One of the biggest challenges in ACL rehabilitation is getting a satisfied quadriceps strength. Previously research has shown quadriceps deficits in knee extensor strength, neuromuscular control, and proprioception remain up to 2 years after surgery (Nagelli and Hewett 2017), and these deficits are also present in the contralateral non-injured leg (Chung et al. 2015; Lepley et al. 2015; Negahban et al. 2014; Zult et al. 2017). The impairments are likely a result of aberrations in the sensorimotor system following an ACL injury and ACLR (Needle et al. 2017; Nyland et al. 2017). Therefore there is a need to find an optimal way to target quadriceps weakness in the early phase after ACL reconstruction due to knee pain, effusion, and concerns about graft elongation when loading the quadriceps van Melick et al. 2016.
Potential for minimizing its negative effects of immobilisations after injury or after surgery. When we look at the mechanism of how it is meant to work, most of the evidence supports a neuromuscular mechanism with both cortical and spinal effect and it seems to improve cortical neural drive. But it also seems to have an effect on spinal excitability
Recently, cross-education training has been proposed as a neurophysiological phenomenon where an increase in strength is achieved within the operated limb following strength training in the healthy contralateral limb. It is hypothesized that this effect is a result of a cortical and spinal level that improves the neural drive allowing the muscle inactive muscle to facilitate positive adaptations in brain activation, leading to improved muscle recruitment. Thereby supporting the implementation of cross-education exercise during the initial stages after ACLR to improve post op quadriceps strength recovery.
Papandreou et al. 2013 found in 42 patients that cross-education in addition to standard care resulted in a quadriceps strength-sparing effect and reduced asymmetry at 8 weeks post-surgery. The program consisted of an eccentric exercise program applied to the quadriceps of the non-injured leg knee. 5 set of 6 reps at 80% of 1RM were performed in leg extension machine during a 8 week intervention program
Similar findings were reported in an RCT by Harput et al, 2019. In this study, 48 patients were randomly divided into three groups when they reached four weeks post surgery: (1) concentric CE (n = 16); (2) eccentric CE (n = 16); and (3) control (n = 16). All groups followed the same post-surgical rehabilitation program for their reconstructed limb. Additionally, the two experimental groups followed 8 weeks of isokinetic training for the uninjured knee at 60°/s for 3 days per week for 4 weeks. Quadriceps strength of both knees was greater in concentric and eccentric CE groups compared to control group during the 12th- and 24th weeks post surgery (p < 0.05) and the strength gain was 28% and 31% in concentric and eccentric CE groups, respectively, when compared with the control group. Concentric and eccentric CE had similar effects on quadriceps strength recovery. The authors concluded that Cross-educational should be integrated into ACLR, especially in the early rehabilitative phases to restore quadriceps strength.
However, Zult et al. 2018 did not get the same promising results. They included 43 patients for supervised training of the non injured leg, twice a week, from week 1-12 post ACL surgery with 3 sets of 8-12 RM (gradual exposure) with 1-2 min rest between sets. This was compared to a 26 weeks standard rehabilitation program.
The isometric MVCs did not differ between the experimental and control group, although the experimental group at week 26 scored 3.4 Nm/kg indicating that quadriceps strength was recovered well. A torque of at least 3.0 Nm/kg is related to good patient-reported outcome (Pietrosimone et al. 2016). One explanation of why Zult did find the same result as the other studies could be that the intervention group only performed cross-education training twice a week. This illustrates that there might be a dose-response relationship in cross-education training, recommended at least 3 times per week. Although further research should be conducted to investigate the long-term effects of cross-education training in ACLR.
In a narrative review by (Bucktorpe and Della Villa, 2019), their advice is to include strength training for both limbs as part of the ACL functional recovery process. For example, 6 sets of leg press for the injured side at 12 repetition maximum (RM) would be complemented with 3–4 sets of 3–5RM for the uninjured side.
Buckthorpe M, Della Villa F. Optimising the ‘Mid-Stage’ Training and Testing Process After ACL Reconstruction. Sports Med. 2020 Apr;50(4):657-678. doi: 10.1007/s40279-019-01222-6. PMID: 31782065.
Harput G, Ulusoy B, Yildiz TI, Demirci S, Eraslan L, Turhan E, Tunay VB. Cross-education improves quadriceps strength recovery after ACL reconstruction: a randomized controlled trial. Knee Surg Sports Traumatol Arthrosc. 2019 Jan;27(1):68-75. doi: 10.1007/s00167-018-5040-1. Epub 2018 Jun 29. PMID: 29959448.
Papandreou M, Billis E, Papathanasiou G, Spyropoulos P, Papaioannou N. Cross-exercise on quadriceps deficit after ACL reconstruction. J Knee Surg. 2013 Feb;26(1):51-8. doi: 10.1055/s-0032-1313744. Epub 2012 May 15. PMID: 23288773..pdf
Zult T, Gokeler A, van Raay JJAM, Brouwer RW, Zijdewind I, Farthing JP, Hortobágyi T. Cross-education does not accelerate the rehabilitation of neuromuscular functions after ACL reconstruction: a randomized controlled clinical trial. Eur J Appl Physiol. 2018 Aug;118(8):1609-1623. doi: 10.1007/s00421-018-3892-1. Epub 2018 May 23. PMID: 29796857; PMCID: PMC6060748.
Zult T, Gokeler A, van Raay JJAM, Brouwer RW, Zijdewind I, Farthing JP, Hortobágyi T. Cross-education does not improve early and late-phase rehabilitation outcomes after ACL reconstruction: a randomized controlled clinical trial. Knee Surg Sports Traumatol Arthrosc. 2019 Feb;27(2):478-490. doi: 10.1007/s00167-018-5116-y. Epub 2018 Sep 4. PMID: 30182287.