I bought a physioball last night for the core performance workouts, so I can do prehab and the physioball workouts at home. The book recommended a 55 cm ball for my height (5'7"), the boxes they came in said I fit in the range for the 65 cm ball (I got that one).
Here's the thing: in the pictures in the CP book, the physioball they're using is proportionally way bigger than the one I got. My physioball is too small for me to get in some of the positions illustrated in the book. For example: for one of the letter stretches in the prehab unit, it shows the woman with her arms fully extended to the floor. On the 65 cm ball, I'm too close to the ground to fully extend my arms. I can still inflate the ball more to make it a bit firmer, but it's already fairly firm, and I'm having enough trouble balancing on it for some of the exercises right now.
So, should I look into getting a larger one, or is this one the way to go, and I should just deal with it?
I had that same problem and I just made due with what I had. For a majority of the excercises, the ball I had was perfect. There were just those few that didn't work...
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"It's what you've got inside that matters. The details and technological things will take you only so far. You still have to pedal the bike. Some people are always looking for the magic secret. There's no secret. Just bust your ass." -Dave Zabriskie
Can you post a picture of the exercise you are describing? I am having a hard time picturing it.
I have 2 balls (stop laughing) and the large one is 65cms. I have never had a problem with it being too small, but maybe I have not done this exercise. How tall and big are you? Again, I am 5'9" and sometimes I feel it is too big for me...
From Core Performance, it's a "letter" stretch. I forgot exactly which one, but basically you make a letter with your arms: "Y" "T" "W" You lay over the ball, facing the floor, with your chest off the top of the ball (ie, you're resting on the ball with your abs). In the starting position, the picture has the model with her arms fully extened basically straight down towards the ground. My hands hit the ground before I can fully extend my arms. And looking at how big the physioball looks compared to the models (they use a woman and a man) mine is proportionatly smaller.
There's also one where I'm laying on it facing the ceiling. My knees are supposed to be bent 90 degrees, but the ball is too low for that; my hips are below my knees, and I'd have to thrust out more to get them @ 90 deg.
I'll try putting more air into it tonight and see how much that will help. It'll be fun to fall off it a few more times!
Sounds like you would be better of using a 75cm ball then. Thera-band makes some nice balls and they have a 75cm model. Very nice quality ball and it is anti-burst.
Also more info...
Q. What is the proper inflation size of the exercise ball?
A. Each ball is designated by centimeters of height, from 45 cm (17.5") to 85 cm (33.5"). Inflate the ball until the top of the ball reaches the designated height from the ground; use a tape measure to compare the height of the ball. If the ball has too much air, it will be very hard and unstable; too little air causes it to "bottom out" and become too stable. Correct inflation should be firm (neither too hard nor too soft) that retains its roundness.
Warning: Never inflate you ball greater than its labeled maximum diameter, Do not inflate when the temperature is greater than 90º F/32º C or less than 65º F/18º C
You'll find that many of the balls 'vary' even though the cm's are labeled the same. Just inflate the heck out of it and let it stretch out a bit....it will get softer as you use it without losing size.
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Robert dos Remedios, MA, CSCS,
HCC (Hartman-Cosgrove Certified)
Director of Speed, Strength & Conditioning
College of the Canyons, CA http://www.canyons.edu/departments/pe/strength
"NO CHAMPION HAS EVER ACHIEVED HIS OR HER GOAL WITHOUT SHOWING MORE DEDICATION THAN THE NEXT PERSON; MAKING MORE SACRIFICES THAN THE NEXT PERSON; WORKING HARDER, TRAINING, AND CONDITIONING HIM / HERSELF MORE THAN THE NEXT PERSON; ENJOYING HIS / HER FINAL GOAL MORE THAN THE NEXT PERSON"
If you're 5'7", you just haven't fully inflated your physioball yet. A 75 cm ball is pretty large. Even athletes that are a bit taller than 6 feet don't use 75cm physioballs.
:p 
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