You can barbell squat 400lbs for a one-repetition maximum (1RM). How does this ability equate to your 1RM deadlift or a 10RM leg press?
You can bench press 115lbs for eight perfect repetitions. Now you want to use a plate-load chest press device in your workout. How do you determine what resistance to load on the device for a set of twelve repetitions?
Are you able to perform seventeen perfect narrow-grip chin ups? Nice. Now go to the selectorized pulldown machine and perform seventeen perfect pulldowns. Where should you stick the pin on the weight stack to attain those seventeen repetitions?

Inaccurate Comparisons

It's great we live in a time where there are a multitude of devices that can be used to get stronger: time-proven barbells and dumbbells and various plate-loading, selectorized, and other style resistance machines. Variety is good and it's okay to use them all.
There is one problem, though. It was alluded to above. How can you compare or equate your ability with one device (i.e., a dumbbell exercise) with the ability in a different device (i.e., a plate-load machine)? You used a 75lb dumbbell in a single-arm bent-over row. Therefore, should you use 150lbs in a two-arm seated row on a selectorized machine? 
That sounds logical and may be a good starting point when constructing a new workout routine. However, is this comparison accurate? There are some factors that need to be understood in order to make the best choice for your training:

Machine Comparisons

A similar muscle group-targeting machine such as a selectorized chest press may have a different mechanical design as compared to a plate-load chest press in terms of movement arm length, grip factors, the fulcrum point, and thus the angle of force application. They both work, but 100lbs on one device may not be equal to 100lbs on the other device, relative to the number of repetitions achieved. Therefore, you cannot compare them.

Free Weight Comparisons

You can overhead press a barbell with 120lbs for fifteen repetitions to muscular fatigue. Does this mean you can take two x 60lb dumbbells (1/2 of the 120lbs) and press them overhead for fifteen repetitions? No. Because of the specificity issue (two resistances versus one), your effort with the dumbbells would fall short of fifteen repetitions.   

Free Weight vs. Machine Comparisons

Refer to my opening points. Can you compare a barbell result to a machine result? No way. Again, regarding the principle of specificity, 165lbs on a leverage (plate-load) device cannot be compared to 165lbs in a similar free weight exercise.

How to Properly Journal Your Resistance Training

Whatever device you use, record your progression with it - specifically. Resistance is resistance and dependent on how it's manifested gravitationally. Remember to note the following in your training journal:
  • Is it free-standing, weight stacks sliding up guide rods or weight plates loaded on movement arms moving on various-length movement arms? Each one is different than the other.
  • Is the machine cable/pulley, pivoting movement arm or gliding in nature? Again, the amount of resistance used in each situation will be different.
  • In consideration to the aforementioned, is the exerciser pushing or pulling vertically, horizontally, or at a different angle? The angle of force application matters, and comparison of results here will differ.
  • In conjunction with the angle of force application, what is the length of the lever arm, position of the rotation point, or position of the body part being targeted?

The Important Takeaway

  • If you barbell bench press 245lbs, it does not mean you can bench press 245lbs on a selectorized chest press.
  • If you can deadlift 300lbs with a barbell, it does not mean you can deadlift 300lbs on a plate-loading deadlift machine.
  • If you can press overhead 35lb dumbbells twenty times, it does not mean you can press overhead 70lbs on a selectorized machine twenty times.
  • If you can bicep curl 90lbs on a low pulley machine for nine repetitions, it does not mean you can curl 90lbs with a free-weight EZ curl bar.
Variety is good. Use all tools available to get stronger. But understand the devices you use in your program cannot be compared between each other.
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