Temporal Response of Anterior Knee Laxity Following Strenuous Exercise

Research Article

Austin J Biomed Eng. 2016; 3(1): 1034.

Temporal Response of Anterior Knee Laxity Following Strenuous Exercise

Starkel CD¹, Hawkins DA²* and Ashuckian ED³

¹Department of Mechanical and Aeronautical Engineering, University of California, USA

²Department of Neurobiology, Physiology and Behavior, University of California, USA

3Department of Biomedical Engineering, University of California, USA

*Corresponding author: Hawkins DA, Department of Neurobiology, Physiology, and Behavior, University of California - Davis, One Shields Avenue, College of Biological Sciences, USA

Received: April 27, 2016; Accepted: June 27, 2016; Published: June 29, 2016


Routine Anterior Knee Laxity (AKL) testing of athletes can provide clinically relevant information about knee structural changes and has the potential to be prognostic for knee overuse injuries. However, for AKL to be prognostic, we must be able to discern normal AKL temporal responses following exercise from responses indicative of trauma. A Custom Knee Arthrometer (CKA) was constructed and used to quantify the temporal response and variability of AKL among thirty two young adults randomly assigned to an exercise (N=16) or control (N=16) group. Participants in the exercise group completed a 50 minute strenuous bout of lower extremity exercise. Their AKL was quantified immediately before, immediately after, every hour after for five hours, and 24 hours after the exercise. The 16 control participants were tested at similar times, but without exercising. Mean AKL did not change in the control group, but increased significantly in the exercise group (0.63 ± 0.87mm; p = 0.023) following exercise and returned to pre-exercise values within 1 hour postexercise. Individual AKL responses varied within the exercise group with two individuals (~12%) experiencing long lasting knee changes consistent with damage to knee structures. Damage accumulation may be a mechanism of non-contact Anterior Cruciate Ligament (ACL) injuries. Routine knee laxity testing using a biomedical device such as the CKA developed for this study provides a potential for detecting ACL changes and allowing sports medicine professionals to intervene to restore full ACL structural integrity and avoid damage accumulation and subsequent catastrophic rupture.

Keywords: ACL; Arthrometer; Injury prevention; Structural changes


Injuries to the knee and specifically the Anterior Cruciate Ligament (ACL) have been described as an epidemic in youth sports. Estimates of the annual incidence of ACL injuries vary considerably, ranging from 80,000 to 250,000, with approximately 50% of these injuries occurring in athletes 15 to 25 years of age [1]. Approximately 70% of ACL injuries are non-contact in nature and occur during athletic activities that include cutting or landing tasks [2]. Non-contact ACL injuries have been studied extensively by many research groups, and there are multiple theories regarding the mechanisms of non-contact ACL injury. There is no consensus as to a single mechanism and the cause is most likely multi-factorial.

One potential mechanism of non-contact ACL injuries that could be affected by several factors (e.g. movement mechanics, anatomical structure) is overuse, defined as the repeated loading of a ligament and the subsequent mechanical breakdown characterized by a change in the ligament’s biomechanical properties such as stiffness and ultimate tensile load [3]. Repetitive submaximal loading of the ACL may cause microscopic damage to collagen fibrils or fibers, increasing the load on the remaining fibers and making the ligament more susceptible to failure [3,4]. Repetitive loading may also increase the production of inflammatory mediators and degradative enzymes which collectively may damage cells and contribute to overuse injuries [4,5]. The extent of damage sustained by the ACL in two different people performing the same repetitive task will likely vary due to individual differences in the magnitude of loading (caused by different anatomical structure and movement mechanics), and metabolic and inflammatory responses. Further, the extent of ACL damage that accumulates over multiple bouts of an activity will also vary between individuals and result in different structural changes to the ACL.

At 30 degrees of knee flexion, the ACL is the primary structural restraint to anterior translation of the tibia relative to the femur (defined as Anterior Knee Laxity (AKL)) [6,7] and therefore ACL structural changes can theoretically be quantified by changes in AKL. Clinicians exploit this idea to diagnose partial and complete tears of the ACL by performing various knee laxity tests. However, to our knowledge, no one has attempted to longitudinally use knee laxity testing to screen athletes for knee structural changes that could be prognostic of future catastrophic ACL injury.

For AKL to be prognostic for ACL injury, the resolution of the AKL measurement must be sufficient to detect clinically meaningful knee structural changes and the normal AKL day-to-day fluctuations and responses to exercise must be understood. The fact that some orthopaedic and sports medicine doctors can detect partial ACL tears from knee laxity exams, provides evidence that AKL measurement has the potential to provide clinically relevant longitudinal information about knee structural changes. We believe that we can resolve knee laxity changes of 0.5 mm and that this resolution has clinical significance. We can expect that a person having a normal knee laxity of 5 mm who performs activities that lead to 10% damage of the ACL cross sectional area without compromising the material properties of the remaining intact ACL would have a 10% increase in AKL. The AKL change would be larger if the material properties were also compromised in response to the activity. Recognition of the accumulation of 10% or more damage to an ACL can be clinically important and lead to intervention programs that reduce the incidence of non-contact ACL injuries.

To our knowledge, only six published studies have reported the temporal response of AKL following exercise [8-13]. These studies reported varying times ranging from 52 minutes to 5 hours for AKL to recover to pre-exercise values following exercise. Some of these studies briefly mentioned, but without explanation, that there were varied individual temporal responses not reflective of the mean response. Additional research to further characterize the temporal response of AKL post-exercise is needed to explore the potential for using AKL to detect subtle damage to knee structures that could potentially be prognostic for ACL injury.

The goal of this study was to determine if routine AKL screening has the potential to detect subtle knee changes reflective of damage accumulating in knee structures such as the ACL. The intent was not to quantify damage to specific knee structures, as this would be very challenging to do in-vivo, but rather to determine if AKL changes occur that are consistent with knee structural changes, and thus if more expensive and longitudinal studies to track AKL changes and ACL injuries are warranted.

We pursued three aims in this study. First, we tested the hypothesis that AKL increases during strenuous exercise in young adults. Results from this aspect of the study were compared to results from similar studies performed by others to demonstrate the validity of our AKL testing device and testing procedures. Second, we quantified the temporal response of AKL to strenuous exercise. This study provides improved time resolution of AKL measurements compared to previous studies and thus enhances our understanding of the temporal response of AKL following exercise. Third, we explored the variability in AKL changes and recovery times among young adults completing a strenuous bout of exercise to determine if any of the response were consistent with structural damage (i.e. AKL does not return to pre-exercise values). We hypothesized that a percentage of young adults who complete a strenuous bout of lower extremity exercise will experience lasting knee laxity changes consistent with subtle damage to knee structures.



Physically active, male and female, college-aged students 18-25 years of age participated in this study. Participants were excluded from participation if they were an intercollegiate athlete, a competitive club sport athlete, were sedentary, or had any injury or health condition that would prevent them from completing one hour of strenuous exercise. Participants were also excluded from the study if they had any previous knee injuries or had been diagnosed by a health care professional as being hypermobile.

Testing apparatus

All knee laxity measurements were made with a Custom Knee Arthrometer (CKA) developed in our institution’s Human Performance Laboratory (Figure 1). Commercially available knee arthrometers were evaluated prior to the start of this study and their resolution and precision were deemed inadequate for this study. Thus, the CKA was developed and used. The CKA was designed to acquire real time anterior and posterior force deformation data and consists of two force transducers (LC101-100 S Beam Load Cell, Omega Inc, Stamford, CT – accuracy, precision and resolution better than 0.62 N, 0.16 N, and 0.60 N respectively), two string potentiometers (SP1- 4 String Pot, Celesco, Chatsworth, CA - accuracy, precision and resolution better than 0.43mm, 0.11mm, and 0.20mm, respectively), a data acquisition laptop, a custom Lab VIEW virtual interface, and a rack and pinion force application system.