How do I know if my weights are heavy enough? Check your form. This workout involves many repetitions of the same exercise and you will know you are using the correct weight if your form stays consistent between the first part of a repetition set and the end. For example, a row from plank should look the same on repetition number 10 as it does in repetition number two, even if the effort is much greater. If your form is wobbly by the end, drop down the weight amount until you’re able to find consistency. Don’t forget that working with weights is not an all-or-nothing proposition. Your body also provides resistance. Try our 9-Minute Strength Workout for a weight-free option. 

Don’t get us wrong—cardio is important for keeping your body fat down and keeping your heart health in check. (Bonus points if you run or bike, since outdoor exercise is linked to better energy and improved mental health.) But when it comes to building muscle, hitting the treadmill won't help you much. “Every component of exercise, minus cardio, can help with muscle hypertrophy,” which is the scientific term for muscle building, says Michelle Lovitt, an exercise physiologist and trainer in Los Angeles. “Cardio tends to burn calories and puts your body in a deficit, which is great for leaning out, but not building mass.”
Apply the above concept to your hips. When you sit, your hips are in a "flexed" position. Therefore, the muscles that flex your hips are in a shortened state. You probably spend at least a third of your day sitting down. Think about how much time those hip flexor muscles stay shortened. A lot. Over time, they become tighter and tighter until you look like the old man in the picture. So unless you want to look like that, perform the stretches shown below.
Tight hip flexors can also make it harder for your glutes to activate—since they're opposing muscle groups, when one is really tight the other becomes lengthened. When a muscle is more lengthened than it should be, it takes away some of its ability to contract. When your glutes are in this compromised position, it can cause other muscles to do more work than they should, making your workouts less efficient and sometimes, increasing your risk of injury.
How can the muscle progress just because you held a weight for awhile when you could of held a heavier weight for less time? It won’t. It won’t grow because it’s not receiving new tension. Extending the rep by going slower is great, yes, but this slow must be the actual bar speed and not just slow because you can make it slow. You create actual bar speed by making light weight feel heavy. So lift light weight so that the fibers have to switch when it starts to feel heavy. This will increase your strength compared to just lifting heavier right away or all the time. This will help create an actual tempo with actual weights. Remember my example above about how the overall weight after making light weights feel heavy? This is because your muscles have sensed a level of tension that altered its force production so now you have to lift less, yet work harder. Read that again :) this is growth. This is how muscles sense it needs to grow. If you keep the same weight and never increase the weight, then you keep the same tension. This same tension is not enough to create new tension. Remember when I talked about failure? Well, the point where the fatigue of failure comes into play alters as well. It takes less time. That’s the point. Not much time is needed for growth, just break down the muscle as much as it can to a healthy level and do it again. Keep doing it and keep trying to increase the weight.
It has been argued that purposely overtraining for a brief period can be beneficial. One article published by Muscle & Fitness magazine stated that you can "Overtrain for Big Gains". It suggested that if one is planning a restful holiday and does not wish to inhibit their bodybuilding lifestyle too much, they should overtrain before taking the holiday, so the body can recuperate and grow during the prolonged rest period. Overtraining can be used advantageously, as when a bodybuilder is purposely overtrained for a brief period of time to super compensate during a regeneration phase. These are known as "shock micro-cycles" and were a key training technique used by Soviet athletes.[53]