If your incorporating strength training into your weekly fitness routine, it is likely that you care about building muscle/toning, improving strength, reducing injury risk, improving mobility/stability, or likely some combination of these factors. For Christmas my wife and I got a family membership at a local athletic club so we could take the kids swimming during the cold months as well as use the childcare to get some active "date nights" out of it. Being back in a commercial gym after nearly 4 years away training exclusively at my home or training studio I'd forgotten how suboptimal much of the training is that the average person does while training. One concept that will improve most training outcomes is training through full range of motion, particularly the portion where the targeted muscle(s) are in lengthened or stretched positions. Training a muscle in a stretched position and loading it there has a number of benefits.
More muscle: More and more exercise science research is pointing to the superiority of training muscles at lengthened positions. Partial range of motion training at short muscle lengths has routinely been shown to be inferior to full range of motion training (in spite of people often feeling the muscle more with these short ranges). Research comparing full range of motion and partial range at short and long muscle lengths has shown partials at long muscle lengths to be equal to or superior to full range of motion, while both full range and partials at long lengths are found to be superior to short muscle length training. If you want more muscle, more toned/defined muscles, or to change the shape of a particular muscle group then you want to focus on using a full range of motion and prioritizing the stretched portion of a movement.
Improved mobility: A number of studies have found that weight training with full range of motion is equal to or superior to stretching programs when it comes to improving mobility. Likely this is due to some sort of feedback mechanism when you actually have to contract and engage a muscle group in a lengthened position vs simply holding a stretch like traditional static stretching. As an anecdotal example, I routinely have people comment on how mobile/flexible I am. However, I almost never stretch, maybe 5-10 minutes per week, tops. But when I do train, I always strive to take each muscle/joint through its maximal range of motion. In essence if you lift with the maximum range of motion, you are repeatedly doing short mini stretches with an added external load. If we assume a typical set lasts 20-40 seconds, and the standard training program consists of at least 3 sets for each movement/muscle group then you're getting 60-120 seconds of loaded stretching minimum for each exercise.
Injury reduction and increased resiliency: The vast majority of acute injuries happen at long muscle lengths. Muscle tears virtually all occur when the muscle is in a lengthened position and doesn't have the strength to buffer the forces being applied to the tissue. Gradually using heavier loads over time in lengthened muscle positions will help to reduce the likelihood of these injuries and likely make them less severe if they do occur. More mobility and more strength is a staple in many good physcial therapy programs and training at long muscle lengths addresses both of these qualities.
Exercise Selection: To get the most out of this concept we want to select exercises that put the targeted muscles in lengthened positions under load. Search for exercises that stretch muscles, aren't painful, and allow you to be progressive in load over time.
Caution: Training at long muscle lengths is more likely to make you sore. These are also positions where your joints may be more likely to have impingement of tissues and may be less stable initially. When introducing new approaches or exercises into your training program it is always best to start light, and with only 1-2 sets of 6-10 reps. If things feel good over the next 48 hours then you can gradually start to be more progressive over time. If something hurts reduce the load being used until the movement is pain free. If all movement is painful regardless of load consult a physical therapist of medical doctor.