A physics question about a workout.?

Question

According to my bathroom scale, I weigh 221 lbs. On my bowflex, the resistance I use for pull downs is 260 lbs. That means I am pulling more than my own weight downward. Why do my pull downs then not become chin ups? Is it something about the way the flexing rods generate the resistance, or is the resistance being overcome in a direction other than straight downward?

Answer 1

something here is not right ????

On the lat pulldown at the gym, there is a "bar" that locks my knees down to prevent what you have described.  Without that, they would be chin ups.  Now, I don't know anything about the bowflex, but I would assume the rods are providing the same resistance as would actual weights in that you have to exert 260 pounds of resistance effort to bend the bars.  I'm guessing your bathroom scale is correct - so the 260 on the bowflex must either be

1) incorrect

2) actually reflect some conversion to gym weights, such as 10 bowflex pounds = 1 barbell pound.

Try increasing the resistance until you lift yourself off the machine and see what the weight is.

Answer 2

The resistance increases throughout the stroke.  At the very top of your pull down, you're probably pulling only 50 lbs or so.  You don't reach the full 260 lb level until the ends of the "power rods" are almost touching that first pulley.