All muscles have an optimal resting length, which allows them to function correctly, and to generate maximum force output. A muscle which is overactive/shortened has excessive neural drive to it, causing the muscle to be held in a partial contraction when at rest. This shortens the resting length of that muscle. When that muscle (the agonist) is required to activate and generate force, the amount of total available muscle contraction is reduced. Due to reciprocal inhibition these over active muscles also cause reduced neural input to the muscles on the other side of the joint (the antagonsist) (Marieb & Hoehn, 2007). This inhibits the antagonist muscle from correctly doing its job, keeping it in an underactive or lengthened state. A common example of a muscle imbalance is over active hip flexors (which cause a rider to lean forward and perch in the saddle), inhibit the rider’s abdominal muscles and glutes- which help the rider sit back, open their hips, and stabilize themselves in the saddle. This prevents them from achieving a connected seat, prevents the rider from following the horse’s movements, but no amount of yelling “lean back” from the instructor will get the rider to be able to switch off those over active hip flexors so that they can sit back and access the correct muscles. Also, if that rider tries to work out off the horse to strengthen their underactive abdominal muscles and glutes, those muscles will not be fully accessible to them until the over active hip flexor muscles are inhibited and lengthened, quieting their neural signal that is inhibiting the abdominals and glutes from fully contracting (NASM. 2008.).
Muscle imbalances interfere with muscle and joint function, and can lead to pain and dysfunction. They can limit your range of motion and interfere with your ability to perform certain activities. Common causes of muscle imbalances are: chronic poor posture, improperly rehabbed injuries, sitting all day at work, repetitive movements, exercising one muscle group to the exclusion of that group’s antagonist muscle group, or using the incorrect muscles to perform certain tasks- without being aware of it (NASM, 2008). Over time the increased neural drive to these over active muscles can cause muscle adhesions and trigger points, which continue to keep these muscles in a shortened state. This is followed by a cascade of effects that negatively impact the joints effected by these muscles, and neighboring body segments, due to the interconnected nature of the kinetic chain (NASM, 2011).
Click to read some more examples of how muscle imbalances negatively impact riders.