CoreMotion - Recognizing a jump (Swift) - swift

Question

I'm pretty new to CoreMotion but I'm trying to build an app that uses CoreMotion to see if a user is jumping up and down (think skipping rope). It also need to allow the user to be able to hold their phone (no iWatch) however they want (landscape, portrait, strange tilted way, etc.) and still be able to tell if they are traveling up and down regardless. I guess it's a bit like "throw your phone" app, but without measuring the distance.
So I'm using userAcceleration and gravity to see which way is "down" by checking the value of the different gravity axis and it works okey, but it feels like a clumsy way of going about it.
Is there a better way of doing what I'm doing? Basically all the app currently needs is to be able to tell if there is an acceleration perpendicular to the ground (regardless of how you'r holding the phone).
import UIKit
import CoreMotion
class TestViewController: UIViewController {
override func viewDidLoad() {
super.viewDidLoad()
startJump()
}
func startJump() {
if CMMotionManager.shared.isDeviceMotionAvailable {
CMMotionManager.shared.deviceMotionUpdateInterval = 0.01
CMMotionManager.shared.startDeviceMotionUpdates(to: OperationQueue.current!) { (data, error) in
guard let myData = data else {
return
}
var gravX = abs(myData.gravity.x)
var gravY = abs(myData.gravity.y)
var gravZ = abs(myData.gravity.z)
var userX = abs(myData.userAcceleration.x)
var userY = abs(myData.userAcceleration.y)
var userZ = abs(myData.userAcceleration.z)
if gravZ > 0.9 && userZ > 1 {
print("Jump; Z is up")
}
if gravX > 0.9 && userX > 1 {
print("Jump; X is up ")
}
if gravY > 0.9 && userY > 1 {
print("Jump; Y is up")
}
}
}
}
override func viewDidDisappear(_ animated: Bool) {
CMMotionManager.shared.stopDeviceMotionUpdates()
}
}