Training intensity is a continuum, not clearly defined zones. There are more than just easy, threshold, and VO2max paces. You can train at intensities between these common zones. Critical velocity is one of these “in-between” intensities. The effort is harder than threshold training but not as challenging as VO2max. Because of this “somewhat hard” effort, critical velocity is an ideal training intensity for most runners at most points of training.
Critical velocity as a training intensity was popularized by Tom Schwartz, following his success in implementing CV training into his coaching of collegiate athletes. Schwartz refined the practice, but it is present in plenty of coaching theory. Renowned coaches including Greg McMillan and Jack Daniels refer to workouts that target critical velocity as cruise intervals.
The Purpose of Critical Velocity Training
Long-distance runners use primarily slow-twitch muscle fibers (type I). Slow-twitch fibers use oxygen and are more fatigue resistant. Comparatively, fast-twitch fibers (IIa and IIx) generate speed but fatigue quickly. Type IIx uses strictly anaerobic energy pathways and lasts only a very short amount of time before fatiguing.
However, one does not run races – even marathons or ultras – on slow-twitch fibers alone. Type IIa fast-twitch fibers have properties of both slow-twitch and fast-twitch fibers. Comparatively, Type IIa muscle fibers can produce higher output than ST fibers, while still using oxygen as their primary energy pathway.
Critical velocity intervals recruit type IIa muscle fibers. The theory is that if you recruit more type IIa fibers, you will run faster in every distance from 800 meters and beyond. This means that critical velocity training will improve your race times through the 5K, marathon, and beyond.
For long-distance runners, particularly those who are slow-twitch dominant, critical velocity intervals develop speed without placing too high of stress on the body. For fast-twitch runners, critical velocity training can improve speed endurance. Over time it can convert type IIx muscle fibers (purely fast-twitch) into type IIa (combination ST and FT).
Critical velocity training improves speed endurance without accumulating too much fatigue. It’s an ideal training zone for balancing high mileage, training year-round and even specific race prep.
How to Determine Critical Velocity Pace
Generally speaking, critical velocity pace is the intensity you could sustain for a 30-40 minute race. This feels like a “somewhat hard” effort for the duration of the intervals. Precise ranges vary amongst coaches. Schwartz defined it as 30-35 minute race pace. McMillan prescribes a wider range of 25-45 minute race effort, depending upon the duration of the interval and total volume of the workout.
For some runners, 30-40 minute race effort corresponds to 5K-6K pace. For others, it’s in the range of 8K-10K pace. If you have recently completed a race or time trial, you can plug those numbers into a calculator (such as the McMillan one, which will compute cruise interval pace, or the Jack Daniels one, which will provide equivalent race times).
When to Use Critical Velocity Workouts
Critical velocity is a wonderfully versatile training intensity. Once you have laid the foundation of a solid training base through lots of easy running and short, fast strides, you can incorporate cruise intervals.
During the off-season, critical velocity workouts can maintain fitness without placing too high of a demand on the body. Since you do not reach VO2max, you produce less lactate and accumulate less fatigue. Off-season CV workouts are best done with short intervals and a lower total volume of intervals. These workouts often take the shape of fartlek runs, with a focus on time and effort.
As you approach a race, you can progress the work. You can lengthen the duration of the work interval, decrease the recovery interval, and increase the total volume.
Using Critical Velocity Runs in Race Specific Training
For 5K runners, critical velocity workouts bridge base building and specific training. Cruise intervals are most beneficial if one is a slow-twitch runner coming to the 5K from longer distances. Critical velocity training will develop enough fast-twitch fibers to prepare for the demands of specific 5K-pace running. For 5K runners with a finish time in the range of 30-45 minutes, critical velocity will also double as goal pace training.
For 10K and half marathoners, cruise intervals serve as a staple through even the sharpening phase. Beyond race-specific training, shorter critical velocity workouts can build fitness during early season training. As competition approaches, workouts become longer in duration with shorter recovery in between each repetition. 800m and 1K cruise intervals are a common use of critical velocity training for these distances.
For marathoners, many coaches recommend critical velocity workouts during the earlier weeks of marathon prep (six to twelve weeks prior to the race). Some training approaches include cruise intervals through the peak weeks of training, in order to maintain speed without too much fatigue. Critical velocity workouts can range from 2-minutes up to 8-minutes in duration. During peak training, the total volume of the workouts increases.
Even ultra runners will benefit from critical velocity. Critical velocity workouts will maintain running economy even throughout high volume training, without accumulating too much fatigue to affect upcoming runs.
Sample Critical Velocity Workouts
With any critical velocity training, you want to be mindful of pacing. The intervals may be shorter, but that does not mean you run them as fast as you can. The effort of each interval should truly feel sustainable for a 30-40 minute race. Assess your effort frequently. If you start too fast, then scale back. These workouts should not be fatiguing; if you struggle to finish, you ran too fast.
For all of these workouts, include a 10-20 minute warm-up and cooldown.
Early Season/Base Building:
After a focus on aerobic development and running economy, short critical velocity intervals serve as a bridge to faster running.
8-10 x 2 minutes at CV effort, 2-min recovery jog
Go-to CV Fartlek:
You can use this workout in virtually every phase of training. When starting out, begin with fewer reps and a longer (2-3 min) recovery jog. As you progress, increase the number of reps first. Then, you can shorten to a 1-1.5 minute recovery jog.
6-8 x 3 minutes at CV effort, 1.5 min easy jog
Combo Cruise Intervals:
Schwartz often incorporates very short fast intervals at the end of a critical velocity session. These short ~30 second intervals maintain leg speed without accumulating much fatigue.
6-8 x 800m at CV pace, with 90-second recovery jog
3 minutes easy
4 x 30 seconds fast/30 seconds easy
Like all workouts, critical velocity workouts should be used in conjunction with other quality sessions. Just training at critical velocity will make you only good at running at your critical velocity. All types of workouts have value in a well-rounded training plan.
Do you use critical velocity/cruise intervals in your training?