Free weights or machines. What works better for strength training?

Training with free weights or machines. Which one is actually better?

The muscle fundamentally doesn't care whether it is trained with free weights or machines. A muscle simply adapts to a training stimulus. The greater the muscle fatigue (as long as it occurs within a sensible tension duration), the greater the anabolic response will be. Good results can be achieved with both tools, provided certain points are observed. Training with free weights, however, comes with one clear disadvantage.
Using the biceps as an example, you can clearly see the disadvantage of free weights compared with machine training. When we let go of the dumbbell, you will notice that it always falls towards the floor (gravity). This is a clear disadvantage, because the load always acts in the same direction, namely towards the floor. The forearm, however, rotates around the elbow joint.

Muscular torque depending on the angle of the joint

Figure 1: Muscular torque depending on the angle of the joint, e.g. for the dumbbell exercise “elbow flexion of the forearm” (“biceps curl”).

Figure 1 shows the muscular torque depending on the joint angle for the dumbbell exercise “elbow flexion of the forearm” (“biceps curl”). The blue area shows the muscle strength potential that is actually used. The orange area therefore shows the strength potential that is not used. The unused potential arises because the dumbbell only provides a linear resistance, while the forearm rotates around the elbow joint. Training machines with a cam therefore compensate for this disadvantage, as they are able to produce direct and varying rotational resistance. The muscle's strength potential can thus be optimally exploited at all joint angles.

Figure 2: Comparison of the extension torque during free weight training of the biceps with the extension torque during machine training of the biceps. (Gottlob 2011)

As you can see in Figure 2, the extension torque with the arm almost fully extended is close to zero for the free weight exercise. During training on a strength machine, the resistance therefore still acts on the biceps even when the arm is extended.
In this case, it would make sense to choose the iso contraction training variant for the dumbbell exercise, as the resistance remains constant during the static contraction.

The following are the further advantages and disadvantages of free weights and strength machines

Whether you choose free weights or machines is fundamentally irrelevant.

If you build less muscle than you want, this is hardly due to the machine.

If muscle building does not work as you would like, this can be due to a lack of protein intake.
You can read here how you should structure your protein intake.