This answer is simple. Work Capacity. Simply defined as an individual’s ability to perform as much work as possible in a prescribed amount of time. Or an individual's ability to perform a prescribed amount of work as quickly as possible. 

While most “dedicated” lifters will possess a strong and muscular physique, what really separates the men from the boys, or the “dedicated” lifter from the truly dedicated lifter is their capacity to perform physical work in a set amount of time. 

You may think of work capacity as “the ability to keep moving, lifting and carrying objects at a steady pace for a set period of time” or the ability to move, lift, and carry an object or set amount of objects as quickly as possible.

The individual who can perform the most work in a set amount of time or can complete a set amount of work the fastest, possesses the greatest work capacity.

For example, let’s say we have two lifters who both squat 405lb for their 1-rep max, a simple way to determine which lifter has the greater work capacity would be to test how many squats at 365lb (90% of their 1RM) they can perform in 10 minutes. As you can guess, the lifter that performs the most reps in 10 minutes has the greater work capacity and will make better long-term progress simply because they can perform more work. 

Many lifters look strong and fit, but possess very poor work capacity. In layman’s terms, they are “all show, and no”. Improving your work capacity requires high cardiovascular and respiratory output. It’s hard work. Literally. Most lifters, no matter how “dedicated” they think they are, never put in the effort to fully develop their work capacity. Thus, never truly reaching their full potential.