Ankle sprains are the most frequently observed sport-related injury, with a lateral ankle sprain being the most common ankle injury. Chronic ankle instability can arise from proprioceptive and/or neuromuscular control deficits associated with recurring ankle sprains, which is known as functional ankle instability.1 An ankle sprain can lead to feelings of joint instability and, if the instability is experienced over an extended period of time, the injury may be diagnosed as chronic ankle instability (CAI).2 The recurring joint instability characterized by CAI may be due to mechanical factors such as laxity changes in the ligament following a sprain, which is termed mechanical ankle instability. The combination of mechanical and functional factor deficits can lead to challenges in executing lower extremity tasks, which are commonly tested with jumping and/or landing tasks.3
Research has shown that individuals with CAI demonstrate differences in proximal lower limb kinematics and landing kinetics during jumping tasks.3–6 For example, Caulfield and Garrett4 found that participants with functional ankle instability exhibited greater knee flexion 20 ms before landing to 60 ms after landing relative to controls. Similarly, other research has shown that individuals with CAI exhibit significant reductions in muscular activity in the lateral regions of the lower leg during jumping tasks,7 significantly higher angular ankle joint velocities during gait,8 and significantly greater inversion of the ankle before, during, and immediately following heel strike while walking compared to healthy controls.7 The altered biomechanics of individuals with CAI are thought to contribute to the high reinjury rate, with some estimating a likelihood of recurrence as high as 80%,9 which raises the question of what individuals and practitioners could do to enhance motor skills with the goal of reducing the rate of reinjury.
There are a variety of treatment options available for individuals with ankle injuries and CAI,10 such as balance and strength training protocols,11,12 wobble boards,13 and applying braces,14 yet these are not always effective. One reason for this may be that researchers are overlooking cognitive avenues (eg, attentional focus) to investigate how directing attention may affect movement. There are two components to attentional focus: an external focus of attention and an internal focus of attention. An external focus of attention is when an individual directs his or her attention to the effect of his or her movement on the environment.15 For example, an individual balancing on a wobble board might be asked to focus on keeping the board level.16 In contrast, an internal focus of attention would be attention directed to the body part associated with movement,15 such as asking an individual to keep his or her feet level while balancing.16 An external focus is thought to promote more reflexive and automatic movement17–19 and is supported by an abundance of literature demonstrating that an external focus is beneficial for transitory performance and retention of motor skills,20 making the focus of attention framework a plausible candidate to enhance motor skills in individuals with CAI.
To our knowledge, few studies have examined whether the focus of attention influences the motor system in individuals with CAI. Sulewski et al21 had participants complete a 3-day balance training intervention under internal or external focus of attention instructions. The researchers found no effects of attentional focus or training, but suggested that their dependent measures (balance assessment using the Biodex stability system and the Star Excursion Balance Test) were not sensitive enough to detect attentional focus changes (no center of pressure outcomes).21 In contrast, recent evidence indicates that assessing balance control using a force plate and non-linear analyses (sample entropy of a center pressure displacement time series) is sensitive to the effects of attentional focus.22
Another plausible explanation is that the balance task used lacked sufficient complexity to elicit attentional focus differences.23 A variety of studies have shown that complex tasks (ie, those requiring maximal force production24) are improved when an external focus is adopted. For example, standing long jump distance,25–28 vertical jump height,29,30 single-leg jump kinematics,31 sprint speed,32 agility tasks,33 and discus throwing34 have all improved when performers adopted an external focus. Therefore, assessing the effects of an external focus in individuals with CAI using a complex task may determine whether an external focus is beneficial.
Another limitation of previous research using an attentional focus framework to enhance motor skills is the failure to include a retention test to assess if these performance changes are maintained. Thus, the purpose of this study was to examine the effects of attentional focus on single-leg jump performance and motor learning (ie, retention, the extent to which performance is maintained over short- or long-time intervals) for individuals with CAI. We hypothesized that individuals with CAI would exhibit greater single-leg jump distances on their uninjured and injured legs when their attention was directed externally relative to those whose attention was directed internally. Further, we hypothesized that individuals who practiced with an external focus would display superior single-leg jump distances when assessed 24 hours after the practice session (ie, retention) relative to those practicing with an internal focus. Practitioners commonly use performance outcome measures to evaluate motor performance35; thus, single-leg jump distance (cm) was used as the dependent variable.
Fourteen adults (9 males, age: 24.2 ± 3.5 years, height: 179.5 ± 5.5 cm, mass: 84.0 ± 15.0 kg; 5 females, age: 23.2 ± 5.0 years, height: 167.7 ± 9.7 cm, mass: 61.5 ± 9.8 kg) with CAI were recruited to participate in this study. Participants were included if they experienced: (1) a lateral ankle sprain that required crutches or immobilization for at least two days; (2) a recurrent sprain since the initial injury; (3) one or more episodes of rolling or giving way since the initial injury; and (4) pain, instability, or weakness in the involved ankle attributed to the initial injury.21 Participants were not excluded if they reported previous bilateral ankle sprains; however, they were only recruited if they exhibited the inclusion criteria unilaterally and experienced the initial sprain at least 6 months before and were without symptoms (ie, feelings of instability or giving way) in the healthy ankle. The institutional ethics committee approved the project and informed consent was obtained prior to commencing the study.
The current study was conducted in a controlled laboratory environment. A standard tape measure was adhered to the floor and used to assess single-leg jump distance. Athletic tape was used as a start line for participants to stand behind. Participants were randomly assigned to one of two experimental conditions: an internal focus condition or an external focus condition. Prior to completing acquisition trials, participants performed three baseline single-leg jumps for distance on the injured and uninjured leg. During the baseline trials, all participants were asked to: “Jump as far as you can.” Participants then completed 20 acquisition trials in their respective experimental conditions, separated into four blocks of five single-leg jumps alternating legs. Rest periods of 30 seconds were given between jumps and a 2-minute break was given between trial blocks. The number of acquisition trials, the amount of rest between jumps, and the amount of rest between trial blocks was determined by pilot testing to prevent fatigue. Focus of attention instruction, adopted from previous research,36 was provided before the start of every trial block. For the internal focus condition, participants were asked to: “Jump as far as you can. While you are jumping, I want you to think about extending your knees as rapidly as possible.” For the external focus condition, participants were told to, “Jump as far as you can. While you are jumping, I want you to think about jumping as close to the cone as possible.” Congruent with previous research,36 an orange cone was placed 4.57 m from the start line for the external focus condition only (Figure 1). In addition, participants in both groups were asked to read their instructions back to the experimenter from a note card.
Participant completing the single-leg jump task. The orange field cone was only present during acquisition for participants in the external focus condition.
On day 1, all participants completed a 5-minute warm-up on a stationary bike, performed the baseline jumps on each leg, and then completed the acquisition trials. After each jump, the distance was measured from the start line to the back of the participant's heel to the nearest half centimeter. Reminders of what to focus on were administered after every 5 jumps. Following Gokeler et al,31 a jump was disqualified if the participant did not hold his or her landing for a minimum of two seconds. If participants lost their balance, the single-leg jump was repeated. On day 2 (24 hours later), participants completed the 5-minute warm-up on the stationary bike and completed three single-leg jumps on each leg to assess retention. Attentional focus instructions were not provided, and the orange cone was removed for all participants when assessing retention.
All reported values represent adjusted mean ± standard deviations after controlling for baseline performance. Distance jumped was averaged within each phase of testing (baseline, each block of acquisition, and retention) to create a single value for each participant for each testing phase. The Statistical Package for the Social Sciences (SPSS software version 19; IBM Corporation) was used for all statistical analyses. To assess significant performance differences between the two groups for acquisition on the uninjured and injured leg, two separate conditions (internal focus or external focus) × four trial blocks mixed analyses of covariance with repeated measures on the last factor and baseline performance as a covariate were used. To assess retention, separate univariate analyses of covariance were used with attentional focus as the between-patient factor and baseline performance as a covariate for the uninjured and injured leg. Baseline single-leg jump performance was included as a covariate in all analyses to control for inter-individual differences due to the wide range of variability associated with this task (eg, due to leg length or strength differences). An alpha level was set a priori at a P value of less than .05. Partial η2 was used to determine the magnitude of the effect sizes using the criteria of partial η2 = .01 as small, partial η2 = .06 as medium, and partial η2 = .14 as large.37
When controlling for baseline jump distance, there was a significant main effect for attentional focus on the uninjured leg (F(1, 11) = 5.10, P < .05, partial η2 =.32). Participants receiving external focus of attention instruction (146.21 ± 14.62 cm) jumped significantly farther than those receiving internal focus of attention instruction (131.65 ± 17.25 cm) throughout acquisition. The trial block main effect, the attentional focus × trial block interaction, and the attentional focus main effect at retention were non-significant after controlling for baseline performance (all P > .05) (Figure 2).
Mean jump distance on the uninjured leg for acquisition and retention. Values are adjusted for baseline performance on the uninjured leg (133.73 cm). The asterisk (*) indicates that participants jumped significantly farther when receiving external focus of attention instruction (146.21 ± 14.62 cm) than internal focus of attention instruction (131.65 ± 17.25 cm) throughout acquisition (P < .05). Error bars represent ±1 standard error of the mean.
After controlling for baseline performance, the attentional focus main effect, the trial block main effect, and the attentional focus × trial block interaction were all non-significant throughout acquisition for the injured leg (all P > .05). Similarly, no significant attentional focus main effect was observed at retention after controlling for baseline performance (P > .05). Although the main effect of attentional focus during acquisition was non-significant (P = .15), its effect size was large (partial η2 = .18) (Figure 3), as was the effect size for the uninjured leg (partial η2 = .32).
Mean jump distance on the injured leg for acquisition and retention. Values are adjusted for baseline performance on the injured leg (128.91 cm). No significant differences were observed accross conditions for the injured leg. Error bars represent ±1 standard error of the mean.
Attentional focus research has robustly demonstrated the benefit of directing attention to the effects of an individual's movement rather than movement production.20 The purpose of the current study was to examine the effects of attentional focus on single-leg jump performance and learning for individuals with CAI. Our findings show that an external focus of attention does not universally translate into enhanced performance in a clinical population. Intrinsic factors38,39 (eg, body mass index,40 range of motion, postural stability,41 and proprioceptive factors42) should be considered when developing best practices for using attentional focus to enhance motor skills in clinical populations.
Commensurate with previous research,26,27,33 our findings from the uninjured leg showed the commonly observed utility of adopting an external focus of attention. Specifically, single-leg jump distance was greater in the external focus group compared to the internal focus group during the acquisition phase. Although it is plausible that individuals with CAI use compensation strategies in their uninjured limb to overcome deficits in their injured limb, that did not influence their enhanced performance when adopting an external focus. This deviates from previous literature that only showed biomechanical changes and not an increase in distance jumped when adopting an external focus in the same task.31 These findings indicate that an external focus of attention framework can lead to different results in various clinical populations; thus it should not be broadly adopted by clinicians and practitioners until intrinsic factors within each population are addressed.
When examining the performance of the injured limb, a large effect size was observed, with the external focus group jumping farther during the acquisition phase. Although the main effect was not significant, it highlighted another important feature of our hypothesis. Adopting an external focus significantly enhanced performance in the uninjured limb, but the magnitude of the enhanced performance was less when the participant switched to his or her injured limb. In this respect, participants served as their own control group, which allowed bilateral performance to serve as a marker of attentional focus utility between an uninjured and injured limb in the same person. The suppressed findings in the injured limb suggest that the ankle injury may not have allowed them to switch from an internal to an external focus as instructed. It is plausible that individuals with CAI have a heightened focus on limb position during activity to reduce the risk of reinjury. Thus, their injury may have placed too much cognitive demand on them to fully adopt an external focus of attention.
The enhanced performance of the injured limb suggests that participants may not have fully adopted an internal or external focus. It is reasonable to suggest that any residual tissue, ligament, or neuromuscular deficits from CAI may not lead to the same recruitment of muscles or coordination that is available in the uninjured limb. Alternatively, it is possible that the uninjured limb was also not performing to the capability of an individual without a history of CAI. Patients with CAI exhibit bilateral deficits in central nervous system function (eg, bilateral alterations in corticomotor excitability),43,44 but our study design precluded examining this possibility further without a true control group (absence of CAI in either limb).
Although performance changes were observed in the hypothesized direction, retention was not observed in either condition. This was surprising because previous studies have shown that an external focus of attention can lead to retention.45,46 These findings were congruent with Sulewski et al,21 who did not find differences between attentional focus conditions for individuals with CAI who participated in a 3-day balance training program. The authors suggested that their measurements may not have been sensitive enough to detect postural control differences resulting from their manipulations. Gokeler et al31 showed that knee flexion increased when participants with anterior cruciate ligament reconstruction adopted an external focus. Although our participants may have modified their movement strategies, their movements were not detected by our behavioral metric.
One limitation of this study is the absence of a control group because it is difficult to ascertain if an external focus augmented performance or if an internal focus suppressed performance. However, we consider this finding to be clinically relevant. Further, we did not implement a transfer test, but our failure to find differences during our retention test indicates that no differences would have been observed if a transfer test was implemented. Finally, although we recruited participants who self-reported unilateral ankle instability, we did not exclude those who had a history of bilateral ankle sprain. Because central processing mechanisms are unknown after an ankle sprain, it is possible that a history of bilateral ankle sprains or bilateral CAI may alter the central processing mechanism, which could have influenced our results.
Implications for Clinical Practice
Our results showed that adopting an external focus of attention may not uniformly enhance motor skills in all individuals with CAI. For our CAI population, a significant enhancement in single-leg jump distance was observed in the uninjured limb when an external focus was adopted. In the injured limb, enhanced performance was also observed in the form of a large effect size when comparing the external and internal focus groups, albeit not significant. Performance was not retained for either group or limb. The current study has practical implications for CAI rehabilitation because the addition of an external focus instruction can augment performance in individuals with CAI. Although not as effective on the injured limb, the immediate performance improvements on the uninjured limb using an external focus would likely produce motivation that could enhance an individual's expectancies for future performances. Wulf and Lewthwaite47 proposed that an external focus may increase dopamine availability by strengthening the synaptogenetic processes to augment memory consolidation.48 Our significant findings for the uninjured limb of individuals with CAI provide further support for using external focus for injury prevention and rehabilitation.49,50 Future studies could replicate the current study with a unilateral CAI group and a matched control group to detect if differences exist. In addition, researchers may consider implementing a more challenging protocol (eg, triple hop test) with longer training durations (eg, multiple days) to potentially augment the external focus effect on injured limbs.
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