Active Voice: Restoring Knee Function to 'Normal' After ACL Reconstruction — Is It All About Quadriceps Strength?

By David R. Bell, Ph.D., ATC

Viewpoints presented in SMB commentaries reflect opinions of the authors and do not necessarily reflect positions or policies of ACSM.

David R. Bell, Ph.D., ATC, is a researcher and assistant professor in the Departments of Kinesiology and Orthopedics and Rehabilitation at the University of Wisconsin–Madison. He teaches in the athletic training program and serves as the director of the Wisconsin Injury in Sport Laboratory. His research focuses on improving outcomes following traumatic knee injury and identifying risk factors for injuries in youth athletes.

This commentary presents Dr. Bell's views on the topic of a research article which he and his colleagues had published in the October 2016 issue of
Medicine & Science in Sports & Exercise® (MSSE).

Each fall semester I am reminded of the impending start of college football season, as the University of Wisconsin Marching Band hones their pre-game and halftime shows within ear shot of my office window. This ritual also serves as a reminder of how my career started — as an athletic training major almost 20 years ago. My early experiences were critical in my development as clinician, and later as a researcher — providing me first-hand opportunities to observe the impact of musculoskeletal injuries on athletes. I'll never forget the first time I was on the sideline with an athlete who had suffered an ACL injury, and the emotion I felt when informing the athlete and her parents of this life-altering injury. I also will never forget the joy I felt when that athlete stepped back onto the field after successful surgery and rehabilitation.

While rehabilitation after ACL reconstruction is multifaceted, one of the most critical outcomes consistently identified in the literature is the importance of restoring quadriceps strength. Poor quadriceps strength has been linked with poorer surgical outcomes, including the patient feeling worse about his or her knee. Biomechanical studies have demonstrated that patients with quadriceps strength deficits often shift torque demands away from the knee extensors to the hip during functional tasks.

One of the questions we attempted to answer in our study, reported in the October 2016 issue of MSSE, is that of what happens to hip strength in patients with quadriceps weakness after ACL reconstruction. We assessed isometric strength bilaterally in 135 participants (73 healthy and 62 with unilateral ACL reconstruction — approximately 30 months after surgery) involving assessments of both the knee and hip musculature. Those with ACL reconstruction were further subdivided by quadriceps strength into “strong” (LSI = 90%) and “weak” (LSI < 85) categories, based on Limb Symmetry Index scores (LSI = injured limb / healthy limb * 100). We observed that patients with weak quadriceps had increased strength of the hip extensors and tended to have stronger hip abductors. Interestingly, we observed no differences in other variables commonly evaluated in this population, including hopping distance and subjective assessments of knee function.

The take-home point of our study is that patients with poor quadriceps strength potentially compensate for this issue by increasing hip strength. Thus, they can maintain similar function when compared to patients with strong quadriceps. These findings have implications for clinical practice. We assessed patients using inexpensive tools commonly found in the rehabilitation setting and created the strong and weak quadriceps groups using logical clinically based criteria that can be interpreted even by the most novice of clinicians. However, the implications of these findings on long-term knee health are still unclear.

I think back to my first ACL tear patient and wonder: Where is she now? How is her knee? The reality is we do not know what happens to many of our patients after they leave our care. We know even less about how their surgical outcomes influence factors vital for long-term health, including participation in life-long physical activity. This idea also is reflected in the literature, as we know very little about these populations during the time between ACL reconstruction and the development of osteoarthritis. Long-term follow up studies are needed and, while difficult to perform, are vital to understanding the factors most relevant to our patient’s overall health and quality of life.