Bodybuilding means overloading the right movements to grow the right muscles, but how you do this is flexible despite all the dogma out there.
Some aspects of bodybuilding are ironclad though.
Single-joint exercises work multi-joint muscles best. Though compounds are easier on the joints, induce more growth in single-joint muscles, and should usually be prioritized in your workouts, you still must isolate to look your best.
Machines overload just as effectively as free weights, so in theory stimulate growth just as well too.
Some professional bodybuilders, becoming quite strong, want to avoid the risks of handling massive free weights. Transitioning to machines for most exercises, they don’t appear to suffer for it.
There’s nothing special in regards to free weights involving the stabilizers, at least for bodybuilding. The classic exercises don’t work them much besides not contributing to a great physique.
However, friction on machines may hamper the negative phase of the rep for less growth.
Machines can prevent natural movement. For example, the proper arc toward your face on the way up when bench pressing may not be facilitated, which can harm your shoulders.
Overloading the whole range of motion isn’t essential. Dorsiflexion for the tibialis anterior has you feel more tension throughout with your feet high on a closed-chain leg press, yet doing it one-legged, off a stair and leaning back, works too.
Strength curves, or having different points throughout the range of motion more or less difficult, almost never matter.
Thinking otherwise leads many to strive for variety possible with machines, say working their biceps peak with cable preacher curls, in addition to EZ-bar curls for the belly & dumbbell incline curls for the lower portion.
All of these work the same elbow flexor muscle fibers at longer or shorter lengths, perhaps just accumulating volume less tediously.
There could be more disadvantages. It’s too easy to lean back on pull-downs, reducing lat involvement by turning it into a row of sorts, versus a pull-up forcing strict motion that prevents shoulder hyperextension.
All functions can be overloaded with free weights, if done thoughtfully (and even without cable pulleys), so machines are never required for a healthy trainee.
However, the ease-of-use that machines provide can be significant, even vital, depending on your circumstances.
Reasons
Muscles were designed by evolution to overcome the pull of gravity rather than to work against machine resistance, so the biggest gains you will make in building size and strength will come from pumping iron—using a barbell and dumbbells.
– Arnold Schwarzenegger
Easily load knee extension & flexion.
Though just one benefit, machines are so important for single-joint overload at the knees that this alone justifies them.
Sissy squats & dumbbell lying leg curls feel awkward. You may need to do higher reps than preferred, pre-fatigue with squats or stiff-legged deadlifts, and involve unwanted muscles substantially if not using machines.
The sissy squat almost feels like a closed-chain pull-over for the upper body, and the leg curl involves the adductors just to avoid dropping the weight.
This draws attention away from what should be your real focus: performing enough close-to-failure reps with heavier & heavier weights.
For leg extensions & curls, seated vs. standing vs. lying makes little difference… just extend or flex your hips enough respectively to prevent over-shortening that leads to cramping.
Reduce axial stress on the spine.
Free weight movements can stress your back, either directly or in order to maintain your positioning.
A chest-supported row or a machine-based reverse fly, instead of a free weight row or bent-over lateral raise, unburdens your spine, which applies to leg presses instead of squats too.
Keep in mind that sparing your back means not working the lower back muscles, so it’s not recommended to limit free weight movements completely here.
Vary muscles isolated.
Machines can help on a case-by-case basis depending on your split routine structure.
The hyperextension doesn’t involve your upper body, working the lower body like a stiff-legged/Romanian deadlift, yet without the riskiness of a good morning most overloading the vulnerable stretched position.
The pull-over machine hits the lats due to external rotation, unlike the dumbbell version, but working the long triceps head versus the single-joint elbow flexors (which can feel overtrained if using lots of exercises with them).
Include useful cardio options.
A rowing machine and the Airdyne/assault bike involve both the upper & lower body significantly, getting your heart rate up without relying on your legs alone.
These options, among other machines, exert less impact forces on the joints as well.
Like working the lower back, some but not too much stress here is best, but you can still consider these depending on how you are holding up.
Train safely.
This again is a case-by-case analysis depending on your needs at the moment.
If you injured your leg so you can’t stabilize on a free weight movement, whether acute or chronic, the machine equivalent would let you train the prime movers hard.
A leg press may work better than a back squat if it hurts your shoulders just to reach back and hold the barbell, especially with a safety bar being less commonly available.
Bodybuilding Machines Are Tools
Machines are simply tools, even if less versatile on average for normal trainees, along with free weights to build muscle.
Avoid convictions like that all free weight selections are better. As you get stronger, the popular barbell exercises present their own dangers!
Instead, focus as specifically on your goals as possible to achieve what you desire, never adhering to some ideal for its own sake.
Bodybuilding is an unnatural activity that doesn’t work muscles as they evolved to perform, and therefore has its own requirements to pursue best.
Personally, I feel leg extension & curl machines are far more practical than alternatives so to be almost required, yet these are the only machines I value.
So what machines are best for bodybuilding? Probably very few in most circumstances, but only you can decide this according to your needs (and not some rigid philosophy)!