The Mechanism of Alternating Tension and Relaxation
Why does sequentially tensing and relaxing muscles produce such a pronounced effect? The physiological explanation is related to how the muscular system works. When a muscle actively and maximally contracts for several seconds, it is followed by a phase of post-isometric relaxation – the muscle automatically transitions into a state of deeper relaxation than it was in before the contraction. Simply put, after a strong contraction the muscle fibers let go and remain less tense than usual for a short time. Jacobson utilized this property: by intentionally tensing muscle groups and then abruptly relaxing them, we achieve more complete relaxation than by trying to relax directly without a prior tension phase. It is very important to focus on your sensations during this process: feel the contrast between the state of tension and the subsequent relaxation. It is this contrast that creates a vivid feeling of relaxation that is remembered by the nervous system.
Alternating muscle tension and relaxation also engages different parts of the nervous system. During brief periods of tension, the sympathetic nervous system is activated (responsible for the “fight or flight” response): heart rate increases, blood pressure may rise slightly – the body is primed for action. But as soon as the tension is released, the parasympathetic system takes over, which is responsible for rest and recovery: the heart rate slows, blood vessels dilate, breathing calms. This transition to an internal “rest mode” is subjectively experienced as relaxation, calm, even drowsiness. Thus, alternating tension and relaxation serves as a kind of controlled exercise for the autonomic nervous system, training its flexibility. Over time, practicing progressive relaxation increases the body’s ability to switch from a stress mode to a rest mode. The person becomes less reactive to stressors, and their nervous system recovers more quickly after challenges.
It’s important to note that the technique teaches relaxation voluntarily, at will. In the beginning, you have to perform a special exercise – tensing muscles and then relaxing them – but gradually you develop the ability to quickly release muscle tension without the preliminary effort phase. In other words, later on it will suffice to notice the first signs of tightness in your body and consciously give the command to relax – and your body will respond by shifting into a calmer state. This habit is extremely valuable in daily life: it allows you to prevent an acute stress reaction at the early stages, to “shed” accumulated tension before it grows into intense anxiety or panic.