Anticipation of landing leg masks ankle inversion orientation deficits and peroneal insufficiency during jump landing in people with chronic ankle instability

Ankle inversion orientation and peroneal activation insufficiency may contribute to lateral ankle sprains during landing in chronic ankle instability (CAI); however, how anticipation alters these factors is neglected. This study aimed to assess the impact of anticipation on joint orientation and muscle activity during landing in individuals with CAI. Fifteen participants with CAI and 15 healthy participants (control) were recruited to perform single‐leg landings after bilateral countermovement jumps when the landing limb was specified before (planned) or after (unplanned) take‐off. Joint angle (hip, knee, and ankle) and electromyography (gluteus medius, rectus femoris, biceps femoris, gastrocnemius lateral head, tibialis anterior, and peroneal longus) were collected and analyzed with 2 (groups) × 2 (conditions) statistical parametric mapping ANOVA. In the unplanned condition, the CAI group demonstrated a less plantarflexed (maximum difference [MD] = 9.5°, p = 0.047) and more inverted ankle joint (MD = 4.1°, p < 0.001) before ground contact, along with lower peroneal activity at ground contact compared to the control group (MD = 28.9% of peak activation, p < 0.001). No significant differences between groups were observed in the planned condition. In conclusion, anticipation may mask jump landing deficits in people with CAI, including inverted ankle orientation and reduced peroneus longus activity pre‐ and post‐landing, which were observed exclusively in unplanned landings. Clinicians and researchers need to recognize the impact of anticipation on apparent landing deficits and consider the implications for injury prevention and rehabilitation strategies.


| INTRODUCTION
Prospective cohort studies showed those with a history of lateral ankle sprain had a 3.5 times greater risk of another lateral ankle sprain over 3 years compared to those with no history of lateral ankle sprain, and that up to 40% of people develop chronic ankle instability (CAI) when assessed 1 year after their first ankle sprain. 1 CAI is characterized by a history of lateral ankle sprains and residual and chronic symptoms, recurrent lateral ankle sprains and/or perceived instability. 2According to the Hertel and Corbett model, 3 individuals with CAI exhibit a spectrum of motor-behavioral, sensory-perceptual, and pathomechanical impairments after the initial lateral ankle sprain.Specifically, motor-behavioral deficits involve various movement deficits during functional tasks.Understanding these movement deficits in different functional tasks helps to identify contributing factors to recurrent lateral ankle sprains.
Most weight-bearing sports produce significant landing forces during running, jumping, and cutting movements. 3nilateral landing tasks, which impose large and rapid impulse loads to the ankle complex during initial contact, are commonly used to assess movement deficits in a simulated close-to-injury circumstance. 4,5Increased inversion angle 6 and decreased activation of peroneal muscle 7 during initial contact are two identified impairments contributing to lateral ankle sprain during landing.The typical sequence of inversion ankle sprain 8 is: 1) That an inverted ankle landing position generates twisting torque around the ankle joint, 2) coupled with reduced peroneal muscle activity so leads to overstretching and sprain of the ankle lateral ligaments.Previous studies showed that individuals with CAI exhibited greater ankle-inversion angles, 6 as well as less peroneus longus muscle activity 9 than healthy individuals, which may be associated with recurrent lateral ankle sprains during landings.Moreover, some proximal adjustments, including reduced hip flexion angle 9 and knee flexion angle 10 to attenuate loading during jump landing were also observed.
However, those regularly pre-planned movements (unilateral jump landing 4,5 or drop landing 11 ) cannot fully simulate the unexpected situations 12 arising in sports performance, for example, athletes in the field need to process various visual stimuli (e.g., the position of teammates or opponents) while simultaneously monitoring and spontaneously adjusting their movements to sudden unexpected changes. 13To mimic the challenges arising from the unpredictable sports environment, a task with a visual cue shown after the jump landing was designed to examine lower limb biomechanics in unplanned condition. 14here is limited evidence about how individuals with CAI react in this time-constrained unplanned jump landing task.Thus, investigating joint orientation and muscle activity during unplanned jump landings where the landing leg will be shown after take-off will help to understand the underlying adaptations to deal with unexpected conditions and develop a more targeted and patient-oriented rehabilitation program for individuals with CAI.
This study aims to identify whether ankle orientation and muscle activity deficits were altered by anticipation during jump landing using statistical parametric mapping among individuals with CAI and controls.It is hypothesized (a) that unplanned jump landing places individuals with CAI at a greater risk of lateral ankle sprain in terms of joint orientation and muscle activity than planned landing, and (b) that individuals with CAI demonstrate increased ankle inversion angles and reduced peroneal activity pre-landing and post-landing than controls in both conditions.

| Participants
Fifteen participants with CAI and 15 participants without any history of lateral ankle sprain (controls) were recruited in the study (Table 1).The specific inclusion criteria for the CAI group were determined based on the International Ankle Consortium's position statement 2 and included (i) the first lateral ankle sprain occurred at least 12 months ago (ii) reports of the previously injured joint "giving way" and/or recurrent sprain and/ or "feelings of instability", (iii) Cumberland Ankle Instability Tool (CAIT) 15 scoring <24, FAAM-ADL score <90%, 19 FAAM-Sports score <80%, (iv) participating in >90 min or more of physical activity per week which involved jump landing movements.Apart from the general requirements based on the International Ankle Consortium's position statement, 2 another requirement of jumping activity was used in this study.(v) countermovement jump (CMJ) height equal or higher than the height of knee joint.This jump height ability was required to ensure an approximate 500 ms flight time that allowed for processing of the visual stimulus indicating landing leg after take-off in the unplanned condition. 16ubject exclusion criteria 2 include (i) history of a fracture in the lower extremity, (ii) history of previous surgery (e.g., joint structures, nerves, and bones), (iii) acute injury to musculoskeletal structures of lower extremity joints in the past 3 months, and (iv) a history of neurologic disorders.Specific inclusion criteria for the control group include (i) no history of lateral ankle sprain injury in their lifetime, (ii) a score of 100% on the FAAM-ADL and FAAM-Sports, (iii) same requirements of activity and jumping height as the CAI group.All participants provided written informed consent, as approved by the Research Ethics Committee of Loughborough University (LEON number: 7841) before participation.

| Instrumentation
The study was conducted in the Computer Assisted Rehabilitation Environment (CAREN) environment developed by Motek Medical, which is comprised of two force plates (Bertec, USA) and 12 motion capture cameras (Vicon Bonita, Oxford Metrics, UK).The motion capture cameras sampled at 200 Hz following a Vicon Active wand calibration.A customized CAREN program was designed to create the unplanned jump landing condition by automatically providing a visual stimulus specifying the landing leg after take-off (with a latency of 100 ms) from the force plate.The visual stimuli were presented on a large screen (4.05 m × 2.50 m) using a long throw data projector (Hitachi, CP8AW100N).EMG signal data was collected by DELSYS wireless dynamic EMG sensors (Trigno™ Wireless Systems, Delsys Inc., USA) at 2000 Hz.All kinetic and EMG data were sampled at 2000 Hz and synchronized through the Vicon software.

| Procedure
All participants were dressed in comfortable sports shorts and shirts and shoes.Anthropometric data including height, mass, and age were recorded for each subject.A total of 26 reflective markers (9.5 mm in diameter) were fixed to the following locations on both legs: Anterior superior iliac spine, posterior superior iliac spine, greater trochanter, lateral epicondyle, medial epicondyle, line on the axis of the femur, lateral malleolus, medial malleolus, line on the axis of the lower leg, first metatarsal head, fifth metatarsal head, second toe, and heel.Then, the skin was shaved and cleaned with 70% alcohol.EMG sensors were attached to the gluteus medius (GM).rectus femoris (RF), biceps femoris (BF), gastrocnemius lateral head (GL), tibialis anterior (TA), and peroneal longus (PL) according to the SENIAM guideline. 17ll participants first performed a standardized warm-up (30 jumping jacks) procedure and three test jumps, and those whose jumping height during any test jumps below their knee joint was excluded from further experiment based on visual screening by the researcher, ZC.H.Then, participants meeting the minimum requirements of jump height performed 30 bilateral CMJs in place followed with either planned (n = 15) or unplanned (n = 15) single-leg landings onto the force plate with the landing leg specified in a random order controlled by the CAREN software (Figure 1).In the planned condition, the participants received visual information depicting the F I G U R E 1 Graphic demonstration of the jump landing protocol.In the planned condition, the participants received visual instruction sign ("LEFT" or "RIGHT") before the jump via the screen.For the unplanned condition, the sign was automatically shown on the screen 100 ms after take-off.
requested landing leg before the jump.In the unplanned condition, this information was automatically provided only after take-off (with a latency of 100 ms).A successful trial was characterized by a jump height over 30 cm (detailed calculation method provided in the data processing section), landing on the correct leg (relevant in unplanned trials only) and maintaining a stable single-leg stance without touching the ground with the free leg, leaving the force plate, or touching the ground with the hands for at least 10 s following ground contact. 16The arms could be used to equilibrate the postural sway immediately after ground contact, but the hands were repositioned on the hip as soon as possible.Participants were familiarized with the jump during the laboratory session and were excluded if they could not jump over the height of the knee joint after practice by visual screening.The jumping height was adjusted from 30 cm 13 reported previously to ensure comparability between trials and participants.The first three successful trials on an injured limb during planned condition and unplanned condition were collected for the group comparisons.

| Data processing
All dependent variables were identified during the period from 200 ms before initial contact (defined as first frame with ground reaction force above 15 N after the jump 15 ) to 200 ms after initial contact for the three successful trials on the injured side in the planned and unplanned landing and were exported to Visual 3D (C-Motion).
Kinematic markers trajectories were filtered using a fourth-order low-pass Butterworth filter and a 10-Hz cutoff frequency. 18The cut-off frequency of 10 Hz was determined by residual analyses 5 for all landing trials.A static calibration trial in the anatomical position was recorded to define neutral (zero) joint angles using previously described methods, 19 and this calibration file was applied to all landing and jumping trials.Local coordinate systems for the ankle, knee, and hip were defined using the International Society of Biomechanics (ISB)'s recommendation. 20Next, a rigid link model (foot, shank, thigh, and pelvis segments) was created using the static calibration.Ankle, knee, and hip joint angles were computed with a Euler X-Y-Z order of rotations [i.e., flexion (+) and extension (−), adduction (+) and abduction (−), and internal rotation (+) and external rotation (−)].The vertical distance between the toe marker and the force plate was used to measure the jump height and only trials with jumping height higher than 30 cm (producing uniform flight times more than 500 ms 16 ) were used for further analysis.
EMG data were filtered with a zero-lag 20-450 Hz bandpass, fourth-order Butterworth filter, and smoothed using a root mean square (RMS) algorithm with a 100 ms moving window.Each muscle was normalized to the maximum across the three selected successful trials of the peak RMS activation. 21

| Sample size
A power analysis was performed for sample size estimation based on the peroneal longus activation magnitude reported previously(CAI group, 3.58 ± 1.07% maximal voluntary isometric contraction (MVIC) vs. control group, 6.48 ± 1.26% MVIC). 22By setting a 95% statistical power with an alpha level of >0.05, a minimum requirement of six participants in each group to achieve the actual power being 0.9711.

| Statistical analysis
Shapiro-Wilk tests were used to assess the normality of data.All SPM analyses were implemented using the open-access SPM1D code (www.spm1d.org) in MATLAB R2019a (The MathWorks, MA, USA).To compare joint angle and EMG differences between groups, a two-way repeated measures SPM ANOVA to identify any group (CAI vs. control) by time (planned vs. unplanned) interaction effects.First, SPM{F} statistic was calculated from the mean joint angles or EMG RMS value across conditions and groups.Second, the temporal smoothness of SPM{F} based on its average temporal gradient was estimated.Subsequently, the threshold of SPM{F} was computed using random field theory, 23 above which only α = 0.05 of the data would be expected to reach had the test statistic trajectory resulted from an equally smooth random process.Any clusters of SPM {F} that exceeded this threshold were considered significantly different.Individual probability values were calculated for each suprathreshold cluster, which indicates the probability that a cluster of a given height and size could have resulted from an equivalently smooth random process.If any interaction was observed, a Bonferroni post hoc analysis was used to correct for multiple comparisons.The maximum difference (MD) between groups was calculated for each result that was different.

| Lower-extremity joint kinematics
Significant group × condition interactions was found on the hip kinematics and ankle kinematics (Table 2 and Figure 2).During the unplanned landing, the CAI group showed group differences around the ankle joint and hip joint.More specifically, CAI individuals exhibited less plantarflexion (Figure 2C, MD = 9.5°, p = 0.047) from −180 ms to −25 ms prior to initial contact and more ankle inversion (Figure 2F, MD = 4.1°, p < 0.001) from −142 ms before to +23 ms after initial contact.Moreover, there was greater hip flexion (Figure 2A, MD = 10.1°,p = 0.024) from −200 ms to −62 ms and less hip abduction (Figure 2D, MD = 7.9°, p = 0.015) from −170 ms prior to landing to 0 ms.No joint angle differences between groups were found during the planned landing condition.

| Lower-extremity EMG
Significant group × condition interactions was found on the GM activity (p < 0.001, F = 6.86) and PL activity (p < 0.001, F = 7.23) (Table 2 and Figure 3).The CAI group demonstrated higher GM activity from −80 ms to 60 ms  3A, MD = 37.6%, p < 0.001) and lower PL activity from −16 ms to 75 ms (Figure 3E, MD = 28.9%,p < 0.001) compared to the control group in the unplanned landing condition.No EMG activity differences were found in the planned landing condition.

| DISCUSSION
To our knowledge, this study is the first to examine the effects of a time-constrained unplanned jump landing task on joint orientation and muscle activity in individuals with CAI while previous studies primarily focused on jump landing in pre-planned conditions.Our key finding indicates that landing without anticipation (unplanned landing) exacerbates landing deficits, such as increase in ankle inversion orientation and reduced peroneal longus activity before and after landing, which contribute to the recurrence of lateral ankle sprains in individuals with CAI.These findings align with our initial hypothesis.However, in contrast to our second hypothesis, no difference between groups was observed during the planned landing.It is possible that anticipation and the ability to plan a landing may mask jump landing deficits, hence these were only apparent in the unplanned condition.
The first major finding is that unplanned conditions induced inverted and dorsiflexed ankle position and less PL activity before and after landing for individuals with CAI.More specifically, a larger ankle inversion angle (MD = 4.1°) was found from 142 ms before landing to 23 ms after landing.An ankle inversion angle of 14° at foot strike has been reported to incite an event of ankle inversion sprain due to medially deviated ground reaction force and subsequently an inversion moment arm along the ankle joint center. 6,8The results of this study indicated that without anticipation, individuals with CAI presented greater ankle inversion angle pre-and post-landing, which, given that participants were able to undertake some practice jumps and were landing on a flat, non-slip solid surface, may increase the risks of lateral ankle sprain during unplanned landings in sporting contexts.Additionally, relative to the control group, the CAI group adopted a less plantarflexed position between pre-landing from 185 ms till post-landing 25 ms.The interpretation on the relationship between plantarflexion and the cause of lateral ankle sprain is still controversial.Some researchers 9,24 reported that a high degree of plantar flexion at initial contact may incite a lateral ankle sprain due to the loose-packed position and increased stress through the ankle joint.The less plantarflexed position observed in the CAI group might be due to a voluntary self-defense mechanism to avoid what they perceive as an unstable position. 25While some authors 26,27 also reported that inversion of the ankle joint can proceed irrespective of plantarflexion or not.Further studies are needed to understand how the kinematic patterns in people with CAI are influenced during landing and how this contributes to recurrent injury in this population.
Apart from kinematic adjustments, lateral ankle sprain frequently results from excessive inversion when the peroneus longus muscle and lateral ankle ligaments cannot F I G U R E 3 Lower extremity EMG signal comparison.The electromyographic (EMG) signals were recorded from 200 ms before landing to 200 ms after landing.The EMG curves were presented as mean values and standard deviation clouds.The "0" on the horizontal axis indicates the event of touching the ground."-" indicates before initial contact while "+" indicated after initial contact.GM, gluteus medius, RF, rectus femoris BF, biceps femoris, GL, gastrocnemius lateral head, TA, tibialis anterior, PL, peroneal longus.The CAI group showed higher GM activity and lower PL activity than the control group during unplanned landing.
enough resistance against inversion movement during 28 For this first time, this study found up to 28.9% less PL amplitude deficits after landing compared to the control group in the unplanned landing condition only.This is consistent with previous research showing 25% less PL EMG activation during the transition phase from landing to cutting after initial ground contact than controls. 4As the primary muscle to produce eversion movement, 29 the insufficiency of PL activity before or after landing may fail to stabilize the increased ankle frontal movements due to removing the anticipation, which will impose greater risk on the ankle joint for CAI.The dominance of these deficits in the unplanned condition rather than the planned condition may result from the inability of the gamma motoneuron system to adjust the sensitivity of the sensory receptors within the injured ligament, resulting in decreased activity to rapid lengthening of the peroneal musculature in the unplanned condition 30 or the preprograming of gamma motoneuron system making them more sensitive and quicker to elicit a reflexive action via feedforward control in the planned condition. 31The physiological mechanism behind the differences between planned movements and unplanned movements requires further clarification.
The study's second important finding is the proximal adjustments in the unplanned condition.CAI group adopted a more flexed and adducted hip joint position during unplanned landings, consistent with the proximal changes reported in previous studies. 4,5,10Increased hip flexion may help attenuate the impact of ground reaction force (GRF) and maintain balance in the sagittal plane. 5,10Furthermore, a less abducted hip angle was observed during unplanned landings, potentially due to the reduced reaction time after the visual signal in the unplanned condition, leading to a more upright position of the femur (less abducted position) 25 and increased GM muscle activity 32 during landing, which may help stabilize the downward motion of the center of mass in the sagittal plane after landing. 25Previous studies demonstrated this adducted position resulted in increased gravitational moment arm and internal hip abductor moment, 33 compensating for the weakness of GM 34 in individuals with CAI.
Interestingly, no group differences were observed during planned single-leg landings, contrary to some previous studies reporting differences during landings, including less plantarflexion angle, 35,36 reduced hip abduction angle, 37 and less PL activity. 38The use of a CMJ instead of a drop landing task in this study may explain the discrepancy, as CMJ landings were generally performed with a more extended lower extremity at ground contact, and involved fewer frontal and transverse planes movements than drop landings. 39Additionally, the CAI participants recruited for this study were actively participating in sports involving jumping and landing activities, which may influence their adaptability and familiarity with repetitive landing experiences, potentially attenuating the variability of landing biomechanics compared to those without landing experience. 40However, some researchers 41,42 have also reported no differences in the sagittal plane and horizontal plane kinematics during jump landing between CAI and individuals without.The similar pattern observed in the anticipated condition could be due to the feedforward motor control adjustments 43 in this population.For example, during the flight phase of a jump, ankle musculature is preactivated to prepare for landing, increasing ankle joint stiffness and attenuate the impact of landing. 44Such preprograming strategy might reduce the distinctions of kinematics when the subjects can predict their movements.Therefore, further investigation is warranted to determine task-specific landing characteristics for individuals with CAI, the kinematic changes in planned conditions, and the role of training experience.
There were several limitations to this study.First, given that the study design was cross-sectional, it remains unclear whether the observed alterations in landing movement strategies among the CAI group were due to an adaptive motor-control alteration to an ankle-sprain injury or were present before the injury.Future prospective studies are needed to clarify the causal effect of CAI on ankle orientation and muscle activity during landing.Second, the study only included CAI participants engaged in sports involving jumping landing activities, which might limit the application and interpretation of the results to the boarder CAI population.Lastly, the study did not include coper participants as another control group, looking further into the movement characteristics of copers might yield valuable information on how to avoid developing into individuals with CAI.
In conclusion, anticipation is associated with landing deficits in individuals with CAI, which may mask the jump landing deficits, including inverted ankle orientation, and reduced peroneal longus activity pre-and postlanding, observed exclusively in the unplanned condition.These deficits in unplanned landing may contribute to the recurrence of ankle sprains.Clinicians and researchers need to recognize the impact of anticipation on landing deficits and consider the implications for injury prevention and rehabilitation strategies.

| PERSPECTIVES
Individuals with CAI had increased inversion angle as well as decreased PL activation at ground contact during an unplanned single leg landing onto the previously compared to unplanned landings by controls with no history of injury.These deficits are associated with increased injury risk 6,7 and were not apparent during planned leg landings.Rehabilitation programs should replicate sporting contexts (variable surface and limited anticipation) to address ankle orientation and muscle activation during landing.Unplanned tasks offer unique insights into how individuals with CAI respond to more naturalistic scenarios.Additionally, individuals with CAI demonstrated proximal changes (increased hip flexion angle and hip abductor muscle activity) when unable to anticipate the landing leg.Thus, interventions aimed at re-programming multi-joint neuromuscular control during unplanned environments should be implemented during and/or after strengthening programs.

F I G U R E 2
Lower extremity joint angle comparison.Lower-extremity (hip, knee, and ankle) joint angle in the 200 ms before landing till 200 ms after landing in the sagittal plane (A-C) and frontal plane (D-F).The curves were depicted as mean values and standardization deviation clouds.The "0" on the horizontal axis indicates initial contact."-" indicates before initial contact while "+" indicates after initial contact.FLEX, flexion, EXT, extension, ABD, abduction, ADD, adduction, DF, dorsiflexion, PF, plantarflexion, IV, inversion, EV, eversion.During unplanned landing, the CAI group had a significantly more flexed and adducted hip position and a less plantarflexed and more inverted ankle position than the control group.

T A B L E 1 Demographics of participants. CAI (n = 15) Control (n = 15)
The effects of anticipation on the kinematics and EMG of lower extremity in both groups.