A CASE STUDY FOLLOWING ACL RECONSTRUCTION

– Written by Martin Buchheit, Mario Sandua, France, Andrew Gray, New Zealand, Karim Hader, Damien Monnot, Jeremy Volante, and France Delafosse, France

INTRODUCTION

Current research suggests that ACL reconstructions should follow a criterion-based process over successive rehabilitation phases^1,2. Today there is increasing information on the progression criteria and methods to rehabilitate the neuromuscular system (i.e., range of motion (ROM), strength, balance, and control) and restore generic and football-specific functional capacities (e.g., jumping, landing, cutting, accelerating, and decelerating)^1,2,3,4 across each of these phases. There is, however, less information and research on the metabolic conditioning aspect of the post-ACL rehabilitation process^5,6,7,8.

Mitigating cardiorespiratory fitness deconditioning or even developing an athlete’s physical fitness to a higher level than before injury during the on-field rehabilitation phase   may be an overlooked factor of a successful return-to-sport process^6,7,8. In fact, higher fitness levels translate into a lower relative internal load for a given workout, which puts players under optimal conditions when returning to train with the team (return to train phase that follows the on-field rehabilitation phase). This may support better decision-making and decrease cognitive load, which may, in turn, facilitate players’ reintroduction to the intense stimuli of football practice. Importantly in this context, this also likely contributes in reducing the risk of re-injury¹.

Lastly, while there is a growing use of technology to monitor function and muscle strength in the gym (e.g., force plates, dynamometers), methods and tools that can be used in the field during the on-field rehabilitation are still limited.

In the present manuscript, we use the case of a recent ACL injury that happened to a Lille OSC player to introduce our post-ACL reconditioning approach during the first rehabilitation and on-field rehabilitation phases. We provide examples of metabolic work programming through various generic and football-specific drills. We also touch on field-based tools and strategies for monitoring fitness and running mechanics that were used to further guide the programming throughout the process.

THE CASE AND THE INJURY

Informed consent was given by the 23-year-old Portuguese international central defender with several years of experience across top European leagues and at the senior international level since 2022. While playing in French Ligue 1, he ruptured his right ACL during a tackle (Figure 1).

The knee injury during a valgus flexion external torsion traumatism was suggestive of ACL tear with associated lesions. Immobilization by rigid splint and crutch discharge was immediate. Due to a positive Lachman test, an MRI was performed 2 days later.. In addition to the ACL, there was a lesion of the proximal attachment of the anterolateral ligament and a low-grade tear of the medial collateral ligament. The lateral meniscus had a radial tear associated with damage to its posterior root and its popliteomeniscal attachment. The bruise of the posteromedial tibial plateau coupled with the hypersignal of the internal meniscocapsular space suggested a ramp lesion. The practice of a contact pivot sport at the elite level made the surgical indication necessary to restore rotational dynamic knee stability. Considering these findings, and after discussion with the patient, 10 days after the injury, reconstruction was performed by 4 strand hamstring autograft with a reconstruction of the anterolateral ligament (Figure 2)⁹.

Arthroscopy also confirmed the meniscal lesions described in the MRI. The lateral meniscus was repaired with a suture anchor, and the ramp lesion was sutured by two absorbable threads. The surgery was performed without issues and the rehabilitation protocol with a non-weight-bearing protocol started immediately.

TIME FRAME OF THE RETURN TO SPORT PROCESS

At the time of the writing, the player had finished the on-field rehabilitation phase and was about to start the return to train phase (Figure 3). Following the day of injury, it therefore would have taken about 7.5 months for the player to return to full training with the team without restrictions. In a 15-year prospective study on 157 ACL cases in male professional football players, the median layoff after ACL reconstruction was 6.6 months to training and 7.4 months to match play¹⁰. The longer layoff period for the current player is related to the associated meniscus tear that worsened the severity of the injury and delayed the return to running by about a month.

In the present manuscript, we focus on the second part of the rehabilitation stages of the overall process; i.e., the first off-leg phases and the on-field rehabilitation (Figure 3), with a special emphasis on metabolic conditioning programming and monitoring.

METABOLIC CONDITIONING PROGRAMMING

Different submaximal and close-to-maximal intensity metabolic conditioning sessions were delivered throughout the return to sport process, with the format and physiological objectives being adapted to the context¹¹. Context refers to the player’s ability or not to perform on-leg activities such as running (first rehabilitation phase, Figure 3), but may also include the overall weekly puzzle and parallel gym, pitch, and technical contents that need to be aligned and integrated with the metabolic conditioning objectives. For example, while reaching high levels of cardiovascular demands (i.e., >90% of HRmax and VO2max levels) is expected during any efficient form of high-intensity interval training (HIIT) sessions, the associated neuromuscular load can be varied to fit the context¹¹.

For instance, a HIIT session with a low neuromuscular load (so-called Type 1 or 3 following the HIITScience terminology¹¹, Table 1) may be preferred at the start of the first rehabilitation phases to allow progressive overall, better “control”¹², proper muscle and tendon recovery, and importantly also, dissociated physiological stimuli to avoid interference of concurrent adaptations¹¹. During the first rehabilitation phase (Figure 3), all the conditioning is performed off-leg anyway. The objective of this phase is fitness preservation/ limiting deconditioning while allowing proper post-surgery recovery and treatment⁸. This implies the use of non-weight-bearing conditioning activities exclusively, mainly involving upper body exercises (e.g., ErgoSki, Table 2 & 3) - the reality however is that football players are generally not too inclined to perform this type of work, and finding alternatives and variations that stimulate the cardiorespiratory system enough is often a challenge.

In contrast, using HIIT with large neuromuscular demands is at some point also required to elicit specific adaptations and challenge the player at a greater level toward the end of the return to sport process (end of on-field rehabilitation, return to train, return to play, and return to performance), using the so-called Types 2, 4 or 5¹¹ (Table 1). It is very important to progress safely with running speeds and intensity of changes of direction¹³, especially in the case of a hamstring graft as in the present case. Hamstring strength is believed to be an important (but not unique) criterion for speed progression¹⁴; we used isometric and eccentric benchmarks to guide the run-based metabolic conditioning programming (e.g., >70% isometric strength to run >15 km/h - Type 1 & 3  run-based HIIT with work/rest ratio >1, >90% to run >20 km/h and -Type 2 run-based HIIT with work/rest ratio <1, and >90% eccentric strength to reach speeds close to max and maximal change of direction intensities, - Type 2 & 4)¹⁵.

During the last return to train/return to competition phases, it’s essentially about load management and offering complementary and tailored (HIIT) sessions to the player. This includes progressively introducing the player into team sessions while still managing his own load - this often refers to “partial training sessions” (e.g., the player takes only part in some but not all drills of the session, plays small-sided games but as a floater), that are either complemented by a tailored gym, HIIT, pitch work, or recovery and treatment¹⁶. HIIT session contents and physiological objectives need to be perfectly adjusted to the team training content (complementary metabolic and neuromuscular demands), so that the player gets what he needs to complete his rehabilitation process. For example, while the player may be subtracted from a small-sided game that may be too “chaotic” for him (not enough control for him at the stage of the process he is at¹²), a similar metabolic and neuromuscular load can be offered via a more “controlled” individual HIIT workout over the similar duration (Type 4, e.g., in the form of short intervals with sharp changes of directions and inclusion of the ball¹³, Table 1 & 3).

This latter phase with load and contents management is often crucial when it comes to ensuring the player doesn’t get re-injured, not only with regard to his rehabilitated knee but to overall muscle injuries. When elite practitioners were recently surveyed on this particular return to train period, they reported the duration of this phase to be inversely correlated with the duration of absence with the team, with player training in an adapted manner (in comparison to the team) for several weeks at minimum for injuries as long as that of an ACL¹⁶.

HIIT SESSIONS

Overall, we are aiming for sets that last at least 4-6 min (long enough to overcome oxygen kinetics and reach the expected energy turnover) and up to 8-10 minutes per series, for a total volume per session of 10 to 18 minutes of total work¹¹. In fact, since HIIT is a highly time-efficient training method, and given the myriad of other training contents that need to be fit into the daily program of the injured athlete, we aim to microdose this content (almost daily). The distribution and programming of the different formats and associated HIIT types per rehabilitation phase are described in Table 2. We also provide below some detailed examples of typical HIIT sessions used through the return to sport process (Table 3).

LOCOMOTOR LOAD PROGRESSION

The progression in terms of GPS-derived (STATSports, Apex, Sonra 5.11, Newry, Northern Ireland) typical locomotor load metrics is shown in Figure 4. Overall, total distance (TD, m), high-speed running (HSR, m), mechanical work¹⁷ (MW, kJ, a compound measure of accelerations, decelerations, changes of direction, and high-speed actions), and Force Load¹⁷ (Fload, scaled step impulse measured from thoracic-mounted GPS-embedded Inertial Measurement Unit, IMU, kg.ms^-1, ADI software, Athletic Data Innovations, New Zealand) volumes were equivalent during on-field rehabilitation to the training period before injury - except the large volume of TD of competitive matches that could not be compensated.

Interestingly, however, when looking at peak intensity periods (Figure 5), while typical team training and match m/min could be equated or even overloaded during the on-field rehabilitation (Upper panel), this was not the case for mechanical work/min (lower panel). This finding is consistent with the fact that run-based HIIT, which was frequently programmed during on-field rehabilitation (Table 2 & 3), allows players to cover a lot of ground over prolonged periods (>150 m/min >5 min)¹⁸. It is however often difficult to overload mechanical work/min at the session level (even with healthy players during team training) in comparison with match demands, and only some specific drills such as small-sided games with few players (i.e., 3v3 or 4v4) are effective for that¹⁸. This highlights the specificity of match demands and confirms that nothing replaces competition when it comes to performing some specific movements at a high pace (e.g., cutting, decelerating, ball interactions).

MONITORING FITNESS

Only maximal tests performed to exhaustion (i.e., measures of maximal aerobic speed, MAS, or the speed reached at the end of the 30-15 Intermittent Fitness Tests associated, V_IFT - together with respiratory gas exchanges or not), or measures of blood lactate accumulation (e.g., individual lactate threshold or the speed at 4 mmol/L) can provide an absolute assessment of fitness¹¹. Implementing those tests is often challenging in the tight schedule of top football players in between domestic league and international fixtures. For that reason, a short submaximal running test is often used as a proxy for changes in cardiorespiratory fitness¹⁹. At LOSC and as in many similar clubs, players run at 12 km/h on a 200-m course set out on the pitch (Figure 6). The heart rate sustained during the last minute of effort (HRex, expressed as a % of maximal heart rate) provides a very simple and practical and indirect marker of relative (generic) conditioning/fitness level²⁰.

If we consider that changes in HRex of about 4% may be representative of a change in MAS of about 0.5 km/h²⁰, in comparison to his pre-injury values (72% and 76% in January 2023), the player had only lost about 0.5-1 km/h of MAS when he repeated the test for the first time (5 months post-surgery, 79% on August 17th, 2023, Figure 7). This magnitude of cardiorespiratory deconditioning is very low and lower than what was reported by Almeida et al. (2018) when comparing injured and control players (difference of 8 to 11 mL/kg/min of maximal oxygen update, VO₂max, which translates into 2 to 3 km/h of MAS, considering the energetic equivalent of oxygen when running). Whether the player had lost more fitness at some point during the first rehabilitation phases (first 4 months) is unknown, since the submaximal running test could not be performed earlier. We believe nevertheless that the important and almost daily micro-dosed off-leg metabolic work done (Table 2, for a total of >60 Ergoski and Ergobike sessions) definitely helped limit the detraining effect of reduced activity; this likely explains the difference versus the data reported by Almeida et al⁵. Overall, at the time of writing, the player had regained fitness levels close to his best to date, as measured at the end of the 2022 pre-season period (Figure 7). This rate of improvement (1 km/h gained within 3 months) tended however to be smaller than the 2-km/h increase in lactate threshold speed reported by Della Villa²¹ during the on-field rehabilitation phase (3 months) of a group of 50 players (mainly amateurs). The exact dose-response of HIIT and MAS/V_IFT improvement is likely individual and context-dependent; 6 to 8 HIIT sessions over 4 to 6 weeks are generally enough to regain/improve MAS/V_IFT by 1.5 km/h²². In the present case, transient episodes of knee pain (as a consequence of the meniscus surgery) delayed the return to running, and the specific contractual situation of the player through the transfer period was another limitation to the psychological and motivational aspects of his rehabilitation.

MONITORING RUNNING MECHANICS

Running mechanics were assessed using GPS-embed accelerometers and the Athlete Data Innovations software¹⁷. More specifically, the session-average difference (%) between left & right step impulses¹⁷ (force x contact time, measured from thoracic-mounted IMU, ADI software, Athletic Data Innovations, New Zealand) was computed during each training and match, and analyzed specifically during acceleration, deceleration, straight-line running (>7.2 km/h), and while changing direction.

Figure 8 shows that in comparison to pre-injury baseline step impulse symmetry during straight-line running, there was a clear period when returning to running when the left side was unloaded (or the right side, overloaded) - which magnitude (and consistency) was greater than the usual day-to-day variability seen before the injury. This asymmetry was associated with reported right knee pain and required the player to stop running for a few days. Upon return and while running on a softer pitch, both the pain and the asymmetry disappeared. While the reason and direction of asymmetries are always difficult to understand²³, in the present case it can be hypothesized that the greater impulse seen on the right side was caused by a longer contact time, itself related to a stiffer leg complex, which was likely caused by lower absorption capacities of the passive structures of the leg (tendons and joints). We also noted that the left stride length tended to be consistently shorter by about 2 cm (data not shown here), showing again that the right step was less effective overall. Interestingly, this lower right leg effectiveness was also confirmed by typical metrics collected during jumps (CMJ) on a force plate²⁴; during this period of acute pain (early August 2023), concentric and landing impulses were recorded to be 6 and 20% lower on the right than the left side, respectively.

CONCLUSION

The analysis of the present case study has allowed us to present a snapshot of our conditioning and monitoring process at Lille OSC when it comes to rehabilitating players post-ACL surgery. Cardiovascular fitness loss after ACL rehabilitation can result in lower baseline fitness levels during mid- and late-stage rehab, which may delay their return to performance in the long run. We believe that professional players can benefit from appropriate fitness preservation/re-conditioning in the early stages of their rehabilitation, which may help them perform better once they return to training with the team.

Key points:

• It is important to regularly monitor various indices of cardiovascular fitness, ROM, strength, and running mechanics during the season so that when an injury occurs, practitioners can have reference points toward which they can direct the rehabilitation process.
• Submaximal runs can be implemented into (team) warm-ups, and re-introduced quickly into the rehabilitation process, as soon as the player can run at 12 km/h for at least 4 min.
• In the present case, the analysis of changes in exercise heart rate revealed that, at the time of starting the on-field rehabilitation phase, the player had only lost 1 km/h of maximum aerobic speed (about 3-7 mL/kg/min of VO2max)
• The important off-leg metabolic conditioning work done during the first rehabilitation phase (Table 2 & 3, Ergoski & Ergobike sessions) have likely helped to mitigate the deconditioning effect of inactivity.
• The metabolic conditioning work done during the 3-month on-field rehabilitation phase (Table 2 & 3, Run-based and Ergobike HIIT) allowed the player to regain his fitness up to pre-injury levels.
• The analysis of peak intensity periods revealed that reaching high mechanical work intensities are challenging during the individual on-field rehabilitation phases - this is mainly related to the specificity of some football movements that happen only when interacting with teammates and opponents, especially during competition (e.g., cutting, decelerating, ball interactions).
• In addition to usual assessments of lower limb strengths and mechanics (e.g., force trace during jumps on force plates), GPS-embed inertial measurement unit and associated analytics (e.g., ADI software) add invaluable sport-specific information to monitor stride mechanics and step impulse asymmetries throughout the rehabilitation process, that can be used to guide phase progression and day-to-day programming (load management and gym/treatment orientation).
• Finally, while these last conditioning phases of the return to sport process may be first seen as heavy performance staff-led (i.e., strength and conditioning, fitness coaches), we need to emphasize the continuous and parallel collaboration that is always required within the multidisciplinary team. Collaboration and hand-in-hand daily work from all practitioners (i.e., physios, doctors, performance staff) are indeed compulsory for a successful return to sports program¹⁶.

Acknowledgments

The authors thank Germain Saniel for his comments on a draft of the manuscript, and Hugo Dufour for his help with the injury screenshots.

Martin Buchheit Ph.D.^1,2

Mario Sandua²

Andrew Gray³

Karim Hader²

Damien Monnot²

Jeremy Volante²

France Delafosse²

1 Type 3.2 Performance, Montvalezan, France

2 Lille OSC, Camphin-En-Pévèle, France

3 Athletic Data Innovations, New Zealand

Contact:  mb@martin-buchheit.net

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Header image by Games Boyes