Single leg vertical jump performance identifies knee function deficits at return to sport after ACL reconstruction in male athletes

Original article written by Argyro Kotsifaki, Sam Van Rossom, Rod Whiteley, Vasileios Korakakis, Roald Bahr, Vasileios Sideris, Ilse Jonkers

Background

Injury to the anterior cruciate ligament (ACL) is common amongst athletes participating in sports that require change of direction, pivoting and landing. An ACL reconstruction (ACLR) is currently considered by many, to be the gold standard management option to restore the mechanical stability of the joint as a pre-requisite for return to sport (RTS)^[1]. Parallel to restoration of structural integrity, there are many physiological, neurocognitive and psychological qualities that should be restored prior to RTS.

Despite the development of RTS guidelines over recent years, there remains a lack of scientific consensus on which RTS criteria should be used to ensure the athlete returns safely to following ACLR, with ACL re-injury rates remaining high in athletic populations^[2]. Previous studies have shown that whole body biomechanical deficiencies and asymmetries can remain amongst athletes, despite successfully completing a battery of RTS tests^[3]. This begs the question, what are we missing as clinicians, as we search for the holy grail of reducing reinjury rates following ACLR?

In professional male football players, ACL injuries are predominantly non-contact in nature and tend to occur during single leg tasks, involving a change of direction or a jump landing. Biomechanical studies have shown the ACL to rupture in a timeframe of approximately 60 milliseconds^[4]. It can be reasonably rationalised therefore, that single leg, reactive testing may provide us with the most useful insight into the athlete’s physical readiness for RTS following ACLR.

Traditionally, a horizontal hop battery has been used by clinicians to assess the single leg reactivity of the injured limb compared to the non-injured limb. These functional tests have been widely adopted as they are easy to administer and interpret. However, these tests have not been able to predict successful RTS 1 year post injury^[5]. Distance hopped has been shown to be a poor measure of knee joint function, with symmetry in distance not necessarily equating to symmetry in knee joint function. Compensation strategies at the hip and ankle may help to achieve acceptable outputs and therefore this may overestimate rehabilitation status. So what other options are there to assess single leg reactive strength following ACLR?

What the authors did

The authors used a marker-based motion capture system and surface electromyography to analyse the performance of 2 jump tests; a single leg vertical jump (SLVJ) and a single leg drop jump (SLDJ).

Participants were tested within 2 weeks of clearance to RTS at approximately 9.5 months post-surgery.

The experimental group consisted of 26 male athletes and the control was made up of 22 healthy male athletes. Outcome measures derived from the jumps were joint kinematics, work, muscle forces and jump height.

What the authors found

Kinematic differences post ACLR

• During both the SLVJ and SLDJ, the following observations were made in the ACLR group:
  • More peak hip flexion
  • More peak ankle plantarflexion
  • More peak trunk flexion
  • Increased anterior pelvic tilt
  • Reduced peak knee flexion

Work

• During the propulsion phase of the SLVJ, work contribution was less in the involved knee and was greater in the involved hip.
• During the landing phase of the SLVJ, no differences were noticed.
• During the absorption phase of the SLDJ, work contribution was lower in the affected knee.
• Overall, there was 75% and 70% symmetry in the work done between the involved limb and uninvolved limb respectively during the SLVJ and SLDJ. 

Muscle Forces

• Hamstring muscle contribution was greater in the involved limb compared with and the uninvolved limb and controls.
• Soleus contribution was bilaterally lower in the ACLR group compared with controls.

Jump Height

• ACLR athletes displayed 83% and 77% symmetry during SLVJ and SLDJ respectively, compared to 100% symmetry in the control group.

Limitations

• Study limited to male athletes only – no description whether recreational, sub-elite or professional athletes were recruited. Difficult to translate findings to subgroup of athletes, and unable to translate findings into a female population.

• Both bone-patella-tendon-bone or hamstring graft subgroups were recruited into the study, and comparisons were unable to be made between groups due to the small sample size. It is widely accepted those undergoing a bone-patella-bone-tendon graft have greater difficulties restoring reactive stiffness of the patella tendon and thus this may have influenced jump outputs more so than those with hamstring grafts.

What this means for coaches

1. Vertical single leg jump tests are useful to evaluate knee joint function following ACLR, prior to RTS. These tests are likely to be more sensitive to knee joint deficiencies than other single leg tests such as the horizontal hop battery.
2. Reduced knee flexion angles on single leg landing tasks have been associated with increased risk to reinjury. ACLR rehabilitation should look to strategies that encourage greater knee flexion work on landing. This may include eccentric quadricep strengthening and restoration of the function and elastic integrity of the patella tendon if a bone-patella-tendon-bone graft has been used.
3. In both the SLVJ and SLDJ, the lateral hamstring contribution was significantly greater in the involved limb than the uninvolved and control limbs. Increased co-contraction of the hamstrings is considered protective against a secondary ACL injury as the hamstrings have a posterior line of pull in a flexed knee and thus might act as an active agonist of the ACL to resisted anterior tibial shear. Heightened hamstring work may pre-dispose the hamstrings to an increased risk of injury following ACLR, and practitioners should ensure high levels of hamstring conditioning prior to RTS.
4. A reduction in soleus work within the ACLR group was observed during the propulsion and landing phases of the SLVJ. This suggests that ACLR rehabilitation should pay particular focus to increase soleus strength. Soleus acts as an active ACL agonist as it reduces anterior tibial shearing forces. Such prescription may help to reduce the chance of re-injury.

Recommended resources

• Podcast – Preventing and rehabilitating ACL injuries – Enda King
• Podcast – Jump analysis, asymmetries thresholds and using the reactive strength index – Matt Jordan
• Article – Return to play, performance, and career duration after anterior cruciate ligament rupture: a case-control study in the five biggest football nations in Europe – Daniel Niederer and colleagues