When you train or workout, there are six or seven (depends who you talk to) training zones that you can workout in. They are as follows:
Zone 1- Endurance or Active Recovery
Zone 2- Aerobic Capacity
Zone 3- Tempo
Zone 4- Threshold
Zone 5- VO2max
Zone 6-7- Anaerobic Capacity
Each one of these training zones has a different physiological effect that your body adapts to. This chart best describes the adaptations:
You can see that the Zone between Zones 3 and Zones 4, called the "Sweet Spot" zone, (highlighted in orange) offers the most physiological adaptations. i.e. the best bang for the buck. This zone looks like Zone 3.5 (below the Threshold zone).
So, why is it called the SST zone? The term or nickname 'Sweet Spot Training' was coined by Frank Overton and Dr. Andy Coggan. The underlying principle of sweet spot training is a balanced amount of intensity and volume that produces a maximal increase in an athlete’s functional threshold power (FTP). In the figure below, the “sweet spot” occurs between a high level/zone 2 and level/zone 4. It is within these ranges that you will build your base the most and simultaneously increase your power at threshold. More bang for your buck, and thus the nickname, “sweet spot”.
As a time crunched athlete, of which I believe 90% of amateur riders are, at least one interval workout per week should be devoted to SST. BTW, I designate an SST workout as an L3/L4 workout. Another interval workout should be a Threshold workout like a 2x10 or 2x15@L4. As you get closer to your racing season, you'll want to add in a VO2max workout such as a 5x4@L5. That is, if you want to increase your FTP and get stronger on the bike. As I said before, these (SST) workouts are the best bang for the buck, and they're not as hard/painful as pure Threshold or VO2max workouts. I don't believe time crunched athletes should do any specific Tempo L3 workouts. Your long ride on the weekends (which you should be riding 3-4 hrs.) incorporates plenty of L3 Tempo miles.
There are plenty of SST workouts on the internet to choose from, just Google Sweet Spot Training Workouts. If you're a Zwift member or ErgVideo user, there are plenty of SST workouts to choose from. My favorite are the Over/Under SST workouts where you're bouncing back/forth between the upper and lower bounds of the SST range. You're really not in L4 Zone long enough to be considered a Threshold workout but you're getting all the physiological effects/adaptations.
Power ON! Coach Rob