To my knowledge, there aren’t strong recommendations for load, as long as the set is subjectively challenging.Time per stretch has mostly been researched in the 0-60s bracket. Within this bracket:
- 2-7s is best for strength.
- 15-45s is good for hypertrophy
- 30-60s is good for improving tendon thickness (if managing tendinopathy, for example)
Of course, there is a lot of grey area between these zones.
Also, just because this is the bracket that has recieved the most research, doesn’t necessarily mean it is the be-all and end-all. There are many great coaches such as Cal Dietz, Joel Smith, Tommy John, Jay Schroeder and Grant Fowler who routinely do loaded stretches for 3-5 minutes. They often cite hypertrophy; improved local muscular endurance and lactate buffering; increased range of motion; improved recovery and mental “sharpness” as major adaptations from such training. I’ve tried it. It’s unbelievably brutal.
Finally, it’s important to differentiate adaptations between overcoming and yielding isometrics. An overcoming isometric is one where you push against an immovable object, whereas a yielding isometric is holding a constant load in a static position. From a nervous system standpoint, there is evidence to suggest that an overcoming isometric is more “concentric,” whilst a yielding isometric is more “eccentric”. Overcoming isometrics are a little bit safer at higher outputs because there is no risk of a weight “crushing” you. Yielding isometrics are better for longer durations because they do not allow your force output to drop as you fatigue (lest you drop the weight). Thus, the “strength” isometrics are best done in an overcoming fashion, the 15-60s isometrics could be done in either fashion and the very long isometrics are best done in a yielding fashion.