When we start exercising, our body is not used to working out and our muscles are not used to contracting in a certain way. Most people work rather inactive jobs and rarely must squat, hinge, or press in their daily activity. The first time you came to the studio, your trainer prepared a session adapted to your fitness level, but you probably started doing movements you had never done before. I can guess you did some dead bugs during your first session (we love dead bugs!). These are unfamiliar movements that your body had never been exposed to, so your muscles were not used to produce the necessary contraction needed to perform the movement.
A lot of people confuse lactic acid with soreness, however, the soreness you feel days after your workout is not due to the lactic acid produced by your muscles. Indeed, the burn you feel in your muscles while you are working out is associated with lactic acid, which is removed seconds after you are done performing the exercise. The soreness you feel days after your workout is associated with micro tears or damaged muscles. It takes a few days for the body to repair the muscles and for the soreness to go away. Some people will feel soreness more than others, so don’t worry if you are sore for a week straight after you come back from vacation, it happens!
After a few weeks of training, you probably noticed your body being less and less sore after your workouts. This is a natural adaptation of your body to training. In other words, it means that you are getting better and that your muscles are getting stronger. As trainers, we adapt each workout to your fitness level, which means that you will progressively feel better through your workout, but it also means that we will increase the intensity or the difficulty of the session to match your current abilities. This is called the overload principle; to create a training effect, your muscles must work harder than they usually do. So don’t get discouraged if it still feels hard, you are doing way more than you did when you first started!