Opening the Kinetic Chain Box

Open vs. Closed Chain

Open kinetic chain describes an exercise in which the foot is not in contact with a solid surface, whereas CKC describes an exercise in which the foot is in contact with a solid surface.

Open kinetic chain exercises continue to gain traction in ACL rehabilitation. However, beliefs still exist that they are dangerous and increase graft laxity. Understandably, clinicians would not want to increase the ACL graft laxity risk. However, inadequate quadriceps strength increases the risk of re-injury. Perhaps clinicians are avoiding an essential long-term rehabilitation tool that impacts athlete well-being.

While avoiding OKC exercises, CKC exercises have gained popularity as they support the functional exercise narrative. Furthermore, there is an increased perception that these exercises are safer than OKC as many clinicians believe that CKC exercises strain the ACL graft less due to increased tibiofemoral joint compressive forces, hamstrings co-contraction, and reduced anterior tibial displacement⁽¹²⁾. However, this is not the case. Climbing stairs and lunges place two to three more strain on the ACL than isometric OKC at higher ranges⁽¹²⁾.

Comparing the peak ACL strains during OKC and CKC reveals similar results (see table 1). However, increasing resistance during the OKC exercise produces increases in strain that do not occur during CKC exercises⁽¹²⁾. Interestingly, OKC knee extension against the 24Nm resistance produces a similar strain to a Lachman test, which clinicians frequently perform postoperatively to assess graft stability⁽¹²⁾. 

Table 1: Rehabilitation exercise peak ACL strains (adapted)⁽¹²⁾

Researchers in the Netherlands conducted a systematic review where they created a consensus statement that suggests that CKC and OKC training can be used to regain quadriceps strength. The consensus states that OKC exercises should only be performed by week four postoperative in a restricted range of movement (ROM) of 90– 45°⁽¹³⁾. This is aligned with a systematic review by researchers at the University of Richmond in Australia. It suggests it is sensible to introduce OKC exercises that minimize ACL graft loading after four weeks (i.e., limiting range)⁽⁸⁾.

Clinicians at the Delaware Physical Therapy Clinic at the University of Delaware introduce long arc quad (LAQ) 90-0° within the first postoperative week (see table 2)⁽¹⁴⁾. It is important to note that they have rehabilitation modifications for different graft types that clinicians must consider when doing ACL rehabilitation. For example, allografts are slower to incorporate, therefore rehabilitation progression is slower, hamstring tendon autografts have knee flexion restrictions that respect tissue healing, patellar tendon autografts might need knee extension ROM modifications to avoid possible irritation, and much more (view full rehabilitation program for more details). Their approach advocates for clinicians to utilize OKC exercise in ACLR rehabilitation programs. To support clinical decision-making, clinicians must measure quadriceps strength and educate athletes on the importance of quadriceps strength for long-term knee health and performance⁽⁵⁾.

Table 2: OKC exercise prescription from the Delaware Physical Therapy Clinic⁽¹⁴⁾

For full protocol, visit sites.udel.edu/ptclinic/professionals/guidelines/

Recently (2024), researchers at the Orthosport Rehab Centre in France aimed to assess the association of OKC exercise for quadriceps and hamstring muscle strengthening using knee extensions. They included 103 recreational athletes who had undergone hamstring tendon autograft ACLR. The control group completed rehabilitation three times per week, including only CKC exercise. The intervention group completed a mixed protocol of OKC and CKC exercises⁽¹⁵⁾.

The OKC exercise protocol focused on the quadriceps and hamstring muscles and was introduced two weeks after surgery without resistance. If the participants had a stroke test value <1+, 0-110◦ knee ROM, no quads lag, and between-leg laxity < 1.5 mm on a 134N laximetry test, the clinicians progressed the exercises at four weeks with progressive external resistance. The intervention program included: 
•    Open kinetic chain exercises (isolated leg extension and seated leg curl) on an isokinetic dynamometer. 
•    Isokinetic strengthening: 10 x 8 reps at a speed of 60°.s− 1, three times per week.
•    The resistance was set at 60% of their maximum voluntary isometric contraction (MVIC), evaluated by hand-held dynamometry once a week at 90°.

The study results showed that early (two+ weeks) OKC with CKC at high volumes and moderate intensity can be used in full ROM (0-100) and seems to be safe as laxity was substantially the same at three and six months⁽¹⁵⁾. Furthermore, at both follow-ups, the intervention group had significantly higher quadriceps, hamstring strength, and limb symmetry index than the control. This study highlights the benefits of OKC exercise, particularly on the hamstring strength symmetry index.

Clinical Implications: Exercise Selection

The evidence and practical recommendations for OKC exercise prescription are not standardized across research study design or real-world practice. Therefore, clinicians need to use their clinical reasoning to make decisions. Depending on the clinician’s degree of experience and confidence, the general recommendations are:

• Unweighted inner range quadriceps (OKC or CKC) exercises without resistance provide an excellent option for improving quadriceps strength and preventing a quads lag (see figures 1, 2, and 3).
• OKC exercises can be performed by week four postoperative in a restricted ROM of 90– 45° without weight for HS-graft and with weight for patellar tendon grafts (see figure 4)^(8,10,13).
• Postsurgical pain, swelling, and limited range of motion in the knee prevent exercising at high knee flexion angles.
• Clinicians must consider graft type when prescribing OKC exercise. More caution is warranted for allografts and hamstring tendon autografts⁽¹⁰⁾.
• Isometric hamstring training at 30, 60, and 90° does not strain the ACL graft⁽¹²⁾. However, clinicians must be cautious of the graft site. Recommendations include introducing isometric knee flexion only after six weeks and dynamic knee flexion after eight weeks⁽¹⁴⁾.
• Clinicians can increase the exercise intensity between 50-100° knee flexion for OKC knee extension as athletes shift into the intermediate phase, increasing to near-full extension by week 12, which may benefit athletes’ quadriceps strength⁽¹²⁾.
• It is critical to monitor joint swelling and pain in the initial healing stages (weeks 0-4), irrespective of exercise choice. 

Conclusion

Anterior cruciate ligament reconstruction rehabilitation is a multi-faceted process. Contention around exercise choice should not overshadow the purpose of rehabilitation: a safe and timely RTS. Irrespective of exercise choice, clinicians must ensure that athletes reach the recommended strength goals. Open kinetic chain exercises are vital to ACLR rehabilitation, regardless of when they are initiated. They isolate muscles and help to overcome any compensations that may impact recovery. Early introduction warrants vigilance on joint health and healing; late introduction warrants the purposeful effort to ensure quadriceps strengthening. Great clinical reasoning allows clinicians to make decisions that positively impact athlete well-being. Open kinetic chain exercises are vital to that decision-making process and shouldn’t be neglected. Combining exercise types may be necessary to rehabilitate athletes back to their previous level of function.

References

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14.    ‘Rehab Guidelines & Protocols’ (2016) Physical Therapy Clinic, 29 January. Available at: sites.udel.edu/ptclinic/professionals/guidelines/ (Accessed: 23 February 2024)
15.    Phys Ther in Sport 66 (2024) 61–66