How to activate your quads
Do you know how to activate your quads? Is neural inhibition impeding your ability to activate your quads or is it something simpler?
I've spent the last number of years both learning muscle control and teaching it. One of the most basic muscle activations that I teach is quadriceps (quads) activation. And I've taught it to people of different ages and different skill levels. And while neural inhibition may be part of the reason you can't activate your quads I'll suggest here that there may be a simpler reason.
First of all, let's assume that there is no problem with the actual wiring leading from your brain to your quads. And let's assume that you haven't recently injured your quads or something related.
If you've just been injured, your brain may have shut down quad activation to prevent further injury. So don't go messing around if that's the case. You'll need to rest. But if that isn't the case, then read on.
The first thing to note is that in order for any skeletal muscle to activate, it needs an opposing force to work against. So for example, if you flex your biceps, your triceps will also activate. Your triceps activate so that your biceps have an opposing force to work against. In the process, your elbow, which is kept still when you "flex" your biceps, becomes stiff or stabilized.
What opposes the quadriceps?
Your hamstrings and/or your gastrocnemius.
If either your gastroc or your hamstrings aren't working then that may be why your quads aren't activating.
They can't activate because they haven't got an opposing force to work against. And sure they may work in accelerated movements, or when doing a weight knee extension, because they've got an external force to work against (accelerated body parts of external weight).
In the absence of those, however, they need the force created by opposing muscle activation to work against in order to activate.
So let's say the calf is the problem. What does your calf, or any other muscle, need in order to activate? A fixed or stable end point.
If you stabilize or stiffen your feet, and in particular your heel, you may find that you can activate your gastroc and that in turn allows you to activate your quads. So if you are doing knee extensions to try to activate your quads, do yourself a favor and stiffen your feet prior to the knee extension.
You're quads may also fail to activate if your hamstrings for some reason are offline.
The hamstrings are a multi-joint muscle. The work on the back of your hip joint and knee joint. As with the calf muscles, the hamstrings also need a fixed endpoint to activate effectively. That could come from activating the caves, particularly the gastrocnemius, since two of it's tendons actually cross the hamstring tendons. However, another option is to anchor the hip bone.
Since the hamstrings attach to the sitting bone which is behind the hip joint, opposition can come from activating the muscles that attach to the ASIC and/or pubic bone, both of which are in front of the hip joint (when viewing the hip bone from the side).
Ironically, one of those hip flexors is a muscle that is part of the quadriceps group, the rectus femoris. However, it isn't the only muscle that can create a downward pull near the ASIC. There is also the sartorius and the tensor fascia latae. In addition there is the gluteus medius and iliopsoas. Another muscles that also may be important for resisting the pull of the hamstrings is the gracilis which attaches to the pubic bone.
Note that given the choice, because of it's relative simplicity, I'd look at the gastrocnemius first.
Suppose you have trouble activating your inner quad, the vastus medialis or more particularly the vastus medialis obliqus. What gives?
The vastus medialis obliqus is a portion of the vastus medialis that attaches to the adductor magnus longhead. This muscle in turn runs down from the sitting bone to attach to the inside edge of the bottom of the femur, just above the knee.
Going back to the idea that muscles need a fixed or stable end point for effective function, then if you want to activate your inner knee, make sure the adductor magnus is activated first.
There are a couple of simple ways to learn how to activate both you adductor magnus longhead along with your vastus medialis obliqus. They've covered in the VMO and adductor magnus course. It includes basic exercises for activating your quads both while sitting and standing, plus exercises for activating your vastus medialis obliquus. (It also comes with a 30 day "sensational" guarantee so that if it doesn't work for you, you'll get your money back.)