Mr Kuenze is from the Department of Human Services Exercise and Sports Injury Laboratory, University of Virginia, Charlottesville, Va. Dr Zinder is from the Department of Exercise & Sport Science, Dr Blackburn is from the Department of Exercise & Sport Science and the Orthopaedics Neuromuscular Research Laboratory, and Mr Norcross is from the Neuromuscular Research Laboratory, University of North Carolina at Chapel Hill, Chapel Hill, NC.
The authors have no financial or proprietary interest in the materials presented herein.
Address correspondence to Christopher M. Kuenze, MA, ATC, Department of Human Services Exercise and Sports Injury Laboratory, University of Virginia, 210 Emmet Street South, Charlottesville, VA 22904-4407; e-mail: firstname.lastname@example.org.
Ankle injuries are common among the athletic population in the United States, with 85% of these injuries being classified as ankle sprains.1 Currently, ankle injuries account for 5% of all sports injuries and approximately $2 billion in annual medical expenses.1 Although the initial sprain may lead to both disability and medical expense, the most concerning effect is the potential for development of chronic ankle instability. Chronic ankle instability is commonly described as self-reported instability, or the feeling of “giving way,” that is present in up to 40% of patients with a history of lateral ankle sprain.2 Although this simple definition has been widely used, current literature has clearly shown that chronic ankle instability is a complex, multifactorial condition that is not yet fully understood.3
Fatigue has been defined as the decrease in the ability to produce force and may be an important factor when attempting to understand the contributions of intrinsic and extrinsic factors that lead to joint instability.4–6 Ankle stiffness (the amount of resistance to external force or perturbation) is considered an effective measure of joint stability and acts as a direct measure of the inherent protective mechanisms to injurious perturbation at the ankle. Overall, joint stiffness is composed of both active (muscle) and passive (tendon, ligament, and joint alignment) components that work in concert to maintain stability prior to and in response to joint perturbation. Although the passive components of stiffness are essential to stability at rest, it has been clearly shown that active contributions are required to maintain stability throughout functional tasks.7
Joint stability is essential to preventing ankle sprain due to excessive motion during activity and is directly affected by ankle joint stiffness.7,8 Therefore, any lack in the ability to effectively produce force or sense changes in force production may decrease ankle stiffness and subsequently decrease the stability of the ankle joint.7,9 It is currently unclear whether the decreased ability to produce force associated with muscle fatigue has a direct effect on stability; however, because of its potential effect on joint stiffness, it can be hypothesized that an alteration in ankle stiffness due to fatigue may lead to increased predisposition to lateral ankle sprain during athletic activity.6
Functional activity can lead to fatigue if it is of sufficient volume, duration, or intensity. During repetitive daily activities and athletic participation, fatigue occurs at both the spinal and muscular levels.4,6,10–12 This fatigue leads to alteration in both muscle activation and force production that may affect the ability of the dynamic restraints to provide maximal stability to a joint. Currently, the effects of chronic ankle instability during a fatiguing activity have not been studied in depth. However, these effects may be important to both the recreationally active and athletic populations due to high occurrence of chronic instability within those populations and its potential effects on injury rates and performance.2,13–17 Therefore, the purpose of this study was to measure the effects of induced fatigue on muscle activation and ankle stiffness in response to frontal plane ankle perturbation in healthy participants and participants with chronic ankle instability.
Forty physically active participants (14 men and 26 women, age range = 21.7±2.5 years, height = 173.3±11.0 cm, weight = 74.7±20.9 kg) were recruited from the population of a Division I university (Table 1). A sample size of 40 participants was deemed to be appropriate and consistent with previous investigations using the inversion-eversion swaying cradle method of measuring ankle stiffness.9Physically active was defined as participating in recreational, club, or varsity athletics at least 3 times per week for a minimum of 30 minutes per session. Participants had no history of severe lower extremity injury that required physician’s assessment within 6 months of testing, no current symptoms of an acute ankle sprain, and no history of ankle fracture and were not currently participating in physical therapy.
Table 1: Participant Demographics Across Stability Conditions
Participants were stratified into 2 groups of equal numbers: those with stable ankles and those with chronically unstable ankles. Participants were included in the chronic instability group if they had a history of ankle sprain to their dominant or non-dominant side that resulted in a score greater than 5 on the Ankle Instability Instrument.18 Participants in the stable group were matched to participants in the chronically unstable group based on gender and dominance of the chronically unstable ankle. All participants were required to read and sign an informed consent document that was approved by the institutional review board.
Ankle Instability Instrument
Chronic ankle instability was defined as a subjective feeling of recurring giving way during daily functional activity following an initial ankle sprain. Screening for chronic ankle instability occurred using the Ankle Instability Instrument, which has been demonstrated as a valid and reliable indicator of chronic ankle instability.18 A cutoff parameter for the Ankle Instability Instrument has been established as a “yes” answer to question 1, as well as a “yes” answer to no less than 4 other yes-or-no questions.18 The free response portions of the Ankle Instability Instrument were used in stratifying the groups in this study.
A Delsys Bagnoli-8 hard-wired electromyography (EMG) system (Boston, Mass) was used, with differential amplification using an 8-channel amplifier. The EMG signal passed through an A/D converter (National Instruments, Austin, Tex) sampling at 1000 Hz. Raw EMG data were collected using DataPac version 2k2 software (Run Technologies, Mission Viejo, Calif) on a personal computer. Raw EMG data were rectified, smoothed, and filtered prior to processing. Data were smoothed via a root mean squared 20-ms sliding window function and filtered via 4th order Butterworth band pass filter (20 to 350Hz) in a custom Matlab (The Mathworks Inc., Natick, Mass) program.
Inversion-Eversion Swaying Cradle
Active ankle stiffness was measured via an inversion-eversion swaying cradle device that has been demonstrated as being valid and reliable.19 Trial-to-trial reliability (intraclass correlation coefficient, [2,1]) has been shown to be 0.96, with a standard error of measurement of 2.05 Nm/rad, and day-to-day reliability (intraclass correlation coefficient, [2,k]) has been shown to be 0.93 and a standard error of the measurement of 3.00 Nm/rad.19
Peroneus longus fatigue was produced via a sustained hold eversion fatigue protocol using a custom-built 500-pound load cell (Honeywell, Golden Valley, Minn) bench apparatus. The apparatus was secured to a padded treatment table in the Sports Medicine Research Laboratory (Figure 1). A small rubber button was secured to the load cell to allow participants to apply compressive force to the load cell via ankle eversion. Force output (pounds) was monitored via a custom LabView (National Instruments) program with a visual display on a nearby computer.
Figure 1. Load cell apparatus setup.
Peroneus longus EMG activity was recorded for 3 maximum volitional isometric contractions (MVIC) lasting 5 seconds each via isometric eversion on the load cell apparatus. Electromyography was used to measure muscle activity of the peroneus longus. The skin was prepared by shaving the area of maximal bulk for each muscle, cleaning the area with alcohol, and lightly abrading the area to ensure good electrode contact and transmission. A bar silver/silver chloride surface electrode (Delsys Inc., Boston, Mass) was fixed onto the point of maximal muscle girth of the peroneus longus muscle belly with 2 bars lying perpendicular to the muscle fibers. Electromyography electrodes were fixed using adhesive collars and athletic tape. A reference electrode was placed over the anterior tibia. Manual muscle tests were used to ensure minimal noise and proper electrode placement.
The participants were then seated with their foot in the inversion-eversion cradle for stiffness assessment. The participant’s hip and knee were positioned in 90° of flexion, and a weight equaling 50% of the participant’s body mass was positioned directly over the ankle being tested, as described in Zinder et al.19 A weighted ball was then dropped onto the corner of the cradle device from a known height to perturb the cradle and produce an oscillation. Participants were asked not to assist or prevent the oscillation from occurring. Electromyography data were recorded for 250 ms before and 3000 ms after ball contact with the cradle device. Five trials were collected for analysis. Peroneus longus EMG pre-activation and EMG amplitude were measured for each trial. Electromyography pre-activation was defined as the mean EMG amplitude 250 ms before initiation of perturbation on the inversion-eversion swaying cradle device. Peroneus longus mean EMG amplitude was defined as the mean EMG amplitude for 500 ms after perturbation of the inversion-eversion swaying ankle cradle.
The sustained hold isometric fatigue protocol was completed following the baseline ankle stiffness measurements. Participants were seated on the load cell apparatus supported by a standard treatment table (Figure 1). The participants’ shank was secured to the apparatus with a strap to prevent hip internal rotation and adduction. A foam block was placed against the lateral aspect of the knee to prevent hip abduction. The participants placed their foot in the position of greatest comfort with the base of their fifth metatarsal in contact with the button extending from the load cell. The participants were instructed to push with maximal effort throughout all trials and were given verbal cues, such as “keep going” and “push harder.” The participants were asked to complete one 5-second eversion MVIC to establish peak isometric eversion force output (N).
Participants were then asked to continuously maintain an eversion isometric contraction at a force equal to 20% of MVIC, which was monitored visually on a custom LabView program on a nearby computer monitor.20 Fatigue was achieved when eversion force fell below 10% of MVIC (50% of target) for 5 consecutive seconds.20 Typically, fatigue was achieved after 15 to 20 minutes of sustained hold isometric eversion. Immediately following completion of the fatigue protocol, the participants were repositioned on the inversion-eversion cradle, and 5 trials were used to remeasure ankle stiffness with the procedures described above.
An a priori level of α < .05 was set as the level of significance for all statistical analysis. Three separate 2 (pre-fatigue and post-fatigue) × 2 (stable and chronically unstable) mixed model repeated measures analysis of variance were used to analyze ankle stiffness, peroneus longus EMG pre-activation, and peroneus longus EMG amplitude between groups at baseline and post-fatigue. Statistical analyses were completed using SPSS version 15.0 software (SPSS Inc, Chicago, Ill).
Six participants (3 stable and 3 chronically unstable) were excluded from analysis due to pre-fatigue stiffness or EMG pre-activation measurements that varied greatly from the mean of their respective groups. These outliers were identified as having standardized residuals greater than 2.0.21
There were no significant differences in fatigue status (F1,32 = 0.05, P = .94) or group (F1,32 = 1.13, P = .19), and there was no significant interaction between fatigue status and group (F1,32 = 1.83, P = .19) for ankle stiffness (Table 2).
Table 2: Ankle Stiffness and Muscle Activation Across Fatigue and Stability Conditions
EMG Pre-Activation Amplitude
There was a significant main effect for fatigue status (F1,32 = 4.22, P = .048). Pre-fatigue EMG amplitude (13.67±12.16 %MVIC) was greater than post-fatigue EMG amplitude (9.33±8.31 %MVIC) (Table 2). There was no significant differences for group (F1,32 = .405, P = .53) and no significant interaction between fatigue status and group (F1,32 = 0.19, P = .67).
There was a significant differences in fatigue status (F1,32 = 6.78, P = .01). Pre-fatigue EMG amplitude (24.64±19.11 %MVIC) was greater than post-fatigue EMG activity (18.24±16.30 %MVIC) (Table 2). There was no significant differences for group (F1,32 = .44, P = .51) and no significant interaction between fatigue status and group (F1,32 = 1.16, P = .29).
In previous studies that have investigated the effect of chronic ankle instability on ankle stiffness, no difference has been observed between stable and chronically unstable ankles.22 Through the use of a sustained hold isometric fatigue protocol, we attempted to induce a lasting fatigue of the peroneus longus muscle to measure the effects fatigue would have on ankle stiffness, a direct measure of ankle joint stability. Therefore, the primary finding in this study was the lack of difference in ankle stiffness regardless of group or fatigue condition. Although the lack of change in stiffness may be counterintuitive due to its direct relationship to force production, which is altered by fatigue, this finding is consistent with our research hypothesis regarding ankle stiffness.23
The inversion-eversion oscillation technique for measuring ankle stiffness takes into account the active and passive structures responsible for maintaining joint stability, as well as the preparatory and reflexive components involved with active stiffness. Although this model gives an overall picture of the ankle joint’s global response to perturbation, it may be difficult to quantify differences between groups due to variability in the strategies used to maintain ankle joint stability. Peroneus longus pre-activation and mean EMG were measured in an attempt to characterize alterations in peroneus longus activation that may help explain the lack of change in ankle stiffness both between groups and across fatigue conditions. The rationale for choosing these measures was an attempt to quantify the preparatory and reflexive activity of the peroneus longus muscle that may have contributed to ankle joint stiffness in response to an inversion perturbation.
Our results showed a decrease in peroneus longus pre-activation and mean EMG amplitude when pre-fatigue and post-fatigue measurements were compared. However, there was no difference in either variable when stable and chronically unstable ankles were compared. This is made clearer when the between group effect sizes and confidence intervals were calculated for the dependent variables at both fatigue conditions (Figure 2). Although the point measurement of effect sizes show positive or negative values in all cases, each of the confidence intervals clearly cross zero and therefore make it unclear as to whether there is an effect of chronic ankle instability on the dependent variables measured in this study. These results indicate that although a significant interaction was not found for any of the dependent variables, it is possible that in increased sample size may provide sufficient power to illuminate an interaction, most notably in ankle stiffness, between participants with and without chronic ankle instability in a fatigued state.
Figure 2. Between group effect size and confidence intervals. Abbreviation: EMG, electromyography.
To our knowledge, no investigation into both pre-activation and mean EMG amplitude during sudden inversion has been completed. Past findings with regard to the lack of difference in mean EMG amplitude between groups in combination with a consistent decrease in post-fatigue EMG values compared with pre-fatigue values shows our findings to be in line with the previous literature.24 Although this does not directly explain the lack of difference in ankle stiffness, there are several studies demonstrating no difference in mean EMG amplitude, time to peak torque, or electromechanical delay between stable and chronically unstable ankles. Results in these studies were highly variable and showed little clinically significant differences between groups.25–29
When no change in mean EMG and no change in time to force generation are considered together, it becomes clearer that the lack of change in activation and force generation of the peroneals may contribute to the lack of difference in stiffness between stable and chronically unstable ankles. Docherty et al9 previously measured the relationship between ankle stiffness and force sense in participants with stable ankles and chronically unstable ankles. Although a positive correlation was found between these measures in participants with chronic ankle instability and not in healthy participants, it remains unclear whether these potential differences in ankle stiffness were significant or clinically relevant.
The primary limitation in this study was the potential inefficiency in the fatigue protocol selected. Fatigue protocols are highly variable throughout current literature. In an attempt to ensure that the decreased peroneal force production associated fatigue would remain long enough to complete ankle stiffness measurement, we chose a sustained isometric contraction protocol. This protocol allowed us to target one muscle while achieving a potentially long-lasting fatigue. Previous investigations using a similar protocol found that maintaining an isometric contraction equal to 20% MVIC was sufficient to fatigue upper extremity musculature.20 However, current literature on the kinetics of gait and posture has shown that the force production demands on the peroneus longus throughout functional activity are highly variable but in many cases exceed 20% of MVIC during repetitive motions.30
Although participants’ fatigue met the criteria set at the onset of this investigation in the current study, an evaluation of the median power frequency data showed inconsistent changes when comparing pre-fatigue and post-fatigue maximal volitional contractions across participants. The inability to confirm fatigue with a more quantitative measure that was independent of participant effort, as well as the low force demands imposed on the peroneus longus, remain limitations of this study, and should be addressed in future research. Also, although not significantly different (P = .104), participants with unstable ankles (80.1±26.6 kg) tended to be heavier than those with stable ankles (69.2±11.4 kg). The axial load in the stiffness cradle is determined by the participant’s body weight and due to this factor there may be potential for the measurement of ankle stiffness in participants with a greater body mass index to be confounded. Although this has not been confirmed in an independent investigation, anecdotally investigators within our laboratory have noted a significant increase in stiffness values for participants with high body mass indexes.
Future research should focus on finding a more specific fatigue protocol that may work to fatigue postural leg muscles in a manner that is modeled after a more functionally fatiguing task. This would allow for greater clarity to be achieved in both ankle stiffness and functional performance measures due to both greater confidence in fatigue of leg musculature that is related to that experienced during athletic activity, and the improved ability to transition future results toward clinical interventions that may aid in preventing injury.
There are several factors that may have affected the outcome of this study, including testing position, insufficient fatigue of the peroneals, and a lack of more global fatigue of ankle musculature. However, when comparing our results with those of previous studies, it is clear that there is no measurable difference in ankle stiffness between stable and chronically unstable ankles when tested in a neutral joint position both pre-fatigue and post-fatigue, despite some alteration in peroneal EMG characteristics. Although this finding may not be clinically relevant on its own, in concert with future research it may allow for a clearer understanding of compensatory mechanisms used by athletes with chronically unstable ankles in an attempt to prevent re-injury, as well as the physiological changes that may occur following fatigue that predispose athletes to both initial injury and re-injury.
Implications for Clinical Practice
Chronic ankle instability is a common problem following acute sprain; however, the effects of fatigue on individuals with chronic ankle instability have not been widely investigated. This investigation attempted to clarify the effects of isolated peroneal fatigue on ankle stiffness (a direct measure of ankle stability) and the preparation and response of the peroneal muscles to inversion perturbation. Although it remains unclear whether a more sports-specific fatigue protocol would have greater effects, after exercise peroneal muscle activity and alterations in ankle stiffness were not present and may not be responsible for the subjective feeling of giving way and repetitive lateral ankle sprains experienced by individuals with chronic ankle instability. Therefore, although peroneal muscle strengthening may remain an integral part of rehabilitation protocols, isolated reduction in peroneal force production does not affect ankle stiffness in individuals with chronic ankle stability differently than healthy individuals and may not be responsible for their relative reduction in ankle stability.
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- Docherty CL, Arnold BL, Zinder SM, Granata K, Gansneder BM. Relationship between two proprioceptive measures and stiffness at the ankle. J Electromyogr Kinesiol. 2004;14:317–324. doi:10.1016/S1050-6411(03)00035-X [CrossRef]
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Participant Demographics Across Stability Conditions
Ankle Stiffness and Muscle Activation Across Fatigue and Stability Conditions
|VARIABLE||GROUP||PRE-FATIGUE||POST-FATIGUE||BETWEEN CONDITION DIFFERENCE|
|MEAN||SD||MEAN||SD||MEAN||LB (95% CI)||UB (95% CI)|
|Ankle stiffness (Nm/rad)||Stable||36.58||7.06||35.03||9.1||1.55||−4.14||7.24|
|EMG pre-activation (%MVIC)||Stable||14.14||12.76||10.72||10.82||3.42||−4.85||11.69|
|EMG amplitude (%MVIC)||Stable||25.19||17.21||21.43||16.69||3.76||−8.08||15.6|