The most obvious reason your legs might not be strong is if you don’t squat. Now, I understand some people have injuries that don’t allow them to squat—these people get a pass. But if you're healthy and free of injury, get your butt in the squat rack! There's no substitute for barbell back squats. Leg presses, Smith machine squats, and leg curls all have their place, but the benefits of these exercises pale in comparison to a properly executed squat.
If you are squatting, that’s great. But are you squatting with a full range of motion? In a full squat, you thighs should be below "parallel position." Full squats have been proven to be superior for strength and muscle-building when compared to their half squat counterpart.
Are full squats bad for your knees? No, according to the science, they are not. Full squats actually help strengthen the structures of the knee and aid in injury prevention.
Not all cardio is created equal. If you are slogging away for hours jogging or walking on a treadmill, you may actually be hurting your strength gains. One study found that combined strength and endurance training can suppress some of the adaptations to strength training. So, how do you get your cardio in without hurting strength gains? The answer lies in high intensity interval training, or HIIT. HIIT can give you a superior cardiovascular workout while preserving muscle mass and strength.
When you impose stress on your body, it will adapt and change to better handle that stress in the future. So, if you go in to the gym and do 4 sets of 10 every week with the same weight, your body will adapt to that stress. But the whole point of lifting is to continue progressing. That means that once your body adapts to 4 sets of 10 you have to change the stressor. This can be adding more reps, more sets, more weight, or cutting down rest periods.
If you're an intense strength athlete who's serious about gaining strength, make sure you're consuming at least 1g of protein per pound of bodyweight to facilitate the muscle adaptation process. That's not typical for every guy—but if you're looking to gain muscle mass and strength, it's crucial.