Over the last 40 years, I've built a half a dozen Leg Curl/ Leg Extension Machines for my, and my friends' home gyms, and at one stage I did up to 275 lbs for 10 reps on the Leg Extensions, and up to 150 lbs for 10 reps on the Leg Curls.
At no time did I ever have pain or an injury from either exercise. IMO, people who have problems with either of these two exercises already have pre existing physical problems.
Also, it's my belief that pulley type machines, where your legs are pulled back and bent under you at the start, maybe harmful. On my machine, the weights are lifted directly on your lower shins, where your legs hang straight down at the start on Leg Extensions.
The biggest problem I've encountered on Leg Extensions, with heavy
weights, is keeping my butt down on the seat. Thus many times I've had
to switch to one leg at a time. Also I
recommend higher reps (10 to 20) because both of these exercises
are isolation movements.
The above photos shows equipment that can be made cheaply and can
be attached to any power rack. Along with a raised bench, the two exercises
shown can be done very easily.
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I built the above attachment for only three bucks ($3.00). It
is just part of my "6-in 1 Bench", which is featured in my book
"The Tight Wad's Guide For A Home Gym"
For more information e mail me at:
cablebar@olympus.net