You have found that it's time to start a new training program. Whether it's a brand new program, the next meso cycle or you're just bored with the current exercises there are a few things to consider when deciding what the new plan is. The first is simply making sure the exercises selected are effective for you. Then making sure that your exercise selection matches your upcoming goal. After that make sure that your split agrees with the choices for that particular day. You don't want to ask for an injury to happen by selecting great exercises to realize that your lower back is getting worked to a significant degree 4 days in a row. There must be adequate time to recover. The exercises must be easily trackable to measure progress. Lastly, they must fit into the workout as a whole.
The exercise is an effective stimulus
Exercises match your goal
Split doesn't overly stress certain muscles
Exercises are easily trackable to measure progress
Workout as a whole must have synergy
Whether an exercise is an effective stimulus or not is a very individual phenomenon. If it were universal then there would be one source of workout information for all and it would be followed by everybody. That is most definitely not the case. There are differences in femur length, torso to leg ratio, height, ape index, injury history and the list just keeps on going so an exercise's analysis on its effectiveness needs to be done for the one person doing it.
The checklist for an effective exercise can be simplified to -
Does the exercise target the intended muscle?
Is the intended muscle's strength the limiting factor?
Is there an easy way to measure progress?
Are joints experiencing minimal to no pain?
Check box one is the simplest. The exercise must work what is intended. If it hits something else then it may still be a good exercise but it's not for this particular workout or section of the workout. If the day's goal is a leg day focusing on Glutes composed of Leg Extensions, Close Stance Leg Press followed by Leg Curls then the point of the day has been missed. Substituting the Leg Presses with Sumo Squats and the Leg Curls with Stiff Legged Deadlifts would be much more in line with the goal. If the day's goal is to do a hypertrophy focused workout for your upper body then maxing out on farmers walks for the first exercise may not be ideal. At the same time though, farmers walks could be great as a finisher to hammer traps and forearms at the end. Don't always throw out an exercise especially when it's a favorite. There is always a way to work in something that you like but depending on what it is there may need to be a compromise somewhere.
An exercise may target the intended muscle but that doesn't mean that muscle is the limiting factor. Three classic examples are the power lifts. Squat, Bench, and Deadlift. Each of these can be done in ways that make them full body exercises (yes, even the Bench Press) and with that comes a lot of training to try and combine all the moving parts into the strongest lift possible. For simplicity's sake we'll take a look at the Squat briefly. Differences in form will change the distribution of how much different muscles are worked. A Narrow Stance High Bar Squat will hit predominantly the quads with appreciable work being done by the glutes, low back and core as well. A Low Bar Squat with a wider stance is going to be much more evenly distributed across the major contributors. Those being the quads, glutes, adductors, core, and low back. The limiting factor could easily be any of those. For a standard leg day the High Bar Squat is probably a better choice. However, if the Low Bar Squat is an absolute favorite and no deviation is desired it can still be done but may have costs in overall fatigue, longer recovery times and needing to choose less demanding follow up exercises to compensate. Still, the simple enjoyment factor is probably more than enough to make it worthwhile.
Tracking gym progress becomes more and more important the longer you have been training. Any machine exercise is generally going to be the easiest to track and compare over time due to the body being limited in form by the constraints of the machine itself. Free weight exercises have a lot more room for form deviation. What this means for tracking is that the form needs to be as close to identical as possible over time. Full range of motion, standardized and repeatable form for exercises are a must for this. Claiming to gain 50 pounds on a Squat over 5 weeks means less than nothing if every week the range of motion decreases another inch. On the flip side if that same person gained 20 pounds on their Squat over 5 weeks but every rep was identical with full range of motion going way beyond parallel then the evidence of progress is for all intents and purposes irrefutable. Progress is slow. Don't fake progress by changing form so the weight goes up. That only slows it more. Be honest, track what is done, and make positive change reliably.
Pain is a necessary part of training. No question on that. There's a fine line sometimes between good and bad pain and it can take a good amount of experience to know the difference. Deep muscle burn that creeps up during a set and dissipates soon after the set is done has a very good chance of being the desired good pain. Sharp and sudden pain has a very good chance that something bad has just happened and stopping immediately would be a very good idea. Things can get a little more nuanced with slow aches and pains. For exercise selection though the important thing is to not just feel what's happening during the exercise but also for the couple days afterward. If during the exercise your joints are just in agony it's a very simple decision to say this exercise is not good. Easy example would be elbow pain during a barbell curl. Sharp pain using a straight bar but no pain with an EZ Bar makes the choice a pretty easy one for the EZ Bar. Another scenario would be that during the workout there's a possibility that bad pain may be there though it's hard to tell. Then for the next 2 days elbow pain happens whenever the arms supinates. So to see if barbell curls were the culprit they're replaced with EZ Bar Curls and then no pain is experienced. Do not stick to an exercise that is known to be causing harm. Adapt and advance to the next one.
Each individual exercise is a part of the whole program. They can't be analyzed by themselves and be deemed good or bad because the context can change things. This is especially true when planning to workout multiple days in a row. If the plan includes training 2 days in a row then having the first day include Deadlifts and Barbell Rows with the following day hitting Squats, Glute Ham Raises and Romanian Deadlifts then the injury risk skyrockets. The exercise selection must have synergy with the rest of the program. We could alter the first day to instead include Seated Cable Rows and Lat Pulldowns so the axial fatigue and lower back demands are greatly reduced. The second day could possibly stay the same but it might be good to substitute the Romanians with a Seated Leg Curl. It depends on the person, experience level, rest of the week and so on. The major takeaway though is that sometimes it doesn't matter how "good" an exercise is when its placement in the program increases injury risk exponentially.
Those are some of the physiological aspects to look at. It's not an exhaustive list but it covers the major points. Sometimes, an exercise is a good exercise because it works well enough but really it's just a favorite of the person doing it and that doesn't count for nothing. Having liked exercises in the program may help get somebody to the gym that otherwise might not, which makes it the most effective exercise of all.
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