Here's a new way to perform back extensions for meatier hamstrings and glutes. Train at home? Train in a gym that doesn't have a back extension or GHR bench? These exercises are great workarounds.
1. Bent-Knee Glute Extension
Taking a few minutes to set this up is worth the glute gains. You can also use a Smith machine, which in a commercial gym setting might be more appropriate.
Think of these as 45-degree back extensions. Set a bench to an incline (30-45 degrees works best) and throw a pad on the end for your hips. With this version though, you'll lock yourself in position with your feet against an immovable bar.
Everything about this position screams glutes. Your toes should be turned outwards and hips externally rotated. This frog-like position places a little more emphasis on the glutes as external rotators. (It's not as ball-crushing as you might think.)
Unlike a typical back extension with straighter knees, your knees stay bent to around a 90-degree angle. This slackens your hamstrings and forces your glutes to work harder as hip extensors. It's one of the reasons why glute bridges and hip thrusts are so effective too.
Lock yourself in with your hip crease on the edge of the bench, then execute by driving your hips into the pad. Keep a slight rounding through your thoracic spine and keep your chin tucked. Your arms can be crossed, or you can hold a plate or dumbbell to your chest to add load. A few sets of 15-25 reps will do the trick.
2. Hamstring Destroyer in Rack
This exercise was popularized by Eugene Teo. Personally, I have no issues doing more than just squats in a power rack, but doing all your assistance work in there too might grind some gears, so I'd suggest using a Smith machine.
Think of this as a horizontal back extension with slightly bent knees. Unlike the previous setup though, you'll need to be further from the bar so there's a greater angle at your knees. This will help get rid of the slack and place more tension through your hamstrings.
With these you're looking to maximally load your hamstrings in their mid to shortened position, which is made possible by the combined bent knee and extended hip (your hamstrings do both). These are also made more effective by your body alignment and foot placement. The devil is in the details with this one.
Your feet should be parallel and the bar should be closer to your toes (versus mid-foot in the previous version). You need to feel secure, but placing the bar closer to your forefoot will increase recruitment of your gastrocnemius. This co-activation of your calf muscles will have an add-on effect, forcing your hamstrings to work harder.
Lock yourself in with your forefoot to the bar and your hip crease on the edge of the bench. Keep a straight back position and initiate the pull with your hamstrings. Press your toes down throughout like you're trying to calf-raise the bar up (you won't). Then try not to squirm as your hamstrings and calves want to cramp.
These are deceptively difficult. If performed correctly, they will rarely require more than your own bodyweight. Before you start adding load, try some isometric holds in the top position to really intensify things.
3. Glute Destroyer
Take the hamstring destroyer, change a few body angles and the way you initiate the exercise, and now you've got an effective way to pump some blood into your glutes.
Turn your toes outward, set your knee angle to 90 degrees (remember, this slackens your hamstrings), round a little through your thoracic spine, and tuck your chin.
Initiate by clenching your butt cheeks and driving your hips into the bench. Imagine someone trying to steal your last dollar from between your butt cheeks in the top position.
Your glutes will be active as both hip extensors and external rotators, working them hard in their shortened position. Pump out 2-3 sets of 15-25 reps, adding weight when appropriate.