When you've accomplished a certain level of proficiency with exercise movements, you will need to look at the next progressions to continue to challenge the body if you are looking to make more change. Some factors that can make a movement harder increase the required mobility, stability, coordination of muscle activation, or speed to complete the movement.

Here are a few to get you started:

Single Leg DeadLift
Master these first:
Romanian deadlift or semi-stiff-legged deadlift
Single-leg hip bridge

The Challenge:
Doing a movement on one side of the body (unilaterally) adds a rotational challenge to an exercise and forces the stabilizing side to work harder to offset the rotation. The Romanian deadlift can be hard enough to keep your chest open by keeping a tight upper back and core activated to keep your spine in neutral, all while shifting your weight onto your hip to create a hinge. Ideally the hip starts square with the stabilizing leg and stays square throughout the entire movement. Another challenge of this movement is loading the weight onto the hip and not loading the quads in the same way that you would with the Romanian deadlift.

What's next:
You can challenge this movement by loading the body in different ways, such as dumbbells in your hands (or in one), weight on your back like a single-leg good morning, or cable loading for some horizontal resistance to load the anteroposterior direction.

Overhead Squats
Master these first:
Back squats
Front Squats
Overhead press

The Challenge:
What makes this movement challenging is the overhead position of the barbell. This requires the additional mobility in the shoulders, pecs and lats to keep the barbell over the centre of your base (area of the floor your feet cover) while the angle between the arms and torso changes. This also requires the stability from the shoulders and back to stay locked out overhead through the full movement so you can work safely and efficiently without energy leaks from sub-optimal mechanics. Factor in that your external load is now above your body and your centre of gravity is now higher from the ground, it makes your torso work even harder. Also, as you distribute all of the weight between the front and back of your body, it is challenging to keep the weight centred in the middle of your feet. Doing all of this while keeping an efficient squat pattern makes this one big move your next tough challenge.

What's next?
Proficient overhead squatting is the gateway to even more challenging movements where you can incorporate all the technical components here with some speed. Having a solid overhead squat gives you the green light to start learning progressions for Olympic lifts (snatch and clean and jerk).

Moving Planks
Master this first:
Dead bug
Front plank
Seated row

The Challenge:
Front planks are hard enough if done well! You should already feel your abs, lower abs, glutes, quads, and back throughout the hold. Now imagine moving side to side or forward and back while managing all of the other muscles you are looking to squeeze.

What's next?
Being able to keep your spine in neutral while stabilizing your shoulders under load leave you ready to be able to handle more challenging core exercises. Ab roll outs, hanging leg raises and plank variations with fewer points of support and/or change in stability of the surfaces.

Working on these types of progressions for your exercises or workouts will give you some training variety for your level of engagement and to move you towards your goals while providing you with new challenges.