You’ve probably seen the viral video of the lady in the leg press machine getting her legs bent inside out, making you question yourself using a leg press machine ever again. Now the belief and myth for leg press, in fact, every exercise, is not to lock out your joints, otherwise you're going to turn into an accordion. The fact is that she wasn’t even paying attention, being distracted by the cameras and the crowd around her, she didn’t respect how dangerous the equipment was. It doesn’t have to be dangerous, if you know what you are doing and how you need to do it, kind of like anything that requires a bit of skill. What I’m going to take you through, is the difference between locking out for strength and control, and time under tension, for muscle mass and endurance/cardio.
Creating ultimate strength is called Myofibril Hypertrophy, tearing the muscle that does the movement of the joint and repairing it through recovery so it is stronger. Placing high loads of weight under short amounts of reps and full range of motion, is needed to create a stronger, functional body, which ultimately means locking out.
The myth is you never lock out. I’m not sure whether this originated from people getting injuries from hyperextension of the joints, or that purely for creating muscle mass, like what most people do whilst at the gym, depends on this technique.
I could go into the science of it, but basically when you take your time with any exercise and focus on form you get yourself the best results and are less likely injure yourself. Training for strength is what’s relative to you in terms of going heavy. You want to be doing between 3-7 reps for men and 5-9 for women, and working between 80-90% of your 1RM (a 1RM is how much you can lift for 1 rep... 1 Repetition Max). This is called Myofibril Hypertrophy (the extreme version of this is powerlifters) and requires a lot of energy (carbs) called ATP which is a highly explosive energy. ATP lasts for about 10 seconds so you want to get the most out of it. When lifting for strength you want to lengthen your muscle from point A to point B with perfect form, and lock out to take time off the tension, so you can reserve energy for another solid lift, and regain control, form, and stability.
Most commonly, as explained in the lock out part above, people are introduced to training in the gym via doing muscle building exercises, with form commonly known as “time under tension”. Creating size, or mass in muscle is called Sarcoplasmic Hypertrophy (think: body-building style training, ie. 'figure sculpting'), this is basically your muscles lengthening and contracting from point A to point B, without stopping, and not going to full lock out (e.g. you're doing a pushup, and you don't lock your elbows at the top, as soon as you're almost at lock out stage you start another rep), which means the muscles are under tension for a longer period of time creating micro-tears through the muscle belly from glycogen, blood and water being pumped into the muscles.
The by-product of muscle from this style of training is know as lactic acid (that burning feeling from working out), which results in fat burning. The Sarcoplasmic Hypertrophy rep range is between 8-15 for men and 10-15 for women at 50-80% of your 1rm. The form on sarcoplasmic is most commonly a bit quicker with the count on reps, so generally doing the exercises quicker, mainly because there is less weight to create injuries, but like all exercise they must be done with caution. Body weight exercises fall under this rule too, but yet again, some are still hard to do, and your technique always depends on what your goals are.
Doing 16+ reps comes in to the cardio/endurance section, where your body is just burning left over fuel, and using your oxygen supplies, making you fitter + leaner. You don’t have to just do 1 style, you can combine all of these in one workout, generally starting with strength (myofibril), size (sarcoplasmic), then endurance.