Quadriceps and Hamstrings Activation Peaks Earlier as Athletes Repeatedly Hop, but There are Differences Depending on ACL Reconstruction Technique

Abstract

Background: After Anterior Cruciate Ligament Reconstruction (ACLR) athletes face the challenge of regaining their previous competitive level while avoiding re-injury and early knee joint cartilage degeneration. Quadriceps and hamstrings strength reductions and neuromuscular alterations potentially related to risk of re-injury are present after ACLR and relate to deficits in muscle activation.

Design: Cross-sectional laboratory study.

Purpose: To examine quadriceps and hamstrings muscle activation during repeated hops in healthy pivoting-sport athletes and those who had undergone ACLR (bone-tendon-bone and semitendinosus graft) who had met functional criteria allowing return to training.

Methods: Surface electromyography (SEMG) was recorded from vastus medialis and lateralis and medial and lateral hamstrings bilaterally during 30 seconds' repeated hopping in male athletes on average eight months after ACLR surgery (5-12 months). All patients underwent hamstring (HS) (n=24) or bone-tendon-bone (BTB) reconstruction (n=20) and were compared to healthy controls (n=31). The SEMG signals were normalized to those obtained during maximal voluntary isometric contraction.

Results: A significant time shift in peak muscle activation (earlier) was seen for: vastus medialis and vastus lateralis activation in the control group, in the BTB group's healthy (but not injured) leg and both legs of the HS group. A significant time shift in peak muscle activation was seen for lateral hamstrings (earlier) in all but the BTB group's injured leg and the medial hamstrings in the control group only. Lower peak activation levels of the vastus lateralis (p\<0.001) and vastus medialis (p\<0.001) were observed in the injured compared to healthy legs and lower peak lateral hamstrings activity (p\<0.009) in the injured leg compared to control leg. Decline in medial hamstring peak activation (p\<0.022) was observed between 1^st and 3^rd phase of the hop cycle in all groups.

Conclusion: Repeated hop testing revealed quadriceps and hamstring activation differences within ACLR athletes, and compared to healthy controls, that would be missed with single hop tests.

Level of evidence: 3.

Keywords: anterior cruciate ligament; electromyography; injury; rehabilitation.

Figure 1.. Mean vastus lateralis (VL) and medialis (VM) activation comparison between the first (red) and the last (blue) period during the 30-second vertical hop task.

Each panel illustrates the mean for the first phase (red) second phase (dotted gray) and the last phase (blue) of the hop task. The Y-axis represents the relative muscle activation compared to maximum voluntary contraction (0-1, vertical scale truncated to 0-0.7 for clarity) and the X-axis represents the entire hop phase with foot contact at 50% (0-100) presented with dotted vertical line. Within each panel, the inset above illustrates the Statistical Parametric Mapping comparing the first and third hop periods. The thick black line depicts the test statistic continuum, which in this case is a t continuum, or “SPM{t}” The red hashed line depicts the critical threshold at alpha = 5%. From a classical hypothesis testing perspective, the null hypothesis is rejected at alpha if the SPM{t} exceeds this threshold, indicated by the shaded grey area where p<0.05. No statistical analyses were made using the 2nd period; it is shown for reference only.

Figure 2.. Mean lateral and medial hamstring activation comparison between the first (purple) and the last (green) period during the 30-second vertical hop task.

Each panel illustrates the mean for the first phase (purple) second phase (dotted gray) and the last phase (green) of the hop task. The Y-axis represents the relative muscle activation to maximum voluntary contraction (0-1, vertical scale truncated to 0-0.2 for clarity) and the X-axis represents the entire hop phase with foot contact at 50% (0-100) presented with dotted vertical line. Within each panel, the inset above illustrates the Statistical Parametric Mapping comparing the first and third hop periods. The thick black line depicts the test statistic continuum, which in this case is a t continuum, or “SPM{t}” The red hashed line depicts the critical threshold at alpha = 5%. From a classical hypothesis testing perspective, the null hypothesis is rejected at alpha if the SPM{t} exceeds this threshold, indicated by the shaded grey area where p<0.05. No statistical analyses were made using the 2nd period; it is shown for reference only.