V02max is always populated with 0.0

I am developing an app, which aims to analyze running workouts including the cardio fitness. Therefore I am trying to read inter alia the vo2max value from HKWorkout.

This works fine for other measures, but V02max is always populated with 0.0:

                    avgWatts = workout
                        .statistics(for: .init(.runningPower))?
                        .averageQuantity()?
                        .doubleValue(for: .watt()) ?? 0
                    
                    vo2max = workout
                        .statistics(for: .init(.vo2Max))?
                        .maximumQuantity()?
                        .doubleValue(for: HKUnit(from: "ml/kg*min")) ?? 0.0

I am the opinion that I also have the permission correctly set:

    func authorizationDetails(){
        let allTypes = Set([HKObjectType.workoutType(),
                            HKObjectType.quantityType(forIdentifier: .activeEnergyBurned)!,
                            HKObjectType.quantityType(forIdentifier: .distanceCycling)!,
                            HKObjectType.quantityType(forIdentifier: .distanceWalkingRunning)!,
                            HKObjectType.quantityType(forIdentifier: .heartRate)!,
                            HKObjectType.quantityType(forIdentifier: .runningStrideLength)!,
                            HKObjectType.quantityType(forIdentifier: .vo2Max)!])
        
        
        healthStore.requestAuthorization(toShare: allTypes, read: allTypes) { (success, error) in
            if !success {
                // Handle the error here.
            }else{
                self.isAuthorized=true
            }
        }
        
    }

I am not sure what the problem is and I cannot find the respective information in apples documentation. Is that quantity type maybe not available in HKWorkout?

I was also trying to use sumQuantity instead of maximumQuantity, but it still does not provide values:

                    vo2max = workout
                        .statistics(for: .init(.vo2Max))?
                        .sumQuantity()?
                        .doubleValue(for: HKUnit(from: "ml/kg*min")) ?? 0.0

In other apps (e.g. Health) I can see the vo2max value, so I assume that the data is actually available.

Thank you in advance for any suggestions.

The unit for vo2max is ml/kg/min not ml/kg*min. It's returning 0.0 because that's what you assign when it's nil. To debug further, I would verify each step of the expression to make sure it's what you expect it to be. Does the statistics call return a value? Does it have an averageQuantity? etc. (But I strongly expect the problem is the last transform, since it's the wrong unit.)

Also definitely make sure that you're looking at a workout that actually has a VO2Max value attached. It often isn't recorded (it has a lot of requirements).

I'm developing an app dedicated for running and also encountered the same issue. I didn't find a solution elsewhere, so I spent time to investigate it myself. This question was raised 6 months ago. I'm not sure if you have figured out a solution, but I've got one and want to share it here for your (as well as others') reference.

The short answer to why it doesn't work with statistics method for vo2Max is that vo2Max doesn't even exist in statistics. I tried allStatistics to see all the keys that can be used to query and didn't see vo2Max.

My best guess is that vo2Max is not recorded with a strong relationship with the corresponding run as we expected. I tried to query it with a predicate created by passing the workout to HKQuery.predicateForObjects just like what I did for heart rates but got nothing. Why? I observered the data in Health app and noticed that vo2Max was recorded automatically some time after the workout has been submitted.

All the above investigation resulted in my solution to this problem. I query vo2Max with the start time as the end time of the workout and set a window to 5 minutes and then take the first vo2Max record. If you look at the time of the recorded vo2Max and the corresponding workout, you'll notice that vo2Max is usually recorded within 1 minute after the workout. So a 5-minute window would be enough, but you can change it as you see fit.

Below is the complete code that I use in my app:

private func queryVO2Max(for workout: HKWorkout) async -> Double? {
    Logger.shared.info("Querying VO2 max for a run from health store")
    Logger.shared.debug("Running HealthStoreManager.queryVO2Max")
        
    guard hasPermission else {
        return nil
    }
        
    // Note: It seems that VO2 max doesn't have a recorded relationship with a workout.
    // The strategy here is to fetch the first VO2 max after the workout end time.
    // Usually VO2 max will be recorded within 1 minute after a workout ends.
    // Here we set a 5 minute window for query.
    let vo2MaxType = HKQuantityType(.vo2Max)
    let timePredicate = HKQuery.predicateForSamples(withStart: workout.endDate, end: workout.endDate.addingTimeInterval(TimeInterval(5 * 60)))
    let queryDescriptor = HKSampleQueryDescriptor(
        predicates: [.quantitySample(type: vo2MaxType, predicate: timePredicate)],
        sortDescriptors: []
    )
    do {
        let queryResult = try await queryDescriptor.result(for: healthStore)
        let vo2Max = queryResult.first?.quantity.doubleValue(for: HKUnit.vo2MaxUnit())
        Logger.shared.info("VO2 max: \(vo2Max.toString())")
        return vo2Max
    } catch {
        return nil
    }
}

One more thing, the unit for vo2max is "ml/kg/min" as mentioned in the Health app. But if you use that to construct HKUnit, you will get an error. "ml/kg*min" is the String you should used to construct HKUnit.

Hope that helps.