They're called isometric exercises (as opposed to isotonic exercies which is where you perform x number of reps). They're good for building strength but you only build strength at the angle you're holding at which isn't really ver practical. They also cause your blood pressure to spike during the hold. Isotonic exercises are generally better to stick to for the bulk of your workouts, although you can throw some isometric stuff in every now and then just to change things up / shock your muscles.
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"contraction" 
Linear Mode
