When you start getting
stronger at barbell curls, one of the first things you notice
is that even when using very strict form, the path of the weight
means a lot of the torque goes into your lower back. Many people
try to get around this by using a weight belt that tightens
everything up to prevent injury. But that's like putting a band-aid
on a headache - it doesn't address the root of the problem,
which is weight out in front of your body and outside your base
of support.
The same is true
with heavy pushdowns. As you build up to heavier and heavier
weights, the resistance is outside your center of gravity and
it puts tremendous torque on your lower back. With pushdowns,
you have to resort to leaning forward, doing pushdowns in a
semi-lunge position or having someone hold you down!
The best way to go
to solve this probelm? Use a dip belt with a couple of heavy
plates hanging from your waist. This will dramatically increase
your body mass and make the weight that is putting the torque
on your back much less heavy, relative to your bodyweight. For
example, if you weigh 180 lbs and you're curling 110 pounds,
that's more than 60% of your body weight out in front of you.
But if you add 90 lbs of plates to your bodyweight, that drops
to 40%. This is a LOT less torque being directed onto your lower
back.
The same is true
for pushdowns. Heavier "body" mass means less torque
on your lower back.
The upsides? Less
back stress and the ability to immediately curl and do pushdowns
with more weight. Your biceps and triceps are still doing all
the work - you just don't have the back stress associated with
it.