If you have been going to the gym for quite a while now, you’ll notice that there are two categories of people — (A) those that don’t follow a workout routine, and (B) those that follow a prepared routine.
People who belong to (A) maybe novices who are just trying things out, or they may also be experienced gym goers who already know what they are doing, the reason they don’t need to follow any rigid plan anymore.
While the people who belong to category (B) has a routine either prescribed by a coach/personal trainer, a workout they created themselves, or stumbled upon the internet, etc.
Regardless of the source of their routine, I noticed that people in category (B) can be further divided into those that follow their routine “as is,” and those that who are flexible.
Flexible gym goers are the ones who substitutes one exercise over the other, just in case they cannot perform the prescribed exercise.
For example, an exercise in your routine is to perform overhead shoulder press using barbells, it just happens that there aren’t any free barbell bars in our gym because other people are currently using it.
What could you do in that situation? You can wait for the other guy or girl to finish using the equipment, or you can substitute another similar exercise like the overhead shoulder press, but using dumbbells instead.
The same principle can be applied if for example you are to use two 20 pound dumbbells to perform three sets of 15 repetitions, but someone else are using those dumbbells. Do you wait for him to finish?
You could try performing three sets of 10 repetitions using 25 pound dumbbells instead, or three sets of 20 repetitions using 15 pound dumbbells.
To sum it up, if you’re desired gym equipment isn’t available for some reason, you can substitute one similar exercise over another one. Also, you can try performing the same exercise, but with a different rep range.