Anyone that has worked with me in the gym will know how I prefer big compound movements over machine/isolated exercises. Even as a bodybuilder the bulk of my training was done using compound lifts like rows, squats and deadlifts. This helped create muscle thickness and density that machine based training couldn’t produce (especially as an all natural lifter).
So to find that a true HIT protocol involves primarily using machines irked me a little at first. But after learning the reasons behind it I was soon converted.
The idea behind HIT training is to take a particular muscle and stress it to the point of absolute failure – so that every muscle fibre within the muscle has been recruited and then fatigued. As you avoid lock out and keep constant tension on the muscle the fibre is ‘always on’, and has no time to rest (even a brief second rest period at lockout may be enough time for some slow twitch muscle fibres to recover).
Combine this with the ‘super slow tempo’ and you now have muscle fibres that are not only actively recruited, but that are recruited at all parts of the lift. There is no ‘bounce’, no speed to help push through weak points, instead is it a slow grind where the fibres have to work at all points of the lift. You cannot cheat a lift if you’re lifting it with a tempo of 6-10 seconds. Nor can you use momentum to push through a weak point.
Now this is all well and good, and the idea behind taking to the muscle to absolute failure should hopefully now make sense.
Quick sidenote – initially when you lift a weight you will recruit your slow twitch muscle fibres, when they fatigue then you recruit your intermediate & some fast fibres, when they fatigue – and only under extreme circumstances – your body will then recruit the remaining fibres in the muscle, at which point when THEY fatigue then there is literally nothing else the body can do to move the weight. Now in contrast, if you were doing a standard set of 12 reps of pushups, every time you lock out your slow twitch muscle fibres get a brief opportunity to recover. Sure you will still hit a fatigue point, but that fatigue may not come as a result of total muscle fibre fatigue.
But this is where machine isolation training trumps compound free weight training.
With a compound movement like a squat, sure you’re working the quads to move the weight, but you’re also recruiting your glutes, hamstrings, calves and torso. And there are parts of the lift where the load on the quads is high, and parts where it’s lower. This is all well and good for overall strength, but if you’re trying to fatigue every single muscle fibre in the quad, you soon realise that a compound lift isn’t such as the squat, may not be the best tool for the job if you want to apply the HIT method of training. Again, any experienced bodybuilder will understand this.
You can see in this EMG activity report how the Quad & Hamstring activation varies at different angles during a squat. Source – www.mikereinold.com
For this reason, a machine that has been designed to target one specific muscle for the entire lift is the perfect way to ensure that you’re keeping constant tension (and constant recruitment) on all the targeted muscle.
If all this seems to confusing, think of it this way – the idea is to fatigue a particular muscle. Machine based training allows you to work a particular muscle through the full joint range, minimising other muscles that could potentially act as support.