In general stair step machines are not bad for your joints, unless you already have knee or ankle problems or are predisposed to developing issues with these joints.
You should avoid stepping motion exercises if you have:
Arthritis
Gout
Chondromalacia (inflammation or irritation of the cartilage under the kneecap)
Plica syndrone (inflammation of the plica tissue on the inner side of the knee)
Benefit of Using a Stepper
If it doesn’t hurt your joints when using a step machine, there are several health benefits gained in using one. First, it is a great form of cardio exercise; a 140-pound person can burn on average almost 600 calories an hour. Because step machines use a vertical motion instead of a horizontal, they require more effort to complete the exercise. More effort equates to a better cardio fitness workout over a given period of time.
Second, because step machines target the calf, gluteal and hip flexor muscles, they are a great way to strengthen and tone your lower body - especially the thighs and buttocks.
Proper Form
As discussed above, using a step machine can be a great way to get both your heart and lower body into shape. But if you use it wrong, it can also cause problems.
The proper form for using a step machine is to:
Grip the hand rails lightly to maintain your balance, if needed. If you have to hang on the handrails to keep up with the machine, you are going too fast.
Stand upright while slightly leaning forward at the hips. A slight forward lean prevents your lower back from overarching and your knees from locking.
Take moderate even steps. Not only does this technique help you burn the maximum number of calories, it is also better for your calf muscles.
Keep your entire foot on the pedal. This prevents you from over-stressing your calf muscles and enables your buttocks and thighs to get the maximum workout.
To work your hamstring muscles more, move your foot until your heel is just off of the foot pad.
Alternatives to Using a Step Machine
If you find using a step machine causes pain in your hips, knees or ankles, there are two good alternatives:
Using an elliptical trainer or
Doing water aerobics/swimming.
Both still provide great cardio fitness workouts, but without the constant stress and pressure on your lower body joints.
When using a step machine, you are bringing your full body weight down on a hip, knee and ankle with each step you take, trying to overcome the resistance of the foot pad to drive it downward. Because you are alternating and concentrating all your weight on one leg at a time, more so than say if you were just walking naturally up a set of steps, the extra pressure can cause problems, especially if you are overweight.