The most common injury that occurs in runners is knee pain. Studies have shown that up to 42% of running injuries occur at the knee. Of all those injuries, patellofemoral pain is the most frequent.1
As a result of its high prevalence, there have been numerous research studies focusing on the sources and contributing factors of knee pain in athletes. Current trends in the treatment of knee pain in runners and promising new research is showing how trunk positions can play a large role in patellofemoral stress.
Running is comprised of four general phases of movement - stance, early swing, mid-swing, and late swing. Of these phases, the majority of knee injuries occur during stance.1 As the foot contacts the ground, the ankle, knee and hip are all loaded with forces as great as three times a runner's body weight.2
These extreme forces are transferred up through the ankle and into the knee. Research has shown that a decreased ability to absorb shock at the patellofemoral joint (PFJ) can contribute to the development of knee pain.3 The goal of preventing knee pain while running should focus on strategies to minimize stress to the PFJ during active loading.
Biomechanics and Anatomy
The quadriceps are made up of four muscles: the vastus medialis, lateralis, intermedius, and rectus femoris. The vastus medialis and lateralis both originate along the linea aspera of the posterior femur. They wrap around the femur and join the vastus intermedius and rectus femoris, and insert into the patella.
As a result of this orientation and insertion into the patella, excessive quadriceps activity can increase patellofemoral stress. Landing with an erect trunk creates an extension torque at the knee and biases quadriceps muscle activity.4 However, landing with a forward-flexed trunk decreases both the extension torque and quadriceps bias. Landing with trunk flexion, in turn, reduces patellofemoral stress and can reduce knee pain.
Previous and Emerging Research
Until recently, trunk position has received little clinical and research attention. The majority of the focus of knee pain research has been on lower-extremity mechanics. However, in the past several years, studies have begun to emerge that analyze trunk position and its relationship to the knee.4-7
A runner entering deceleration during stance phase with a forward-flexed trunk (A,C) and an upright trunk (B,D).
In 2009, Blackburn et al. performed a jump-and-land study to determine whether trunk flexion decreased ACL tear risk. In the study, participants stepped off a box and performed a single-leg landing. The participants performed two trials - one with an upright trunk and one with a flexed trunk. The study showed that trunk flexion during landing reduced quadriceps activity and, more importantly, force on the knee.4
Powers et al. is testing the concept further. Preliminary results indicate that trunk flexion during running reduces PFJ forces. The study is analyzing PFJ stress during running with a self-selected trunk position compared to a forward-trunk lean position.8
Test the Concept, Spread the Word
Stand up tall and lean your trunk backward. You should feel your quadriceps muscles and anterior tibialis muscles start to engage to prevent you from falling back. Now lean your trunk forward. You should feel your calves, hamstrings and gluteals engaging to hold your body upright.
This is biomechanics in action. The backward trunk lean provides an extension torque on the knee, leading to recruitment of anterior musculature, while a forward trunk lean provides a flexion torque on the knee, leading to recruitment of the posterior musculature.
Once research that is currently underway becomes published, it will lead to a wave of additional studies. These studies will likely focus on trunk mechanics while running and its relationship to PFP.
However, it often takes many years for validated research to become incorporated into common clinical practice. Currently, this is a new concept that's not widely known. So get the word out. Try it yourself and on your patients and see what you think. You may be pleasantly surprised with what you find.
1. Taunton JE, Ryan MB, Clement DB, et al. A retrospective case-control analysis of 2002 running injuries. Br J Sports Med 2002;36:95-101.
2. Cavanagh PR, Lafortune MA. Ground reaction forces in distance running. Biomechanics Laboratory, The Pennsylvania State University, University Park, PA.
3. Heino BJ, et al. Patellofemoral stress during walking in persons with and without patellofemoral pain. Med Sci Sports Exerc. 2002;Oct;34(10):1582-93.
4. Blackburn JT, Padua DA. Sagittal-plane trunk position, landing forces, and quadriceps electromyographic activity. J Athl Train. 2009;Mar-Apr;44(2):174-9. doi: 10.4085/1062-6050-44.2.174.
5. Blackburn JT, Padua DA. Influence of trunk flexion on hip and knee joint kinematics during a controlled drop landing. Clin Biomech (Bristol, Avon). 2008;Mar;23(3):313-9. Epub 2007;Nov 26.
6. Yu B, Lin CF, Garrett WE. Lower extremity biomechanics during the landing of a stop-jump task. Clin Biomech (Bristol, Avon). 2006;Mar;21(3):297-305. Epub 2005;Dec 27.
7. Kulas AS, Hortobágyi T, Devita P. The interaction of trunk-load and trunk-position adaptations on knee anterior shear and hamstrings muscle forces during landing. J Athl Train. 2010;Jan-Feb;45(1):5-15. doi: 10.4085/1062-6050-45.1.5.
8. Powers CM, Hsiang-Ling Teng S, Kai-Yu Ho KY. Utilizing a forward trunk lean during running decreases patellofemoral joint stress. University of Southern California, Los Angeles, CA.
Jared Vagy is a physical therapist and a runner in Santa Monica, Calif. He received his DPT from the University of Southern California. The author thanks Shawn Goodman for editorial assistance.