Val Slide leg curls alone are a great exercise to work the posterior chain, particularly the hamstrings. The problem with them is that once people get stronger with them and are able to do anywhere between 12-15 reps per set they do not think about progressing them. They just keep plugging away at the standard Val Slide leg curl.
Well there are ways to progress them. You can use added resistance by placing a weight on top of the slides. You can add resistance by placing your heels inside one end of a band and having a partner anchor the other end of the band. You can progress them them by going to single leg curls or eccentric single leg leg curls. Or here is another way to progress them that I saw done by strength coach Ben Bruno. All you are going to do is perform an isometric barbell bridge while performing the leg curl action. Your hamstrings will be working overtime here as you use them to keep your hips up against the added resistance while flexing the knees. In fact my hamstrings were working so hard the first time I did these that I went into full on hamstring cramp mode. And that was only with a bar weight of 65 pounds. Take a look.