For a lot of moderate to highly experienced lifters, finding that slight edge can make a big difference in what you look like and how you perform in the gym. Sometimes these things come from the head of an inventor like Mark Bell and his Slingshot, or sometimes they can be deemed "bro-science" like fasted cardio. Sometimes the slight edge you may need can be found hidden in a peer-reviewed journal that no self-respecting meat head would be caught reading. This may be the case when it comes to weighted stretching.
Although it has risen to prominence in recent years, thanks to the work of Dr. Jacob Wilson, weighted stretching may be an underrated and easy means to increase the size of stubborn muscles. The research around weighted stretching and muscle hypertrophy dates back to the ´90s. In the early 1990s, a study was done on birds that had nothing short of amazing results. In this study, Dr. Jose Antonio attached a weight to the wing of young quail, and over the course of a month, he progressively increased the weight. After a month, the level of muscle hypertrophy (growth) was measured. The bird's wing where the weight was attached had grown by over 300 percent (318 percent to be exact). After studying the results more closely, Dr. Antonio discovered evidence of hyperplasia. A controversial topic in the strength and conditioning field, hyperplasia is the growth of muscles not through the increase in size of the fibers (hypertrophy) but through the increase in number. This is something that is still not conclusively observed in humans, but could open the door for insane increases in size and strength if it can be tapped into.
Further studies on both birds and humans have shown that weighted stretching can increase muscle size, but not muscle strength. Why does this weighted stretching cause such an increase in muscle size? As of now, the best theories are that stretching causes an increase of stress/fatigue on the muscle fibers that causes them to break down more and subsequently be rebuilt stronger and larger. Weighted stretching in between sets may also work as a means to occlude the muscle of blood (a topic for another article), which will lead to more muscle damage and more repair.
How can you incorporate weighted stretching into your workout program? My advice is to pair weighted stretches in between isolated single muscle exercises like a calf raise or bicep curl. Hold a light dumbbell or add resistance so the weight keeps constant tension on the muscle as you stretch for 30-45 seconds. Weighted stretching is similar to blood flow resistance training in that it may burn more while you are stretching than while you are performing your actual sets. It's just mind over matter.
If you are feeling like you should be getting more out of your workouts, try throwing in some weighted stretches in between your accessory work. It may be that boost you need to continue to make solid progress and get closer toward your goals.