Abstract
Objectives:
Clinical practice guidelines for rehabilitation following ACL reconstruction (ACLR) emphasize the use of functional and strength testing, patient reported outcomes, chronobiological age, and physician expertise. While there are no current internal measures used to aid in the decision of a safe and timely return to sport, recent literature has suggested that muscle oxygen saturation (SmO2) may be an important marker to indicate the internal physiological recovery of the quadriceps following ACLR. The goal of this study is to answer the two questions: 1) Do healthy athletes have a difference in SmO2 between legs when subjected to aerobic and strength-based exercises? 2) Do patients post-ACLR differ from healthy athletes, and can we track the recovery of normal physiology over 1 year?
Methods:
Fifty healthy athletes and 39 surgical athletes from the ages of 14-22, were recruited to participate in clinical trial NCT05488054. Healthy athletes had no history of knee surgery, while surgical athletes had history of isolated ACLR without contralateral limb injury and/or surgery. Healthy athletes participated in one research trial, while surgical athletes participated in a trial at 6-, 9-, and 12-months post-op. At each trial, a non-invasive and fully wireless Moxy™ muscle oxygen sensor was placed over the athlete’s vastus medialis (VMO) muscles bilaterally. Bilateral muscle oxygen saturation was collected during the following exercises: 1. Fan-bike TABATA exercise, 8 intervals of 20 seconds cycling followed by 10 seconds’ rest. 2. Fan-bike 1-minute max effort. 3. Isometric holds at 60 degrees flexion for maximum time 4. Unilateral single-leg leg-press for max reps (goal 8-10 reps). 5. Bilateral leg-press for max reps (goal 8-10 reps). Moxy™ sensors would collect data continuously over the course of each trial. Upon completion of the trial, data was transferred to a laptop for statistical analysis. Statistical analysis was only completed for bilateral exercises (TABATA bike, minute max effort bike, bilateral leg press) to minimize time spent exercising as a confounding variable. For all analysis, individual readings of right and left VMO were paired by time and participant to minimize impact of random fluctuations in sensor sensitivity between days. All statistical analysis was done using R studio version 2022.12.0+353 with the ‘dplyr’, ‘gapminder’, ‘ggplot2’, and ‘reshape2’ packages.
Results:
Fifty healthy participants successfully enrolled and completed muscle oxygen testing. Healthy participants consisted of 21 male and 29 female athletes, with a mean age of 20.1 years old. Sensor tracings for two patients were removed due to recording error, leaving 48 for statistical analysis. Figure 1 represents raw continuous data produced by Moxy™ Sensor in a healthy athlete. In healthy athletes, all exercises were distributed normally around 0% difference in O2 saturation between the right and left limbs (Figure 2A and 2B). Paired t-testing showed no statistical difference in mean SmO2 of right and left limbs during TABATA (p = .19, 1-minute fan bike p = .8, and bilateral leg press p = .31). Mean difference and 95%CI are reported in Table 1. 39 surgical participants enrolled in NCT05488054. At the time of submission, 28 participants completed 6-months post-op testing, 18 participants completed 9-months post-op testing, and 5 participants completed 12 months post-op testing. At 6-months post-op the participants surgical limbs were found to have significantly higher SmO2% during all exercises when compared to the contralateral healthy limb (tabata: p<2.2E-16, 1-minute fan bike: p<2.2E-16, leg-press p = 2.8E-13). At 9-months post-op, participants surgical limbs continued to record significantly higher SmO2% readings during exercise compared to the healthy limb (tabata: p<2.2E-16, 1-minute fan bike: p = 2.2E-6, leg-press p = .048). At 12-months post-op, SmO2(%) readings were no longer statistically significant for leg-press (p =.20) and 1-minute fan-bike (p=.09) but did differ with tabata (p < 2.2e-16). Figure 3 highlights data for the surgical cohort at 6-, 9-, and 12-months post-ACLR. Mean difference in SmO2 trended towards 0% from 6 to 12 months and are reported in Table 1 with direct comparison to healthy athletes.
Conclusions:
Overall, this study answers the two main questions. Question 1) Do healthy athletes have a difference in SmO2 between legs when subjected to aerobic and strength-based exercises? Testing and analysis of 48 healthy athletes without history of surgery show no statistical difference between right and left limbs during exercise (Figure 2A and 2B). Exercise-specific means are distributed around a difference of 0% SmO2 between legs (Figure 2A) and raw tracings exhibit tight overlap between right and left limbs. Question 2) Do patients post-ACLR differ from healthy athletes, and can we track the recovery of normal physiology over 1 year? Surgical athletes do differ from healthy athletes, and these differences do normalize over time. In surgical athletes, raw tracings highlight that quadriceps on the surgical limb are not able to deoxygenate normally when compared to contralateral healthy limb (Figure 3). A visual gap between tracings is seen in the raw data during activity. While shorter exercises (1-minute bike and leg press) see a return of normal physiology by 12 months, exercises of longer duration remain statistically different (Figure 3 – TABATA at 12 months). This data suggests that while athletes improve over time and may reach external parameters for return to play by 9-months post-op, internal muscle physiology continues to recover up-to and potentially beyond the 12-month time point. Further testing is required to validate these findings, and correlation with data such as quadriceps size, return-to-play testing results, and PRO’s can help further delineate improved return-to-play parameters.
