So if it’s natural and normal, what’s wrong with isometric action?
Nothing at all, most of the time. The system works so well that many people will never experience an injury from an isometric action. Isometrics are even given as rehabilitation exercises after injury to prevent muscle loss due to inactivity when normal activity is too painful to carry out. Sometimes isometrics are also prescribed to rehabilitate tendinitis (aka tendinopathy) and muscle tears.
But imagine that you’re lifting a 25kg weight or holding your whole body weight while climbing and let’s think about the forces going through the muscles. These larger forces easily cause injury. So we need to think about what might be going on when we are using our bodies under maximal load.
What actually causes the injury during an isometric activity?
Even if an isometrically contracted muscle appears not to be moving, the forces exerted on muscles, joints and tendons are actually higher than in a normal concentric contraction such as bending your arm (https://www.ncbi.nlm.nih.gov/pmc/articles/PMC5746261/). This means that if an isometrically contracted muscle is suddenly loaded with more force, it is likely to experience a strain or a tear.
What is it about climbing in particular that causes isometric strains?
Climbing is a sport in which an extremely high level of maximal muscle contraction is used to hold onto bits of rock (or plastic if indoors). Because of the force of gravity – about 9.8 m/s2 – the effort to pull oneself upwards is pretty substantial. In our early climbing days our shoulders and arms feel like lead the day after a climb because we rely too much on upper body strength. As time goes on we learn to move in a more sophisticated way and we utilise our abdominal core and our legs, but as we progress and want to climb more difficult routes, this is where injury is more likely.
When a muscle is isometrically contracted in its most elongated position (a straight arm holding a very small hold), the force going through the muscle fibres is much greater than when the arm is bent. If your foot slips at this point, the resulting shockwave which goes through the muscle attachment points leads to tissue failure, and that equals injury. In the image below, the climber’s left arm is the one at risk of injury should she slip while pulling on this hold. The right arm, bent, is much less likely to experience injury.