Stamina Amount goes down if player is standing still?

Hello!

I am currently trying to make a Stamina System for the Player, so that it won't be able to run forever. This works, however, if the player holds down shift without moving, the stamina bar goes down. I thought it may have been an issue caused by the UserInputService as it states that as soon as the key is held down to use stamina, however I am quite unsure and i am also unsure how to fix it.

Here is my script:

local UserInputService = game:GetService("UserInputService")
local Bar = script.Parent:WaitForChild('Background'):WaitForChild('Bar')

local Player = game:GetService("Players").LocalPlayer

local NormalWalkSpeed = 16
local NewWalkSpeed = 20

local Power = 10
local IsSprinting = false

repeat wait() until Player.Character

local Character = Player.Character

UserInputService.InputBegan:Connect(function(Input, GameProcessedEvent)
	if GameProcessedEvent then return end
	if Input.KeyCode == Enum.KeyCode.LeftShift then
		Character.Humanoid.WalkSpeed = NewWalkSpeed
		IsSprinting = true
		while Power > 0 and IsSprinting do
			Power = Power - 0.03
			Bar:TweenSize(UDim2.new(Power / 10, 0, 1, 0), 'Out', 'Quint', 0.1, true)
			wait()
			if Power <= 0 then
				Character.Humanoid.WalkSpeed = NormalWalkSpeed
			end
		end
	end
end)

UserInputService.InputEnded:Connect(function(Input, GameProcessedEvent)
	if GameProcessedEvent then return end
	if Input.KeyCode == Enum.KeyCode.LeftShift then
		Character.Humanoid.WalkSpeed = NormalWalkSpeed
		IsSprinting = false
		while Power < 10 and not IsSprinting do
			Power = Power + 0.03
			Bar:TweenSize(UDim2.new(Power / 10, 0, 1, 0), 'Out', 'Quint', 0.1, true)
			wait()
			if Power <= 0 then
				Character.Humanoid.WalkSpeed = NormalWalkSpeed
			end
		end
	end
end)

If I could get some help with this, it would be great. Thanks!