Working hard in the pool doesn't guarantee working at VO2max. The difference comes down to calculated pace, repetition duration and recovery times. Concrete protocols, poolside CSS test and 4-week progression.
Your swimmers are coming out exhausted. Two sessions of 10×100m per week for six weeks. Times aren't moving.
The problem isn't the effort put in. It's the precision: working "hard" in the pool doesn't guarantee working at VO2max. The line between a session that genuinely develops the aerobic ceiling and a session that tires without targeting that adaptation is thin. It comes down to effort durations, recovery times and a calculated pace.
That line comes down to three variables: effort duration, recovery time, and a calculated pace — not one estimated by eye.
VO2max is the maximum oxygen flow the body can consume, transport and use during intense effort. It is expressed in mL per kg of body weight per minute (mL/kg/min). The higher this number, the more powerful the aerobic "engine": the swimmer can maintain a fast pace without tipping into anaerobic metabolism.
In swimming, the VO2max measured in the pool is typically 10 to 15% lower than that measured on a treadmill for the same individual. Three reasons explain this gap. The horizontal position improves venous return and lowers peak HR by about 10 bpm. Muscle mass engaged is smaller: arms dominate over legs. And ventilation is constrained: you don't breathe when you want. For a club coach, this shifts field benchmarks. Not lab benchmarks.
Source: Holmér I. (1972), Journal of Applied Physiology, 33(4), 502-509.
These figures are useful as benchmarks, but in practice no club coach will measure VO2max in the laboratory. What you need is a reliable field proxy.
The most accessible method for club use is the CSS test (Critical Swim Speed). Developed by Wakayoshi and colleagues in 1992 and widely validated since, it is based on two maximal efforts at different distances.
CSS gives you the threshold pace. The vVO2max (the speed at which VO2max is reached) sits about 10 to 15% above it. In practice: take your swimmer's CSS pace and add 5 to 8 seconds per 100m to get the target pace for a VO2max session. A swimmer with a CSS of 1'26"/100m will be working at VO2max around 1'18"–1'22"/100m, depending on their form and repetition distance.
To stimulate VO2max, two conditions must be met simultaneously: intensity must be sufficient (95–100% of swimming max HR), and effort duration must allow reaching and maintaining that intensity for at least 2 to 3 minutes cumulative per repetition. This is where most "high-intensity" protocols fail: too short, or with too little recovery, they never actually bring the swimmer up to their VO2max zone.
Format: 4 to 6 × 300–400m · Recovery: 2 to 3 minutes · Pace: estimated vVO2max
This is the classic protocol, the most studied. Repetitions long enough for the body to reach maximum oxygen consumption, with short recovery that maintains cardiac demand between repetitions.
Concrete example for an intermediate club group: 5×300m departing every 6'30"–7', at a pace 5 seconds faster than their CSS per 100m. Effective recovery of approximately 2'30"–3'. Early repetitions feel manageable. The last two are hard. If the swimmer holds pace to the end, the session is well calibrated. If pace drops by more than 5% from the fourth repetition, reduce the number of repetitions or extend recovery by 30 seconds.
Format: 8 to 12 × 100m · Recovery: 20 to 30 seconds · Pace: estimated vVO2max
Here, the short recovery prevents HR from dropping between repetitions. The cumulative effect builds progressively: early repetitions are at Z3–Z4, later ones approach VO2max, the final ones slightly exceed it. This is a more lactically demanding protocol, but very effective for accumulating time at high intensity.
Format: 20 to 30 × 25m · Departure every 45–50s · Pace: controlled sprint (90–95%)
Note: this protocol does not replace Protocols A and B. A single 25m does not reach VO2max (see the kinetics section above). What works here is the cumulative effect: over 25–30 repetitions with only 20 seconds of rest, HR never drops. It settles into the target zone from the 6th or 7th repetition and stays there. This is a VO2max stimulus through accumulation, not through isolated repetitions. For mixed-level groups, it's the most practical option.
A single session doesn't develop VO2max. A structured block is needed, long enough to induce adaptations, but calibrated to avoid fatigue accumulation.
Progression is made by first increasing volume (number of repetitions), then reducing recovery, then increasing pace. Changing three variables at once in a single week is a common mistake that leads to overtraining or quality degradation.
Two VO2max sessions per week maximum. The 48-hour rule is a real scheduling constraint. If you train your group three times a week, you can only fit one VO2max session in. Two VO2max sessions require a minimum four-day window: Monday and Thursday, or Tuesday and Friday. Wednesday and Friday is too short. A VO2max session performed on a body that hasn't fully recovered doesn't produce the expected adaptations. And cumulative fatigue over three weeks looks exactly like a performance plateau. Swimmers train. Nothing moves. The problem isn't the program: it's the timing.
Before your swimmers' next high-intensity session: run the CSS test first. Fifteen minutes, two efforts, one formula. You'll know whether you're training them at VO2max or training them to suffer. They are not the same thing, and the difference shows up over eight weeks.
The first measurable adaptations appear after 3 to 6 weeks of targeted training. Gains observed over 8 to 12 weeks generally range from 5 to 15% depending on initial level — beginners progress fastest, trained swimmers progress more slowly but continue to improve over time.
VO2max is the aerobic ceiling: the maximum intensity at which the body can function using oxygen. The lactate threshold is the intensity at which lactate production exceeds the body's elimination capacity. The threshold generally represents 80–90% of VO2max. In training, developing the threshold (Z3) allows swimming fast for longer; developing VO2max (Z4) raises the absolute ceiling and makes threshold effort feel easier.
Use the CSS test (Critical Swim Speed): swim a 400m and a 200m all-out (with 30 to 45 min of recovery between the two). Calculate CSS = 200 / (T400 – T200) in meters per second. The VO2max pace is 5 to 8 seconds per 100m faster than your CSS pace. This benchmark is more reliable than percentages of max HR, because heart rate rises more slowly in swimming than in running.
Sources
320+ active coaches · 7,500+ sessions · Since 2017
In Padlie, you can assign a zone to each block and keep the history of your VO2max sessions week after week. Free plan, no credit card required.
Try for free →