I come a across a lot of people in my job as a Personal Trainer who have either hit a plateau in their training or have never made any progress at all. I ask the normal questions:
"What does your workout look like each time you come to the gym?"
"Are your sets and reps the same every workout?"
"Do you choose the same exercises every workout?"
"What are your diet and sleep habits like?"
The answers to these questions allow me to find the faults in your training that are holding you back. Very rarely do I have to ask anything further than these to figure out the problem. The issue that a majority of people struggle with, is that they have no basic knowledge of what is going on in the body when they exercise.
Now I'm not saying that you have to be an expert in the fields of exercise physiology, or that you have to have some fancy certification or go through a huge course. I'll admit that there's still a lot that I don't know, but I believe that many people just show up to the gym with no plan and just hope that what they do works.
Here's the basics of what you need to know to help you keep training for a long time in order to see continual improvement.
When you exercise, you are introducing a stress to your body. If you are doing aerobic exercise, you are putting stress on the body's ability to utilize oxygen for energy (ATP) production. If you are resistance training, you are stressing the body's ability to create force. This is the first part of Newton's Third Law of Motion: this is the action.
When a stress is applied to one of the body's systems, it responds by making an adaptation that accommodates that stress. It doesn't want to be caught off guard next time that stress rears its ugly head. Imagine getting caught in the rain without an umbrella because you didn't check the weather forecast. What do you from then on out? You check the weather before you leave for your day. The body does the same thing when you exercise. This is the second part of Newton's Third Law of Motion: this is the reaction.
This is something that I find a lot of people miss. Either they do not stress the body enough to cause an adaptation and therefore they do not need recovery, or they do more than their body can recover from and therefore hinder the ability to adapt. Let me put this first: You can only train as much as you can recover from (this is called Max Recoverable Volume, or MRV), or as hard as you can recover from. This differs from person to person, and special considerations are given to different sexes and age groups, but the principle is true across them all. Often times, we need to plan our recovery into weeks called deload weeks, where volume or intensity is reduced in order to allow recovery before a new stress is introduced. Programming and deload weeks is a different topic to cover, but this is something that a lot of people overlook.
This is the process of training/exercising, regardless of training status or experience. Naturally, if you're just starting an exercise routine after having not done much physical activity in the years prior, your body will be stressed more easily than someone with even 1 year of exercise under their belt. But the body is a beautifully designed machine that makes quick changes to meet the demands it's placed under, so this is something that is beneficial to know from the get go.
So before you walk into the gym next time, think about what your body is doing, and ask yourself if your routine is going to fit into the plan of Stimulating, Adapting, and Recovering.