Hi Randy, You know I am not a coach or even a very experienced lifter, but I have been thinking about this very problem/issue. I hope some of the experienced coaches weigh in on this. However, for my own edification if nothing else, let me lay out the framework in which I have been thinking about this issue.
There is a relationship between 4 variables which determines how much a given workout increases your strength: Genetic Potential (GP), Current Strength Level (5x3RM) (CSL), The Minimum Adaptation Stress (MAS) level which will cause a adaptation, and the actual Workout Stress (WS) of the workout in question.
If the weight you are using (the Workout Stress) is bigger than your Current Strength Level, you will fail the last reps of the last set. If the Workout Stress is less than the Minimum Adaptation Stress then you will not gain in strength. So your WS needs to be between your CSL and your MAS.
- The closer the WS is to CSL the more adaptation takes place during recovery.
- For novices the MAS is very low, so almost any workout causes adaptation
- Initially when the Current Strength Level is far below the Genetic Potential a workout close to your CSL will generate a very big increase in DSL by the time of the next workout.
- As your CSL starts to get closer to your GP a workout close to your CSL won't increase the CSL as much for your next workout.
- Genetic Potential is a function of age and as you get older it declines. Your recovery ability also declines. Linear Progression for older people will be at a slower rate.
So to set up your questions in this framework: You have a high GP (Genetic Potential), so you have been able to add 10lbs to your WS (Workout Stress) every workout and your CSL (Current Strength Level) has increased enough each workout to accommodate that 10lb increase.
Suddenly you had a missed rep. The slope of you "linear progression" has flattened out. A workout near your CSL no longer produces a strength increase of 10lbs. If all other recovery factors are the same, then it is due to you getting closer to your Genetic Potential (GP).
So, recognizing this, in subsequent workouts you need to decrease the increments in your WS. Thus your question: Why not use 7.5lb increases instead of going all the way down to 5lb increases?
This is where some experienced coach input would be helpful!
Based on all of the reading I have done on this site, plus reading Rips books, my stab at the answer is that it has been the broad experience that the biggest part of linear progression during the novice phase for most people proceeds at 5lbs per workout. Initially, if you have a high GP and start with light weights, you can have a 10lb LP but soon you will fall down to a 5lb LP, and finally a 2.5lb LP. My impression is that if you try a 7.5lb LP it won't last too long and by dropping to the 5lb LP you will have a nice long period of solid LP.