The other aspect of cam design that doesn’t get discussed much is the effect of fatigue on the strength curve for the muscle or movement. As you perform reps and get fatigued, the strength curve changes shape. You get weaker disproportionately in the most contracted position. So, at best, the cam will only be perfect for one of the reps in the set.
Here is how the David Exercise Machine company describes the issue:
Muscle fatigue causes the reduction of strength with each passing repetition. This is a very well known fact and can be felt by every exerciser.
What is not so well known is that the muscle does not fatigue in a proportional manner at all muscle lengths. In fact the muscle fatigues in a disproportional manner in shortened position.
The graph demonstrates that the “strength curve” produced by the muscle changes as the fatigue progresses.
This phenomenon is better understood if we do one single, maximum repetition. Since the load is very high, the movement is very slow. What happens is that during this one repetition, muscle fatigues. So at the end of the movement muscle is more fatigued than in the beginning, which is logical. In the same manner fatiguing happens also with a set of repetitions.
I just find it interesting that’s he older Nautilu s machines usually had some big counter weights but the newer Nitro type machines don’t seem to yet the movement is basically the same thing? How did they do away with the need for the counter weights ?
Maybe later designers decided that the degree of improvement in the resistance curve from proper counter weighting was not big enough to warrant the impact on the cost or the space required for the machines?
2ST Preacher Curl has a counterweight on the movement arm. Looking at the picture of the NITRO Curl, I think that cylinder shape attached to the cam just behind the axis of rotation is the counterweight. Could be wrong.
Yes that looks to be a counter weight but it seems so small compared to the big protruding counter weight on my compound bicep which is roughly the same motion? I guess they figured out how to shrink the counter weight size? I’m also wondering if the movement arms on the older machines were much heavier than the newer machines that had lightened it up with lighter metal so it didn’t need as much counter weight?
Scott
Definitely looks lighter than my multi arms. Also just 1 fused arm instead of weights for 2 independent arms. The cams on the newer machines are also different
If I were designing a cam I would design it so that there were no particular hard parts of the movement . It would be equally hard at the very beginning , middle and end. Best of all would be a cam that was variable so you could make it harder where ever you chose it to be harder. If I recall some machines havevsomething like that but I can’t remember what they are ?
Scott
Maybe you are thinking of the Strive line of machines? They have weight horns at several positions, which allow you to emphasize different parts of the range of motion (beginning, middle, or end).
At one point I had some extra cams I took off machines I took parts from. On my BNTA the cam was way to aggressive so I took a similar shaped cam and drilled multiple holes in the center and moved it from one to the other hole trying to find where the feel was even. Unfortunately I got cut short in my experiment for other reasons and just put it back to normal. In reading Hutchins book he talks of working in the prototype shop with Scott Legear . I’ve always thought that would be the coolest job building and testing prototypes !!
Scott
Many years ago when I was in my heyday of my muscle building I got a job at Heartline Fitness in the fabrication shop helping this Romanian welder make machines. I the short time I was there I learned a lot about building exercise machines. I was in heaven until I realized the shop had poor ventilation and when I’d come home it was like working in a coal mine, my nose was black from welding smoke and paint. I remember the show room was full of Nautilus machines people traded in. Had I stayed I could have had any machine I wanted. As crazy as it may seem I guess I valued my health more than exercise machines so I quit.
Scott
Yes, that is the former Strive Plate-loaded line with 3 different weight horns. They also made Selectorized machines that have an adjustable cam. Similarly, you can adjust the cam to make it harder in the top, middle, or bottom. I own the Strive selectorized Leg Press. It’s a good leg press. The only machine I like where the seat moves instead of the foot plate. It has a pretty small footprint and a 510 wt. stack with 5 lb increment slide-ons. Both the selectorized and plate-load line are still being made. The new company name is PRIME. Can be found at primefitnessusa.com. They also make an interesting adjustable Trap Bar.
Not crazy at all. Quite insightful, actually. I’ve been a welder/fabricator for a while and have left a few places for exactly that reason.
Anyhow, that’s an interesting take on how you’d like a cam to function. I’d have to brush up on cam designs to fully understand how much mechanical advantage takes place and where in the rotation it occurs. I know that some of the features of our bodies have developed over a really long time though and can be really difficult to counteract.
Thanks for the link to Prime Fitness. Some interesting looking stuff, and made in Pennsylvania. Nice to see that.
The trap bar is neat. Looks similar to the Kabuki and Eleiko bars, but with the novelty of the adjustable handle height. But at $775, well out of my price range. I wonder how well the adjustable handles would hold up in a commercial setting, with a lot of big dudes pulling big weights???
My old gym had a Strive leg extension, plate loaded with 3 loading points. Since I typically only did one set on the leg extension, I always used the middle point. Never got around to trying the 1 and 3 position.
I’ve never seen the version with an adjustable cam. I suppose you just rotate it forward or backward to shift the peak load to a different point in the ROM?
I think back quite often how that could have been the greatest job ever! I love to weld and make things and I was making things I love, exercise machines. I tried to bring it to their attention that it wasn’t a safe environment and let’s install some fans etc but they didn’t care. The welder was used to conditions in Romania and as bad as it was here it was 10 times better than a Romania factory where they’d shoot you for little reason.
Scott
Yes Sir, that’s exactly how it works. It has a handle with holes in the cam to lock it in place.
It actually has more than just the 3 positions, it has settings in-between the top and middle and between the middle and bottom. Setting #1 loads the middle of the range, But, honestly it feels pretty even throughout the range. Setting #2 loads the end of the range and it really ramps up close to lockout. Setting $3 loads the bottom (beginning) part of the rep. I typically do a set to failure at #1, rest 60sec, then same wt set at #2 to really hit the quads. I get plenty of glute work with the back pad reclined all the way, so I feel no need to use #3. The Strive protocol was to do a set at each of the 3 positions.
