In strength training, trainees use many different metrics to analyze their progress. A common one is soreness. Trainees frequently associate soreness with success. They do some lunges, some squats, and some pushups, and they're sore the next day. They get a satisfaction out of this and continue doing what they do. Soreness, however, is an abysmal metric for strength training. The first reason is obvious: it's not quantitative. The second is not so obvious: it just doesn't work well.
There are two types of soreness, immediate soreness after the exercise or workout and delayed onset muscle soreness (DOMS) generally felt one or two days after the workout. DOMS is the metric trainees generally refer to when they talk about soreness. The reason trainees think it is an effective metric is simply a lack of understanding of the underlying mechanisms: they don't know what causes DOMS.
The current prevailing hypothesis is that DOMS is caused by inflammation that is the result of damage to the myosin filaments and Z disk in the sarcomere of the muscle. This damage is caused primarily by the eccentric portion of a lift, meaning that the muscle belly is lengthening under tension. An exemplary lift is the bench press to illustrate this.
In the bench press, you lie down on a bench, chest facing the ceiling. You take the bar in your hands and lower it to your chest. When it touches your chest, you push the bar away from your chest until your elbows are locked out (fully extended). The first portion, the lowering, is the eccentric portion. The second portion, the pushing away from the chest, is the concentric portion.
If this is the source of soreness, then why does this damage not cause strength progress after recovery and repair of the injured site? This is because strength increases are primarily caused by increased cross section of the muscle belly. This can be accomplished by increasing myofibrils which is caused by damage and recovery of the myofibrils. Note the disconnect in these two mechanisms; Z disk damage and myosin damage were not mentioned in this paragraph. Figure 1 displays the breakdown of a muscle.
Figure 1: Constituents of Muscle by Order
Soreness will certainly result in the goal of strength progress; it would be tiresome to reset the bench press or squat every time one needed to do another concentric rep. This doesn't mean that soreness should be the goal though. Foolish trainers will (knowingly or not) specifically choose exercises that cause soreness. Lunges are a perfect example. If a trainee does 100 lunges without weight, they will be extremely sore the next day. If they do three sets of twenty lunges, they might be reasonably sore, but this does not mean they will be productively stronger after recovery.
I tell prospective clients "get noticeable results in two weeks or your money back" for a good reason. It's because significant progress can be made in two weeks for a novice when productive exercises are chosen and smart programming is used. Doing 3x20 lunges and burpees and then running on a treadmill for 20 minutes four times a week will not produce noticeable results in two weeks. It WILL produce soreness, but not noticeable growth in strength.
A superior metric for progress is actual strength increase. This is shown in barbell training as an increase in weight loaded on the bar. Doing three sets of five reps at 135lb today and then doing three sets of five reps at 140lb in two days is a clear sign that progress has been made. When a trainee adds five pounds of weight to the bar every other day for a month straight, and they are squatting 70lb more than when they started, strength progress is obvious and quantifiable. Do not think soreness is an indicator of progress when you work out. Instead, do several carefully selected barbell exercises and increase the weight on the bar every workout until you can't anymore. When this happens, you will have made a great deal of strength gains (if you rested long enough between sets and eaten enough).