According to the GAS (general adaption syndrome) an organism will respond to stress in stages.
For simplicity, we’ll use a novice lifter as an example.
Stage 1 – Alarm/shock phase - During a squat session a novice lifter squats 3 sets of 5 reps at 100lb. To overcome the weight on the bar the body releases the hormone cortisol. Muscle fibers tear, bones, tendons and ligaments are stressed and the body’s nervous system is requested to recruit higher threshold motor units. This acute episode shocks the body.
Stage 2 – Resistance phase - This stage is known as the recovery stage and it produces changes in hormonal production, increasing structural protein (cross sectional with of the muscle fiber), increasing in metabolic protein (the creation and break down of specific proteins) and increasing neuromuscular efficiency (creating a new motor skill). A mixture of physical growth, physiological adaption and movement efficiency are the premises for this supercompensation effect. The novice is now capable to lift 3x5 @ 105lbs. When the novice does so and the process starts over again.
This stress/recover/adapt cycle processes differently under different conditions. For a novice lifter this entire process can happen after a single episode of weight training. For an advanced athlete who is already strong and has accumulated years of adaption this process is much slower requiring increase the stressor over a month or longer to force a training adaptation.
Stage 3 – Exhaustion phase - This is classified as over training. This is when the lifter exceeds the capacity to recover. Signs that a lifter is in this phase are a drop in performance, chronic soreness, injury, depression, mood swings, trouble sleeping, and elevated heart rate (its not limited to this list). Avoid this stage at all costs. A novice lifter can recover from this training debt in less than a week in contrast to an advance lifter who may require upwards of 4 weeks of reduced training loads to restore homeostatic equilibrium.
We all have the ability to adapt. The overload event(s) must be enough to disrupt hemostasis and cannot cause more damage than the body is capable of recovering from. As the body gets good at healing it accumulates fatigue. Over time this accumulation builds and must be dissipated appropriately to avoid overtraining and allow for strength to increase.