The article briefly introduces the rationale behind ACL-Reconstruction (ACL-R) and discusses why commonly applied biomechanical approaches should be balanced with psychotherapeutic methods due to the relationship between poor psychological outcome measures and return to play. ACL-R poses significant and unique challenges for successful recovery and return to play for many athletes due to various psychological, cognitive, and social challenges. As a result, the article conceptualises a solution in the form of using heart rate variability biofeedback (HRVB) as a complementary technique to psychological interventions to improve outcomes in the RSI-11 and TSK-11 and encourages research in this area to confirm the positive impact of HRVB on the successful return to play in this population.

• Anterior cruciate ligament

• ACL reconstruction

• ACL- Return to Sport after Injury

• Heart rate variability biofeedback

• Tampa Scale of Kinesiophobia

Donnelly D, Georgiadis E (2024) Heart Rate Variability Biofeedback in ACL Rehabilitation to Optimise Psychological Readiness for Successful Outcomes in Returning to Sport. Ann Sports Med Res 11(1): 1222.

ACL: Anterior Cruciate Ligament; ACL-R: ACL Reconstruction; ACL- Return to Sport after Injury (RSI); TSK-11: Tampa Scale of Kinesiophobia; HRVB: Heart Rate Variability Biofeedback; ITB: Iliotibial band; MSK: Musculoskeletal; MDT: Multidisciplinary Team; HRV: Heart Rate Variability

Anterior cruciate ligament (ACL) injury is one of the most common procedures in sports medicine [1]. This article will briefly outline the rationale for ACL reconstruction,while discussing potential neurophysiological mechanisms that contribute to worsened psychological state and return to play outcomes. The article will then propose a solution for Physiotherapist’s to improve psychological outcomes in the form of heart rate variability biofeedback (HRVB) to ensure that Physiotherapy interventions concomitantly address functional and psychological outcomes in rehabilitation after ACL reconstruction (ACL-R).

The rationale for ACL-R is usually based on biomechanical reasoning. The surgical procedure usually involves taking an autograft from the individual’s hamstring, patella or adductor tendon to replace the damaged ligament, and is increasingly performed alongside Iliotibial band (ITB) tenodesis to further reduce recurrent ACL rupture [1]. The tendinous tissue is then proposed to go through healing, proliferation and maturation [2] so it is able to perform its functional role of providing stability against anterior tibial translation and internal rotation [3].

The rehabilitation process usually results in excellent outcomes in knee stability and muscle strength following a welldesigned rehabilitation process [4]. Despite excellent physical objective outcomes, there appears to be a disconnect between registering good physical outcomes and successful return to sport. Several meta-analyses discovered that compromised psychological readiness increases the risk of an unsuccessful return to sport [5-8]. Moreover, an observational study (16) found that greater self- reported fear reported in the Tampa Scale of Kinesiophobia (TSK-11) correlated with lower single leg hop performance, quadricep strength and increased risk of a re-rupture within 24 months. The importance of positive psychological outcomes are further supported by (9;10) who found that better ACL- Return to Sport after Injury (RSI) outcomes are strongly and significantly associated with return to sport. Faleide and colleagues (33) also found that their cohort of active sporting participants were less likely to return to sport with a low score on the ACL-RSI scale [11]; scores 0 to 100; low responses > 47), and that this was more predictive than functional outcomes.

Recent evidence supports the notion that functional neuroplastic changes occur in people with ACL injury [12]. Research shows a link between ruptured ACL and greater anterior cingulate gyrus activity levels which helps to explain increased emotional tone and subsequent fear perception can increase sensitivity and pain intensities when exposed to passive visualisation tasks that evoked their fear of harm in other musculoskeletal (MSK) conditions [14]. Therefore, clinicians should view ACL injuries not only as a musculoskeletal pathology, but also as a neural lesion with neurocognitive and neurophysiological aspects [15]. This could help to explain kinesiophobia and clinical outcomes discovered by Paterno and colleagues [16] who found that greater self-reported fear and lower TSK-11 scores worsened functional outcomes and return to sport.

While protocols such as the Melbourne protocol offer a comprehensive framework to bolster outcomes post ACL-R, it is reasonable to state that Physiotherapists spend less time addressing techniques to tackle the two psychological outcome measures included in the protocol. The previously mentioned TSK-11 and ACL-RSI scales measure an individual’s subjective psychological state which relates to catastrophizing and kinesiophobia status. While this shows a clear understanding of psychological involvement and the importance of patient reported outcome measures, there is an absence of psychological strategies within Physiotherapy ACL protocols to the authors knowledge, which means that Physiotherapists might be ill equipped to address associated psychological or emotional fears. This is compounded by research which indicates that many Physiotherapists believe that they lack the skills to induce psychological change [17].

While professional outfits have the luxury of a psychologist within their multidisciplinary team (MDT), many semiprofessional and amateur athletes do not have access to such professionals. In the absence of a qualified psychologist, and given that Physiotherapists usually have the most contact time with athletes post-operatively, it seems reasonable to propose feasible psychotherapeutic strategies that can be used within the clinical environment to complement conventional rehabilitation [17].

Current literature seems to suggest an abundance of reasons for the offer of psychological support to athletes undergoing ACL-R rehabilitation, with negative thinking patterns contributing to anxiety, fear and depressive symptoms during this period acting as a major obstacle towards the quest for a successful return to sporting action. Presented with such a challenge, the solution seems to relate to an effective application of cognitive restructure methods while retaining personal control, competence, and relatedness, with the overarching goal to achieve adaptive behavioural responses towards a successful return to competitions [18-20]. Within such psychological support, certain techniques seem to help the successful return to play, with goal setting, relaxation, imagery, positive selftalk, adaptive modelling and counselling showing increased effectiveness and predominant position as mental techniques/ procedures of choice [21].

One solution that can significantly support the effectiveness of these psychological techniques and strategies, is in the form of heart rate variability biofeedback (HRVB). HRVB is a method to increase Heart Rate Variability (HRV). During HRVB individuals learn to breath slowly, at a rate close to 6 breaths per minute that induces various positive results in the autonomic nervous system via two reflexive mechanisms, the Respiratory Sinus Arrhythmia, and the Baroreflex heart receptors [22]. Such slow breathing induces the activation of the parasympathetic system -mediated by the Vagus Nerve- creating feelings of relaxation and calmness. Additionally, HRVB has been linked to significant positive effects on cognitive, social and physiological components of the human system, with a recent meta-analysis supporting modest by highly significant positive effects on a large array of physical, cognitive, behavioural and cognitive conditions [23].

Associating the reality of athletes recovering from ACL-R to studies specifically assessing important components of mental health, improved HRV outcomes via HRVB are associated with better emotional wellbeing [24], lower levels of worry [25], lower anxiety [26] and better regulation of emotions [27]. While no direct evidence exists which examines the relationship between HRVB interventions, ACL-RSI and TSK-11 to the authors knowledge, there is research which demonstrates a link between compromised HRV and other musculoskeletal pathologies including neck pain [28]. Interestingly, there is evidence showing that improved HRV in response to HRVB improves symptoms in chronic neck pain [28]. Catastrophizing and kinesiophobia are common terms associated with neck pain [29] which parallels the language used in the ACL-RSI and TSK-11 scales that includes nervousness, fear and feeling afraid. A more germane article showed mixed results in that a medium to large effect size was found in the HRVB group versus control in a group of 28 athletes for improvements in catastrophizing and psychological distress, although the results were non-significant [30]. Despite this, HRVB could offer a solution to address catastrophizing and kinesiophobia, as HRVB has been shown to alter functional brain connectivity in emotional and executive regions of the brain [31].

HRVB can be used alongside other psychological methods to address the factors mentioned throughout this article since this method can be easily learned, increasing the effects of interventions aiming towards improved psychosocial functioning. Based on the results of [23] meta-analysis, HRVB is one of the best additions to the skill set of practitioners working in the settings of behavioural medicine, mental health, and sport psychology. A recent meta-analysis also confirmed the significant positive effects of HRVB on improving depressive symptoms in various psychophysiology conditions showing its potential to improve psychological well-being and quality of life indices for populations in need [32,33]. Hence, future research can assess the effects of interventions combining HRVB with other psychological techniques such as cognitive restructuring, psychosocial interventions and counselling, to establish whether it could improve ACL- RSI and TSK-11 scores and subsequent successful return to play.

ACL-R poses significant and unique challenges for successful recovery and return to play for a large number of athletes due to various psychological, cognitive, and social challenges. This wellexplored phenomenon requires the design of a carefully crafted psychological intervention which requires additional resources and expertise. In this short perspective paper, we propose the addition of HRVB as a complementary technique to psychological interventions taking place during the rehabilitation period from ACL-R. Recent studies and meta-analytic evaluations support the efficiency of HRVB to reduce negative psychological responses in various psychophysiology conditions, supporting mental health and well-being of populations in need. Future studies need to confirm the positive impact of HRVB on the successful return to play both as an adjunct to existing psychological interventions and as a sole component of interventions aiming for positive mental, emotional and social responses during ACL-R rehabilitation and return to play stages.

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