Anterior cruciate ligament (ACL) tears are a common injury with up to 200,000 ACL reconstruction (ACL-R) surgical procedures performed annually.1 For an ACL-R athlete, sports medicine professionals determine eligibility for return to sport (RTS) based on several factors. Hopping assessments, specifically single-leg maximal hop and hold (SLH), have commonly been included.2–4 Body-worn inertial measurement units (IMUs) have been utilized to establish a strong correlation between tibial acceleration and peak landing forces during a vertical jump5; however, only one study has reported on their use during SLH.6 Using a trunk-mounted accelerometer via double-sided tape (DST), Williams et al6 reported that a measure of postural sway to quantify hop landing balance had moderate-to-excellent reliability.6
The integrity of fixation between accelerometer and body influences the measurement signal because of noise introduced from device resonance.7 As the vertical ground reaction forces are considerably larger during hopping8 than running, it is unclear if these large landing forces would cause device oscillation inducing an increased and faulty acceleration magnitude. To mitigate measurement noise, the most accurate fixation method uses intracortical bone pins9; however, this method is not feasible from a sports medicine perspective. With appropriate fixation and digital filtering, the acceleration signal from a skin-mounted device adequately corroborates with actual bony accelerations.9 The majority of reports have fixated accelerometers through some application of double-sided tape10 or elastic bands.7 Currently, there are no studies that have analyzed the influence of fixation method on acceleration magnitudes during the SLH landing phase.
Therefore, the purpose of this study was to evaluate the influence of fixation method of a lightweight IMU device mounted to the tibia during SLH landing. The most commonly reported fixation method (DST) was compared with a silicon strap with Velcro adhesion (SS). It was hypothesized that SS would produce comparable acceleration magnitudes to DST fixation.
Methods
Design
A single cohort, repeated-measures design was used to investigate the relationship among hop distance, peak tibial acceleration (PTA), time to PTA and peak tibial acceleration slope (AS; dependent variables), and tibial IMU fixation method (DST and SS; independent variable) during performance of the SLH.
Participants
Participants were free of any current or previous lower-extremity injuries in which medical care was sought, and they had also engaged in regular exercise for the previous 3 consecutive months. Testing procedures were approved by the Sacred Heart University’s institutional review board. All participants (10 females and 6 males; 20 [0.9] y; 1.67 [0.08] m; 66.0 [8.5] kg) signed a health history form and provided informed consent.
Procedures
A lightweight (9.5 g) IMU (Blue Trident; Vicon, Auckland, New Zealand) was utilized; it featured both low-g (±16 g) and high-g (±200 g) triaxial accelerometers. Measurement tape was used to assess ankle circumference and mark a location 3 cm proximal to medial malleolus.10 Two fixation methods were studied as follows: (1) DST10 and (2) manufacturer-supplied SS11 (Vicon; Figure 1).
—A lightweight (9.5 g) and portable inertial measurement unit (Blue Trident) with triaxial accelerometers was attached to the distal tibia 3 cm proximal to the medial malleolus with silicon straps and Velcro adhesion.
Citation: Journal of Sport Rehabilitation 30, 5; 10.1123/jsr.2020-0309
Anthropometric measures (height, weight, and ankle circumference) were collected, including leg dominance, which was defined as the leg used to kick a ball.2 The participants completed a standardized warm-up (jumping jacks and bilateral walking lunges) before being given a verbal and visual SLH demonstration. Specifically, the participants were instructed to hop as far forward as possible and hold the landing position for approximately 3 seconds while maintaining their arms behind their back.
The IMU device was attached to the dominant leg in a counterbalanced order. Ankle circumference thresholds (small: ≤19 cm, medium: 19–24 cm, and large: ≥24 cm) were utilized for proper fitting of SS, which was iteratively tightened as “tightly as tolerable” by the participant via Velcro adhesion (Figure 1).
Following 2 SLH practice trials, the participants completed 3 trials while maintaining their hands behind their back12 and holding the final landing position for approximately 3 seconds. Hop distance was measured for each trial, and 30 seconds of rest was granted between trials. Participants had 5 minutes of rest between the attachment method conditions. Data collection was initiated using the Capture.U application (1500 Hz; Vicon).
A MATLAB script (MathWorks, Natick, MA) filtered acceleration data (x, y, and z axes) with a 60-Hz, fourth-order, dual-pass Butterworth low-pass filter before resultant acceleration was computed as
—Representative resultant acceleration data for a participant during the landing phase of the single-leg hop. Three dependent variables were determined for each trial: PTA, time to PTA, and acceleration slope.13 PTA indicates peak tibial acceleration.
Citation: Journal of Sport Rehabilitation 30, 5; 10.1123/jsr.2020-0309
Statistical Analysis
Descriptive statistics were calculated for all dependent variables from the maximal hop distance trial from both fixation conditions. Repeated-measures analysis of variance statistical tests assessed differences in hop metrics based on fixation method. Bonferroni pairwise comparisons post hoc tests were performed to evaluate the effect of the fixation method. The significance level was set a priori at α = .05. The effect size was determined by the partial eta-squared (
Test–retest reliability was assessed using 2-way mixed-effects intraclass correlation coefficients (ICC3,k). An ICC value ≥.9 was considered a “high” level of agreement, ≤0.7 was “poor,” and between 0.7 and 0.9 was “questionable.” The standard error of the mean and minimal detectable change (MDC95%) were used to assess natural trial variability for both attachment methods. To explore the relationship between hop distance and PTA, a Pearson correlation analysis was performed on the maximal hop distance trial. All statistical analysis was performed in SPSS software (version 25.0; IBM Corp, Armonk, NY).
Results
Descriptive statistics for all dependent variables are provided in Table 1. Mauchly’s test of sphericity indicated that the sphericity assumption was violated (P < .05); therefore, a Greenhouse–Geisser adjustment was used. Repeated-measures analysis of variance based on attachment method revealed no significant main effect (F1,15 = 1.42, P = .252,
Descriptive Statistics for SLH Test With Both Fixation Methods
| DST (n = 16) | SS (n = 16) | |||
|---|---|---|---|---|
| Hop Metrics | Mean | SD | Mean | SD |
| Hop distance, cm | 102.9 | 24.9 | 103.8 | 25.5 |
| PTA, g | 27.22 | 7.94 | 26.21 | 10.48 |
| Time to PTA, ms | 9.50 | 2.48 | 10.17 | 2.54 |
| AS, g·ms−1 | 2846.53 | 1131.95 | 2505.19 | 1585.72 |
Abbreviations: AS, acceleration slope; DST, double-sided tape; PTA, peak tibial acceleration; SLH, single-leg hop; SS, silicon straps.
Both fixation methods had “high” ICC values (DST = .916 and SS = .936) for PTA (Table 2). At the 95% confidence level, the MDC for PTA was 5.7 and 6.6 g, respectively, for the DST and SS methods. These values represent 22.0% and 25.2% of the 3 trial mean. For peak tibial AS, the SS fixation method produced more reliable results (0.941) versus DST (0.868). At the 95% confidence level, MDC for AS was 802.8 and 978.4 g·ms−1, respectively, for the DST and SS methods. These values represent 31.6% and 38.8% of the 3 trial mean. The only hopping metric wherein fixation methods substantially differed in ICC values was time to PTA. The DST produced poor reliability (0.397) and SS produced questionable reliability (0.768).
Reliability and Variability of SLH Metrics With 2 Attachment Methods
| Hop Metrics | ICC | SEM (%Mean) | MDC95% (%Mean) |
|---|---|---|---|
| Hop distance, cm | |||
| DST | .976 | 6.1 (6.3%) | 16.8 (17.4%) |
| SS | .973 | 6.1 (6.2%) | 16.8 (17.3%) |
| PTA, g | |||
| DST | .916 | 2.1 (7.9%) | 5.7 (22.0%) |
| SS | .936 | 2.4 (9.1%) | 6.6 (25.2%) |
| Time to PTA, ms | |||
| DST | .397 | 0.79 (7.5%) | 2.18 (20.7%) |
| SS | .768 | 0.58 (5.7%) | 1.6 (15.9%) |
| AS, g/ms | |||
| DST | .868 | 289.6 (11.4%) | 802.8 (31.6%) |
| SS | .941 | 352.9 (14.0%) | 978.4 (38.8%) |
Abbreviations: AS, acceleration slope; DST, double-sided tape; ICC, intraclass correlation coefficient; MDC, minimal detectable change; PTA, peak tibial acceleration; SLH, single-leg hop; SS, silicon straps.
The relationship of maximal hop distance to PTA demonstrated a significant moderate positive correlation for both attachment methods (DST: r = .72, P < .01 and SS: r = .77, P < .01).
Discussion
The results of the current study demonstrate that both fixation methods produce comparable acceleration metrics during SLH. The main advantage of utilizing SS versus DST when attaching a wearable tibial IMU device is in the ease of use. Once the SS is sized appropriately, the device can more easily be tightened by the athlete and does not require additional materials.
Although maximal SLH distance is typically utilized to aid in RTS decision making, most protocols involve repeated SLH trials. When analyzing across all trials, the statistical analysis revealed that both fixation methods had similar “high” reliability for PTA with a low standard error of the mean (DST = 7.9% and SS = 9.1% of mean). This corroborated the 1-week and 6-month excellent reliability in detecting PTA during running with the same device10 and represented improved reliability than that reported for detecting the landing hop phase with a trunk accelerometer.6 Interestingly, the reliability of SS fixation was slightly improved for determining the peak tibial AS than DST. One possible explanation for this difference is that the SS Velcro may allow the device to be iteratively adjusted to tolerance, which is more difficult than DST. Time to PTA produced either poor (DST) or questionable reliability (SS) obviating its potential use in assessing SLH landing with this device. Based on the current results, PTA is a reliable metric to monitor during the SLH landing phase.
When analyzing across all participants, the mean PTA was 27.22 (7.94) g for DST and 26.21 (10.48) g for SS (range: 12.6–53.5 g), which is substantially higher than for running (7.8–12.9 g).10 Although this is not surprising given the increased vertical ground reaction forces during SLH, it does highlight the necessity of an increased accelerometer operating range for assessing SLH. Mitschke et al14 reported that the accelerometer operating range (±8, ±16, ±32, and ±70 g) significantly underestimated PTA during running when the operating range was less than ±32 g. The device used in this study had an operating range of ±500 g, which was approximately 9.4 times the maximum PTA.
In the current study, Pearson correlation analysis revealed a positive, moderate relationship with approximately 72% to 77% of PTA variance explained by SLH distance. This demonstrates that PTA during the SLH may provide meaningful quantification of the landing phase beyond just monitoring hop distance; however, the MDC of PTA was 22.0% and 25.2% of the mean value, respectively, for the DST and SS attachment methods. These statistics should be considered when determining RTS thresholds for bilateral comparisons of the PTA during the SLH. With reinjury rates between 15% and 23%, an enhanced ability to easily quantify and assess movement quality may aid the sports medicine professional in RTS decision making and potentially lower reinjury rates.15 Future studies should expand upon this work by assessing normative bilateral PTA during the SLH and, subsequently, utilize this device to monitor these accelerations in ACL-R individuals postsurgery to determine its potential as an RTS criteria.
Conclusions
The current study determined that a wearable tibial accelerometer attached via SS can reliably measure PTA and can be used as a viable alternative to DST and elastic bandages. Assessment of PTA can potentially explain the differences of landing mechanics beyond hop distance. Clinicians should consider utilizing wearable tibial accelerometers to monitor rehabilitating athletes and to determine the efficacies of treatment protocols.
Acknowledgments
The authors report no declaration of interests.
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