The applications of residual force enhancement: Eccentric Contraction Training
A fundamental aspect of my research is to use what we learn about muscles to develop effective training strategies for a wide variety of situations. As a former NCAA athlete and current triathlete, I have a generalized interest in sports science. Lately, my focus has been on preventative strategies for non-contact knee joint injuries (ACL tears, knee strains). My muscle research is focused on the molecular mechanism of eccentric contractions (see my titin research page). I am very interested in these lengthening contractions (muscles that are flexing while getting longer) and how we can wield these contractions during workouts to prevent injury, increase successful aging and expedite recovery from reconstructive surgery.
With over 150,000 knee injuries occurring from non-contact athletic events, preventative strategies are warranted to limit knee injury risk. In athletic competitions, knee injuries are most commonly associated with a quick change of direction, or speed change (i.e., cutting in basketball, football or soccer). These cutting events put an incredible amount of force on the knee, with femur and tibial torques rotating the knee joint in opposite directions. Biomechanics studies that focus on the knee-abduction moment (KAM), an index of knee stability, report that the higher the KAM value, the greater the risk of knee injury. Several factors influence KAM values; the most important are knee valgus moment, body mass, and quadriceps muscle strength. In women, KAM values are physiologically higher than in men, at least partially explaining the higher incidence of injury or re-injury in women. Women are 2-8 times more likely to sustain a knee injury then men due to a fundamentally destabilized knee caused by relatively weak muscles and ligaments (even in professional athletes).
Current exercise strategies to increase knee stability focus on agility and strength. Cutting motion torque forces that are applied to the knee joint are usually countered by the muscles that surround the knee (i.e. quadriceps) as a first line of defense. Once the external torques on the knee overpower the leg muscles, the stress is placed on the knee ligaments of the knee joint (ACL, LCL, PCL, MCL), which under enough strain, will tear. Ligaments are slow to heal when strained (due to low blood flow), while torn ligaments never heal. To protect the ligaments from strain, most preventative training strategies focus on strengthening the quadriceps that surround the knee, so that they can counter more of the external knee torque forces. Usually, simple exercises like lunges or squats (weighted or not) are used for this purpose. Agility practice (i.e. rope ladder steps) is also used to build the athlete’s balance and posture, which minimizes external knee torques by proper locomotion form. The last decade of research has seen different approaches developed to target knee strength and increase knee stability.
Most researchers agree that both strength and agility exercises are a good preventative strategy, but the convention, just described, for attaining these goals are time consuming, inefficient and not aggressive enough. Further, since both strength and agility are important, implementing a single activity that increases strength and agility quickly would place a limited strain on precious practice time.
Eccentric overloading workouts (EOW) have shown great potential over the past decade as a muscle, tendon, and ligament enhancer. Eccentric muscle contractions are active muscles that are lengthening (consider a bicep curl, when you lower the weight, that is eccentric contractions). Eccentric contractions can produce more force than concentric contractions (picking up the weight during a bicep curl), allowing greater loads to be placed on a muscle system during eccentric stretch, which further stimulates the muscular mechanisms that build muscle and tendon structures. In fact, only eccentric style workouts have been shown to restructure tendon systems successfully. EOW are unique, in that they maximize eccentrics characteristics to place very large loads on muscles, which rapidly build muscle/tendon systems, as well as proprioceptive mechanisms needed for balance and agility. Furthermore, when compared to conventional practices, EOW are successful with very short bouts once or twice a week. Due to these attributes, personal tainers and coaches have been incorporating them into their workout routines aggressively. Though these workouts show promise, the primary literature on which style of excercise(conventional squats vs. EOW) are the most effective strategy for knee protection. A common EOW implemented by coaches and personal trainers are box-hop jumps (BHJ). For the BHJ, in short, the athlete jumps off an approx. 2 foot box, lands on both feet, squats and then quickly jumps into the air. When landing, the subject lands flat-footed on one foot with a bent knee and then immediately flat-foot hops ten meters, switches feet and hops back to the start position. The amount of repetitions, height of the box and length of the hops can all be altered as the athlete’s performance improves. This exercise uses the fundamental concept of gravity and the athletes body weight to apply forces 3-8 times their body weight on single leg muscle groups in an eccentric fashion. More importantly, the muscle will have a force applied to them that is 1.5-3 times larger than what would be possible in conventional concentric workouts.
–Athletes and scientists are only starting to appreciate the application of eccentric training. You can lift more eccentrically (ex: lowering a dumbbell) then concentrically (ex lifting up a dumbbell) without placing a large burden on the metabolic or cardiovascular system, which has drastic implications for muscle strength, ligament and tendon restructuring and geriatric medicine.