Force Couples occur where two different muscles perform the same action on a joint. This post examines two examples of Force Couples in the body that can make athletes prone to pain and injury, and provides solutions for each.
Force couples are pairs of muscles that have the same effect on a joint. The reason that they are important is that when one muscle in any given pair is more dominant than the other, the risk of an overuse injury in that muscle is higher. When we think about Force Couples we must also consider that each muscle in the pair also has other functions affecting other joints, which further adds to the potential for overuse.
In this blogpost we will examine Force Couples at two major joint systems - one in the lower body and one in the upper body- and talk about simple training solutions to ensure that each muscle in the Force Couple provides a balanced contribution to the joint in question.
Hamstring and Abdominals
The first Force Couple we will talk about are the rectus abdominus muscle and the hamstring muscles. This is a postural force couple because we need both of these muscles to produce a constant, low-grade tension at all times when we are on our feet so that our spine can maintain a healthy alignment. The effect of this Force Couple is to tilt the pelvis backward which we call a, “posterior tilt.” To familiarize yourself with this action, think about cocking your butt outward and creating tension in the low back muscles. This action would be called an “anterior tilt,” and so reversing that action by squeezing the abs and tucking your butt “inward” is what we call a posterior tilt.
The ability to tilt the pelvis posteriorly is important because it acts as a natural restraint to minimize the compression (“anterior tilting”) of the lumbar spine during sprinting, jumping, and overhead actions. Typically this Force Couple relationship presents with the hamstrings being more dominant than the abdominals, and leads to excess activity of the hip-extensor portion of the hamstrings. This potential for hamstring overuse is made worse because of the role the hamstrings also play in flexing the knee and hip extension during sprinting and jumping. Because the abdominals do not contribute to hip extension - but are supposed to help restrain and stabilize hip extension - we have an example of the extra burden the hamstrings have in helping the abdominals maintain the position of the spine while also being heavily involved in locomotion.
To reduce the risk of hamstring overuse, it is critical that the abdominal muscles are trained both statically (think plank and other core exercises where the goal is “don’t move”) as well as dynamically - performing throws with medicinal balls in different planes of motion. This will help the abdominals to hold more tone in general to assist the hamstrings in the action of maintaining a neutral spine position.
Serratus Anterior and Pectoralis Minor
This is a Force Couple of the upper body that can cause a cascading series of movement problems and pain syndromes when the Pectoralis Minor muscle is dominant and the Serratus Anterior is dormant. If you are an overhead or throwing athlete who has experienced shoulder pain at any point, this is an important relationship to look at. Both muscles move the shoulder blade forward (think throwing a punch or a baseball). However, the Serratus Anterior also has the secondary job of rotating the shoulder blade upward – helping to keep the ball and socket close together during overhead actions; while the Pectoralis Minor has the secondary and tertiary job of rotating and pulling the shoulder blade downward.
As you can see, this is a complicated Force Couple! They both share one job, but except for that, they completely oppose one another! Because the Pectoralis Minor has two other major jobs and the Serratus Anterior has one other major job (it does have other minor jobs but they are not as dynamic), the Pectoralis Minor tends to do more work and thus hold more tone. As a result, the Pectoralis Minor often dominates the job that it shares with the Serratus Anterior, which can cause overuse injury to the shoulder.
To correct this, performing soft tissue work on the Pectoralis Minor followed by light activation of the Serratus Anterior can be a small but powerful tool in the long-term maintenance of shoulder health in the athlete populations mentioned.
Although there are other examples of potentially problematic Force Couples, these are examples of two common ones in the lower and upper body that affect athletes the most.
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