k_vert, which is defined as the ratio of the peak vGRF to the spring displacement when the leg spring contracts to its maximum.³¹ The peak vGRF was divided by the mass of the participant to derive the relative peak vGRF.

The respective centres at the ankle and knee joints were the midpoints of the markers placed on the medial and lateral malleolus and lateral and medial femoral epicondyles. The hip joint centre was calculated using a method reported by Kariyama et al.³² The centre of mass and the inertial parameters were estimated based on the method described by Dumas et al.^33,34 Kinetics and kinematics variables were calculated from the 3D coordinate and GRF data. The mean joint torque and power of the three lower limb joints were employed as measures of force exertion. The joint torque was calculated from the inverse dynamics analysis, and torque power by multiplying the joint torque by the joint angular velocity.³⁵ The joint angles of the three lower limb joints at the initial contact, minimum height of the centre of mass and take-off were employed as measures of movement. The participant's step leg, which the participant was more likely to use for a single-leg jump, was used for the analysis. The kinetics data defined extension and flexion as positive and negative values, respectively, for the hip, knee and ankle joints. The eccentric phase was defined as the point at which the centre of mass was the minimum height from the point the vGRF was 10 N or higher after the feet touched the ground. The concentric phase was defined as the point at which the centre of mass height was minimum at the point of take-off. The trial with the highest RSI among the two trials was used for the data analysis.

Statistical analysis

All statistical analyses were conducted using IBM SPSS version 28.0 (IBM, New York, NY, USA). Intra-class correlation coefficients (ICC) were calculated to determine the test–retest reliability of the measured variables. An ICC >0.90 was considered high, between 0.80 and 0.90 moderate and <0.80 insufficient for physiological field testing.³⁶ Data were distributed for all conditions, as assessed by the Shapiro–Wilk test (p > .05). One-way analysis of variance (ANOVA) with repeated measures was used to determine the differences between the parametric data. When ANOVA was significant the p value on the F-test was found, and paired comparisons were used in a Bonferroni post hoc analysis to determine any significant differences. Effect sizes using Hedges’ g were obtained for any significant pairwise comparisons and interpreted using the following scale: less than 0.2, trivial; between 0.2 and 0.5, small; between 0.5 and 0.8, medium; between 0.8 and 1.3, large; and greater than 1.3, very large.³⁷ The alpha level was set to 0.05. All data were presented as the mean ± SD.

Results

The ICC, coefficient of variation and 95% confidence intervals between the first and second RSI measurements are shown in Table 2. The first and second trial ICCs for the RSI were high.

Table 2.

Intra-class correlation coefficient (ICC), coefficient of variation (CV%) and 95% confidence interval (CI) for the reactive strength index (RSI) under each condition.

		INT			EXT			NEUT
	ICC	CV%	95% CI	ICC	CV%	95% CI	ICC	CV%	95% CI
E group	0.780*	17.1	2.04–2.49	0.831*	21.2	2.00–2.43	0.875*	18.1	2.10–2.55
I group	0.918*	24.7	1.50–1.89	0.814*	25.8	1.64–2.09	0.831*	23.0	1.59–1.97

E group = experienced group; I group = inexperienced group; INT = internal focus of attention; EXT = external focus of attention; NEUT = neutral focus of attention; *p < .05.

The mean ± SD scores for all DJ performance variables under each condition in both groups are shown in Table 3.

Table 3.

Mean ± SD scores for each DJ performance variable under each condition.

	INT	EXT	NEUT	Differences
E group
RSI (m/s)	2.276 ± 0.389	2.447 ± 0.520	2.457 ± 0.446	NEUT > INT**
CT (s)	0.213 ± 0.038	0.199 ± 0.028	0.199 ± 0.030	n.s.
JH (m)	0.478 ± 0.072	0.478 ± 0.069	0.482 ± 0.065	n.s.
Peak vGRF (N/kg)	52.42 ± 10.45	56.47 ± 12.20	56.90 ± 12.58	n.s.
k_vert (kN/m)	22.65 ± 6.77	27.48 ± 9.75	27.48 ± 8.88	NEUT > INT*
I group
RSI (m/s)	1.697 ± 0.420	1.864 ± 0.482	1.779 ± 0.408	EXT > INT*
CT (s)	0.237 ± 0.056	0.218 ± 0.045	0.228 ± 0.048	INT > EXT*
JH (m)	0.384 ± 0.049	0.390 ± 0.060	0.390 ± 0.043	n.s.
Peak vGRF (N/kg)	45.91 ± 13.34	49.94 ± 12.85	48.60 ± 13.34	EXT > INT*
k_vert (kN/m)	19.75 ± 10.77	22.86 ± 10.32	21.90 ± 12.28	EXT > INT*

E group = experienced group; I group = inexperienced group; RSI = reactive strength index; JH = jump height; CT = contact time; vGRF = vertical ground reaction force; kvert = leg-spring stiffness; INT = internal focus of attention; EXT = external focus of attention; NEUT = neutral focus of attention; n.s., not significant; *p < .05; **p < .01.

In the experienced group, the ANOVA results showed a significant main effect of the condition on RSI (F_2,38 = 5.031, p = .022, partial η² = 0.209). Post hoc test results showed significantly higher values for the NEUT than INT (p = .009, g = 0.42, small). Regarding JH, CT and peak vGRF, there were no significant main effects (JH: F_2,38 = 0.298, p = .744, partial η² = 0.015; CT: F_2,38 = 3.494, p = .062, partial η² = 0.155; peak vGRF: F_2,38 = 2.719, p = .079, partial η² = 0.125). Significant main effects were observed for the k_vert (F_2,38 = 3.742, p = .046, partial η² = 0.165). Post hoc test results showed significantly higher values for the NEUT than INT (p = .032, g = 0.60, medium).

In the inexperienced group, the ANOVA results showed a significant main effect of the condition on RSI (F_2,38 = 4.435, p = .19, partial η² = 0.189). Post hoc test results showed significantly higher values for the EXT than INT (p = .029, g = 0.36, small). For JH, no significant main effects were observed (F_2,38 = 0.546, p = .584, partial η² = 0.028). Regarding the CT, significant main effects were observed (F_2,38 = 4.689, p = .015, partial η² = 0.198). The post hoc test results showed significantly lower values for the EXT than INT (p = .007, g = 0.37, small). Regarding the peak vGRF, significant main effects were observed (F_2,38 = 4.028, p = .026, partial η² = 0.175). The post hoc test results showed significantly higher values for the EXT than for the INT (p = .026, g = 0.30, small). Regarding k_vert, significant main effects were observed (F_2,38 = 3.953, p = .041, partial η² = 0.172). The post hoc test results showed significantly higher values for the EXT than for the INT (p = .007, g = 0.29, small).

The mean ± SD scores of the mean joint torque and mean joint power of the three joints of the lower extremity under each condition in both groups are shown in Table 4.

Table 4.

The mean ± SD scores for the mean joint torque and mean joint power of the three joints of the lower extremities under each condition.

			INT	EXT	NEUT	Differences
E group
Mean joint moment (Nm/kg)	Eccentric	Hip	1.46 ± 0.43	1.42 ± 0.38	1.42 ± 0.40	n.s.
		Knee	1.43 ± 0.46	1.49 ± 0.29	1.43 ± 0.38	n.s.
		Ankle	1.90 ± 0.48	2.01 ± 0.36	1.98 ± 0.43	n.s.

	Concentric	Hip	0.76 ± 0.47	0.53 ± 0.34	0.57 ± 0.30	INT > EXT, NEUT
		Knee	1.44 ± 0.36	1.51 ± 0.29	1.46 ± 0.32	n.s.
		Ankle	2.00 ± 0.33	2.04 ± 0.17	2.00 ± 0.24	n.s.

Mean joint power (W/kg)	Negtive	Hip	−2.94 ± 2.23	−2.41 ± 2.17	−2.62 ± 2.46	n.s.
		Knee	−6.92 ± 2.68	−6.81 ± 1.94	−6.76 ± 2.39	n.s.
		Ankle	−8.79 ± 4.24	−10.45 ± 5.45	−9.68 ± 4.50	n.s.

	Positive	Hip	2.61 ± 1.25	2.17 ± 0.62	2.10 ± 0.71	INT > NEUT*
		Knee	7.35 ± 2.11	7.75 ± 1.68	7.79 ± 2.08	n.s.
		Ankle	11.08 ± 2.87	11.43 ± 2.52	11.56 ± 2.48	n.s.
I group
Mean joint moment (Nm/kg)	Eccentric	Hip	1.02 ± 0.43	0.93 ± 0.41	1.03 ± 0.29	n.s.
		Knee	1.37 ± 0.55	1.34 ± 0.33	1.28 ± 0.33	n.s.
		Ankle	1.48 ± 0.64	1.72 ± 0.56	1.69 ± 0.51	n.s.

	Concentric	Hip	0.66 ± 0.35	0.46 ± 0.26	0.47 ± 0.37	INT > EXT, NEUT
		Knee	1.43 ± 0.49	1.37 ± 0.25	1.43 ± 0.26	n.s.
		Ankle	1.52 ± 0.40	1.70 ± 0.35	1.66 ± 0.26	n.s.

Mean joint power (W/kg)	Negtive	Hip	−2.03 ± 0.92	−1.58 ± 1.08	−1.72 ± 0.91	n.s.
		Knee	−6.13 ± 2.82	−5.84 ± 1.44	−6.18 ± 1.76	n.s.
		Ankle	−7.62 ± 4.85	−9.01 ± 5.47	−8.18 ± 5.24	n.s.

	Positive	Hip	2.12 ± 1.18	1.70 ± 0.80	1.93 ± 0.99	n.s.
		Knee	7.20 ± 4.04	6.90 ± 1.97	6.65 ± 1.26	n.s.
		Ankle	7.38 ± 2.10	8.66 ± 3.01	8.26 ± 2.15	EXT > INT*

E group = experienced group; I group = inexperienced group; INT = internal focus of attention; EXT = external focus of attention; NEUT = neutral focus of attention; n.s. = not significant; *p < .05.

In the experienced group, the ANOVA results showed no significant main effects of the condition on the hip, knee and ankle joint torques during the eccentric phase (Hip: F_2,38 = 0.391, p = .679, partial η² = 0.020; Knee: F_2,38 = 0.363, p = .698, partial η² = 0.019; Ankle: F_2,38 = 0.927, p = .383, partial η² = 0.047). During the concentric phase, there was a significant main effect of the conditions on the hip joint torque (Hip: F_2,38 = 6.339, p = .010, partial η² = 0.250). The post hoc test results showed significantly higher values for the INT than EXT (p = .011, g = 0.56, medium) and NEUT (p = .021, g = 0.47, small). There were no significant main effects on the knee and ankle joint torque (Knee: F_2,38 = 0.987, p = .382, partial η² = 0.049; Ankle: F_2,38 = 211, p = .810, partial η² = 0.011). Regarding the mean joint power, there were no significant main effects of the condition on the hip, knee and ankle joint power during the eccentric phase (Hip: F_2,38 = 0.907, p = .412, partial η² = 0.046; Knee: F_2,38 = 0.061, p = .941, partial η² = 0.003; Ankle: F_2,38 = 1.514, p = .237, partial η² = 0.074). During the concentric phase, there was a significant main effect of the conditions on hip joint power (Hip: F_2,38 = 4.096, p = .039, partial η² = 0.177). The post hoc test results showed significantly higher values for the INT than NEUT (p = .018, g = 0.49, small).

In the inexperienced group, ANOVA results showed no significant main effects of the condition on the hip, knee and ankle joint torque during the eccentric phase (Hip: F_2,38 = 1.513, p = .233, partial η² = 0.074; Knee: F_2,38 = 0.431, p = .583, partial η² = 0.022; Ankle: F_2,38 = 2.492, p = .117, partial η² = 0.116). During the concentric phase, there was a significant main effect of the conditions on the hip joint torque (Hip: F_2,38 = 5.106, p = .011, partial η² = 0.212). The post hoc test results showed significantly higher values for the INT than EXT (p = .010, g = 0.63, medium) and NEUT (p = .026, g = 0.51, medium). There were no significant main effects on knee and ankle joint torque (Knee: F_2,38 = 0.512, p = .527, partial η² = 0.026; Ankle: F_2,38 = 2.240, p = .139, partial η² = 0.105). Regarding the mean joint power, there were no significant main effects of the condition on the hip, knee and ankle joint power during the eccentric phase (Hip: F_2,38 = 2.763, p = .076, partial η² = 0.127; Knee: F_2,38 = 0.187, p = .743, partial η² = 0.010; Ankle: F_2,38 = 2.191, p = .126, partial η² = 0.103). During the concentric phase, there were no significant main effects of the conditions on the hip and knee joint power (Hip: F_2,38 = 1.526, p = .234, partial η² = 0.074; Knee: F_2,38 = 421, p = .571, partial η² = 0.022). There was a significant main effect on the ankle joint power (Ankle: F_2,38 = 3.612, p = .037, partial η² = 0.160). The post hoc test results showed significantly higher values for the EXT than INT (p = .016, g = 0.48, small).

The mean ± SD scores of the angles of the three joints of the lower extremities at each time point under each condition in both groups are detailed in Table 5.

Table 5.

The mean ± SD scores for the joint angles of the three joints of the lower extremities at each time point under each condition.

		INT	EXT	NEUT	Differences
E group
Initial contact (deg)	Hip	144.4 ± 7.7	146.1 ± 6.6	146.2 ± 5.9	n.s.
	Knee	145.6 ± 6.1	145.2 ± 6.7	145.6 ± 6.8	n.s.
	Ankle	100.8 ± 6.6	102.0 ± 8.4	101.5 ± 7.3	n.s.

Minimum height of the centre of mass (deg)	Hip	142.8 ± 9.6	146.9 ± 7.6	146.8 ± 6.7	INT < EXT, NEUT
	Knee	119.6 ± 8.1	122.5 ± 7.5	122.2 ± 6.7	n.s.
	Ankle	73.6 ± 2.8	74.5 ± 3.3	74.5 ± 2.8	n.s.

Take-off (deg)	Hip	167.3 ± 5.3	168.4 ± 5.2	168.5 ± 5.0	n.s.
	Knee	170.5 ± 4.8	169.8 ± 4.7	170.1 ± 4.6	n.s.
	Ankle	129.9 ± 4.6	131.0 ± 4.1	130.6 ± 3.8	n.s.
I group
Initial contact (deg)	Hip	142.9 ± 7.4	143.6 ± 7.5	142.7 ± 6.5	n.s.
	Knee	145.5 ± 5.5	145.3 ± 5.8	146.5 ± 5.9	n.s.
	Ankle	108.3 ± 11.1	109.0 ± 9.0	109.0 ± 10.4	n.s.

Minimum height of the centre of mass (deg)	Hip	137.5 ± 10.2	141.8 ± 10.1	139.6 ± 8.2	INT < EXT*
	Knee	115.0 ± 10.5	118.1 ± 12.4	118.4 ± 9.4	n.s.
	Ankle	73.5 ± 5.9	73.5 ± 5.6	73.4 ± 4.9	n.s.

Take-off (deg)	Hip	165.6 ± 7.6	167.4 ± 8.2	166.9 ± 6.6	INT < EXT*
	Knee	168.4 ± 6.9	169.2 ± 6.2	165.6 ± 15.6	n.s.
	Ankle	128.5 ± 8.1	130.1 ± 7.4	130.8 ± 6.0	n.s.

E group = experienced group; I group = inexperienced group; INT = internal focus of attention; EXT = external focus of attention; NEUT = neutral focus of attention; n.s. = not significant; *p < .05.

In the experienced group, the ANOVA results showed no significant main effects of the condition on the angles of the hip, knee and ankle joints at foot contact (Hip: F_2,38 = 3.121, p = .056, partial η² = 0.141; Knee: F_2,38 = 0.150, p = .861, partial η² = 0.008; Ankle: F_2,38 = 0.761, p = .474, partial η² = 0.038). At the minimum height of the centre of mass, there was a significant main effect on the angle of the hip joint (F_2,38 = 7.617, p = .004, partial η² = 0.286). The post hoc test results showed lower values for the INT than EXT (p = .037, g = 0.46, small) and the NEUT (p = .007, g = 0.47, small). There were no significant main effects on the angles of the knee and ankle joints (Knee: F_2,38 = 2.422, p = .125, partial η² = 0.113; Ankle: F_2,38 = 2.013, p = .148, partial η² = 0.096). At take-off, there were no significant main effects on the angle of the hip, knee and ankle joints (Hip: F_2,38 = 3.168, p = .053, partial η² = 0.143; Knee: F_2,38 = 0.540, p = .554, partial η² = 0.028; Ankle: F_2,38 = 2.461, p = .104, partial η² = 0.115).

In the inexperienced group, the ANOVA results showed no significant main effects of the condition on the angles of the hip, knee and ankle joints at foot contact (Hip: F_2,38 = 0.373, p = .691, partial η² = 0.019; Knee: F_2,38 = 1.172, p = .312, partial η² = 0.058; Ankle: F_2,38 = 0.188, p = .830, partial η² = 0.010). At the minimum height of the centre of mass, there was a significant main effect on the angle of the hip joint (F_2,38 = 5.158, p = .010, partial η² = 0.214). The post hoc test results showed significantly low values for the INT than EXT (p = .007, g = 0.41, small). There were no significant main effects on the angles of the knee and ankle joints (Knee: F_2,38 = 1.475, p = .241, partial η² = 0.072; Ankle: F_2,38 = 0.031, p = .970, partial η² = 0.002). At take-off, there was a significant main effect on the angle of hip joints (F_2,38 = 3.681, p = .035, partial η² = 0.162). The post hoc test results showed significantly lower values for the INT than EXT (p = .012, g = 0.23, small). There were no significant main effects on the angles of the knee and ankle joints (Knee: F_2,38 = 1.309, p = .273, partial η² = 0.064; Ankle: F_2,38 = 1.752, p = .187, partial η² = 0.084).

Discussion

This study aimed to clarify the AFS effects on DJ performance, force exertion and movement according to DJ experience level. This study's main results showed that in the experienced group, the NEUT exhibited higher RSI, smaller hip joint torque in the concentric phase, smaller positive hip joint power and a larger hip joint angle at the minimum height of the centre of mass than INT. In the inexperienced group, the EXT exhibited a higher RSI, smaller hip joint torque in the concentric phase, larger positive ankle joint power and larger hip joint angle at the minimum height of the centre of mass and take-off than the INT. Thus, the results are consistent with the hypothesis that the experienced group with the NEUT and the inexperienced group with the EXT perform DJ with small force exertion at the hip joint, large force exertion at the ankle joint and small flexion at the hip joint, which are necessary to obtain high DJ performance. These results suggest that the AFS affects not only DJ performance but also force exertion and movement and its effects may differ depending on the DJ experience level.

This study's results showed that the AFS affected DJ performance in both groups. In the experienced group, the NEUT improved the DJ performance (Table 3). Furuhashi et al.¹⁷ reported that RSI, JH and k_vert were higher for NEUT than for INT or EXT in DJ experienced athletes, and Bezodis et al.²³ reported that skilled sprinters had faster times in control conditions than in EXT, and this study supports these results. Perkins-Ceccato et al.²² found that skilled golfers improved their golf performance with EXT and inexperienced golfers with INT, suggesting that the AFS effects depend on an individual's skill. Singer et al.³⁸ reported that skilled performers were more successful when they did not consider their body movements. Garfield et al.³⁹ suggested that highly skilled athletes lose consciousness while performing their best. These studies suggest that the AFS effect on athletic performance depends on an individual's motor performance, which may also apply to DJ. Generally, the difference in the effects of EXT and INT on exercise performance is explained by the constrained action hypothesis. The EXT appears to speed up the learning process – or shorten the first stages of learning – by facilitating movement automaticity²⁰ more specifically, a focus on the movement effect promotes utilising unconscious or automatic processes, whereas in INT one's own movements results in a more conscious type of control that constrains the motor system and disrupts automatic control processes. This view is supported by studies showing that there are reduced attentional demands when performers adopt an EXT rather than INT, and a higher frequency of low-amplitude movement adjustments, which is considered an indication of a more automatic, reflex-type mode of control.²⁰ In other words, the reduced attentional demand for exercise makes it more smoothly performed and improves performance. On the other hand, Poolton et al.⁴⁰ suggested that both INT and EXT take up space in working memory in secondary tasks, resulting in performance deterioration. Based on this, the reason why the experienced group in this study improved DJ performance with NEUT may be due to the possible occupation of working memory by the use of INT and EXT when performing automated DJ. Contrastingly, there were no significant differences in the JH or CT between conditions. The RSI was calculated by dividing JH by CT, and both increased JH and shortened CT affected the increase in RSI.⁴¹ Additionally, there is an interrelationship between JH and CT in DJ, such that when one improves, the other decreases.⁴¹ Based on these findings, it is likely that significant differences between the conditions were observed only in RSI among this study's participants because there were both those with improved JH and shortened CT using the NEUT. In peak vGRF and k_vert, there was no significant difference between the conditions in the peak vGRF; however, the k_vert was higher for the NEUT than INT. k_vert was calculated by dividing the peak vGRF by the displacement of the centre of mass height from the initial contact to the lowest point of the centre of mass.²⁵ This suggests that the reason for the higher value of the k_vert for the NEUT is that the displacement of the centre of mass height from the contact ground to the lowest point of the centre of mass is smaller for the NEUT than INT. Additionally, it has been reported that as the k_vert increases, the risk of injuries, such as the anterior cruciate ligament, increases because of the relatively high load on the skeletal muscles.⁴² Therefore, the results indicate that the increased risk of injury should be considered when using the NEUT for the experienced DJ. These results suggest that high DJ performance can be obtained using the NEUT in the experienced group.

In the inexperienced group, DJ performance was higher for the EXT (Table 3). The RSI was significantly higher for the EXT than INT, and CT was significantly shorter for the EXT than INT. Contrastingly, there was no significant difference in the JH between the conditions. A few previous studies have reported that RSI is improved by the EXT for DJ inexperienced athletes,^3,17–19 which is supported by this study's findings. These results support the constrained action hypothesis that EXT is more effective than INT in improving exercise performance and motor learning.² Focusing on the instruction of EXT, in this study, we included objects of varying attention (hot ground, spring and roof). When using EXT, it affects performance at both near and far distances regarding the distance between participants and the object participants are focusing on,⁴³ but the longer the distance, the more the EXT affects performance.⁴⁴ Thus, the improvement in DJ performance in this study may have been due to the fact that the EXTs directed participants’ attention to the ground closer to them and the roof farther away from them. These results suggest that the inexperienced group in this study could perform high DJ performance with EXT containing several objects participants were focusing on. Additionally, the CT was significantly shorter for the EXT than INT, suggesting that the shortened CT improved the RSI. The peak vGRF and k_vert showed higher values with the EXT than INT. Therefore, it is considered that the k_vert was higher with the EXT than INT because the peak vGRF was higher with the EXT than INT in the inexperienced group. Additionally, increased k_vert is associated with shorter CT⁴⁵; therefore, the decreased CT with the EXT may be due to the increased k_vert. Contrastingly, Pedley et al.⁴² indicated that coaching that results in shorter CT and increased k_vert should be restricted to athletes with insufficient strength for the load. Therefore, this study's results indicated that the athletes’ strength-to-load ratio should be considered when using EXT for DJ inexperienced athletes. These findings suggest that the use of EXT can result in high DJ performance in the inexperienced group. The results showed that the AFS affected the DJ performance, and this effect may have depended on the DJ experience level. Particularly, the experienced group showed high DJ performance with the NEUT, whereas the inexperienced group showed high DJ performance with the EXT.

Next, regarding the kinetics data of the DJ, the AFS affected the force exertion in the DJ in both groups. In the experienced group, hip joint torque in the concentric phase and positive hip joint power were significantly lower with the NEUT than INT (Table 4). Wulf and Dufek⁴⁶ reported that EXT rather than INT increased the joint torque of the three lower limbs in a vertical jump. Furthermore, they reported that by using EXT, participants unfamiliar with jump performed results and joint torque similar to that of expert jumpers. Thus, AFS affects a participant's kinetics data in a jump exercise, which turns it into a performance-enhancing force exertion. In jump exercises, such as DJ and rebound jump, aimed at obtaining maximum JH with minimum CT, the contribution of force exerted at the ankle joint among the three lower limb joints was significantly high.^10,11 Furthermore, Hayashi et al.⁴⁷ suggested that the ankle joint can accommodate force exertion faster than the hip joint in the DJ. Therefore, reducing the force exertion at the hip joint and increasing the force exertion at the knee and ankle joints in the DJ are considered rational force exertions to improve DJ performance. In conclusion, the DJ experienced group had lower values of hip joint torque and power with NEUT than with INT, indicating that they were using reasonable hip joint force exertion to achieve higher JH and shorter CT in DJ.

In the inexperienced group, the hip joint torque at the concentric phase was significantly lower with the EXT than INT, and the positive ankle joint power was significantly higher with the EXT than INT (Table 4). The results of the inexperienced group were partially different from those of the experienced group, suggesting that the AFS effect on the kinetics during DJ differed depending on the DJ experience level. The participants in the inexperienced group were not familiar with the techniques of force exertion and movements required to achieve high DJ performance. However, it is considered that with the EXT, the hip joint force exerted at the concentric phase was reduced, and the positive ankle joint power increased, which made it possible to exert a large force in the minimum CT. The previous study reported that it is necessary to increase the force exertion of the ankle joint at the concentric phase to reduce the CT.¹⁰ Therefore, it is suspected that the CT was shortened in the inexperienced group because the force exertion of the hip joint was smaller with the EXT and the force exertion of the ankle joint was larger. Therefore, it is suspected that the CT was shortened in the inexperienced group because the force exertion of the hip joint was smaller with the EXT and larger in ankle joint. Wulf et al.⁴⁸ reported that in vertical jump, EXT has a higher JH than INT, however, EMG activity reduced. They also report that AFS provides optimal neuromuscular activation patterns to enhance performance. Wulf and Dufek⁴⁶ reported that AFS affects a participant's kinetics data in a jump exercise, which turns it into a performance-enhancing force exertion. This suggests that in the inexperienced group, the participants performed rational force exertion to shorten the CT and achieved high DJ performance with the EXT. The kinetics results showed that the AFS can affect the kinetics of DJ and the effect may vary depending on the DJ experience level. Particularly, the experienced group with NEUT and inexperienced group with EXT could be prompted to exert rational force to enhance DJ performance.

Next, regarding the kinematics data in the DJ, the AFS affected the movements in the DJ in both groups. In the experienced group, the hip joint angle at the minimum height of the centre of mass was significantly larger with the NEUT than with the INT (Table 5). Contrastingly, no significant differences were found between the conditions in the angles of the three lower limb joints at initial contact and take-off. Thus, the experienced group performed DJ with smaller flexion after initial contact with NEUT than with INT. Generally, it is important to perform a bounce DJ with a relatively small flexion displacement of the hip and knee joints for more effective power development of the ankle and knee joints.^49–51 The results suggest that the experienced group performed DJ more effectively with NEUT, such as with relatively small flexion of the hip joint, which is a more effective movement for the DJ.

In the inexperienced group, the hip joint angles at the minimum height of the centre of mass and take-off were significantly larger with the EXT than INT (Table 5). Contrastingly, no significant differences were found between the conditions in the angles of the three lower limb joints at initial contact. Makaruk et al.⁴ reported the AFS effects on the DJ movements; they stated that the knee flexion displacement in the DJ was larger with the EXT than INT. The reasons for these results were that the DJ exercise was very difficult for the participant; therefore, the participant did not benefit from the performance enhancement effects of the EXT. However, this study showed that the participants performed the DJ with a small flexion of the hip joint as the hip joint angle at the minimum height of the centre of mass was larger with the EXT than with INT. Therefore, the AFS effect was achieved in the inexperienced group because the DJ was not a very difficult task with a platform height of 0.3 m. Additionally, considering the importance of relatively small hip and knee joint flexion displacements for effective DJ,^11,49–51 we suggest that the movements for effective DJ were performed with the EXT. Furthermore, the hip angle at take-off for the EXT was higher than that for INT. It is considered that high DJ performance was achieved with the EXT by a large hip joint extension during the minimum CT. These results indicate that the inexperienced group performed more effective DJ with small hip joint flexion and large hip joint extension with the EXT. Several previous studies have shown that AFS changed the angle of knee flexion in jumping and landing exercises.^4,52 The changes in kinematic data due to AFS in this study and previous studies can also be explained by the constrained action hypothesis. EXT allows the motor control system to naturally regulate and organise motor actions. Therefore, movements are fast and reflexive.⁵³ Thus, it is possible that in NEUT and EXT, where exercise was performed more smoothly in each group, hip flexion decreased because exercise was performed more reflexively. The results on kinematics showed that the AFS can affect kinematics in the DJ and the effect may vary depending on the DJ experience level. Particularly, the experienced group with NEUT and inexperienced group with EXT can prompt movements to enhance DJ performance.

This study has some limitations. The participants in the inexperienced group were athletes with various specialties. The performance and motion characteristics of the DJ vary depending on the participant's specialised sport.⁵⁴ Additionally, the focus of attention to high DJ performance varies with the DJ experience.¹⁷ Therefore, further research needs to be conducted on each sport and DJ experience level to clarify the relationship between the AFS and DJ performance. Furthermore, this study used Comyns et al.³ as a reference to develop verbal instructions. However, some words were included in the instructions in EXT, and it is unknown which words were effective in inducing EXT. For example, attention to jumping like a spring might have elicited EXT in some participants while it might have elicited INT in others. In addition, the instructions were longer in EXT than in the other conditions, which may have taken up more of the participants’ working memory. Thus, it is also possible that the comparison between EXT and other conditions regarding the effect on DJ became unclear in the DJ experienced group. In the future, it is advisable to determine the content of instructions carefully and adopt instructions that will elicit each focus of attention. However, this study showed that AFS had different effects on DJ performance, kinetics and kinematics depending on the DJ experience level. In particular, the results suggest that DJ performance, force exertion and movement improved more with the NEUT for DJ experienced athletes and EXT for DJ inexperienced athletes than with INT.

Practical applications

A previous study reported that the high performance of DJ experienced athletes and DJ inexperienced athletes was achieved with NEUT and EXT, respectively.^3,16–19 Force exertion and movements in DJ are necessary to achieve high DJ performances^11,48,50 and should be considered to assess the performances of jump exercises.¹⁰ These findings suggest that when using the AFS for DJ, it is necessary to consider that AFS significantly changes not only DJ performance but also force exertion and movement. These findings indicate that it is important for S&C coaches to use AFS for DJ based on the DJ experience level of their athletes.

Footnotes

Acknowledgements

The authors thank all the participants for their participation. This study was supported by the JST SPRING, Grant Number JPMJSP2124.

Declaration of conflicting interests

The author(s) declared no potential conflicts of interest with respect to the research, authorship, and/or publication of this article.

Funding

The author(s) received no financial support for the research, authorship, and/or publication of this article.

ORCID iDs

Yuki Furuhashi

Ryohei Hayashi

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