Is there a way to make a repeat repeat if a variable changes?

I need this code to basically do the following: Have the function not running while I'm at a speed less than or equal to 49. Have the function run when my walk speed changes to more than or equal to 50. I have made the function already, and need someone to give me a loop that can detect if my speed has changed, and then output the function. Here's my code I have:

local sp = script.Parent.Parent.Character:FindFirstChild("Humanoid")
activate = 50
deactivate = 49

function trail()
	wait(0.01)
	local ft = Instance.new("Part")
	ft.TopSurface = "Smooth"
	ft.BottomSurface = "Smooth"
	ft.Anchored = true
	ft.FormFactor = "Custom"
	ft.Size = Vector3.new(0.98, 1.98, 0.98)
	ft.Transparency = 0.7
	ft.Parent = game.workspace
	ft.CanCollide = false
	ft.BrickColor = BrickColor.new("Bright yellow")
	ft.CFrame = CFrame.new(script.Parent.Parent.Character:FindFirstChild("Torso").Position.X,script.Parent.Parent.Character:FindFirstChild("Torso").Position.Y-0,script.Parent.Parent.Character:FindFirstChild("Torso").Position.Z)--Edited
	game.Debris:AddItem(ft,0.8)
	local fla = Instance.new("Part")
	fla.TopSurface = "Smooth"
	fla.BottomSurface = "Smooth"
	fla.Anchored = true
	fla.FormFactor = "Custom"
	fla.Size = Vector3.new(0.98, 1.98, 0.98)
	fla.Transparency = 0.7
	fla.Parent = game.workspace
	fla.CanCollide = false
	fla.BrickColor = BrickColor.new("Bright yellow")
	fla.CFrame = CFrame.new(script.Parent.Parent.Character:FindFirstChild("Left Arm").Position.X,script.Parent.Parent.Character:FindFirstChild("Left Arm").Position.Y-0,script.Parent.Parent.Character:FindFirstChild("Left Arm").Position.Z)--Edited
	game.Debris:AddItem(fla,0.8)
	local fra = Instance.new("Part")
	fra.TopSurface = "Smooth"
	fra.BottomSurface = "Smooth"
	fra.Anchored = true
	fra.FormFactor = "Custom"
	fra.Size = Vector3.new(0.98, 1.98, 0.98)
	fra.Transparency = 0.7
	fra.Parent = game.workspace
	fra.CanCollide = false
	fra.BrickColor = BrickColor.new("Bright yellow")
	fra.CFrame = CFrame.new(script.Parent.Parent.Character:FindFirstChild("Right Arm").Position.X,script.Parent.Parent.Character:FindFirstChild("Right Arm").Position.Y-0,script.Parent.Parent.Character:FindFirstChild("Right Arm").Position.Z)--Edited
	game.Debris:AddItem(fra,0.8)
	local fll = Instance.new("Part")
	fll.TopSurface = "Smooth"
	fll.BottomSurface = "Smooth"
	fll.Anchored = true
	fll.FormFactor = "Custom"
	fll.Size = Vector3.new(0.98, 1.98, 0.98)
	fll.Transparency = 0.7
	fll.Parent = game.workspace
	fll.CanCollide = false
	fll.BrickColor = BrickColor.new("Bright yellow")
	fll.CFrame = CFrame.new(script.Parent.Parent.Character:FindFirstChild("Left Leg").Position.X,script.Parent.Parent.Character:FindFirstChild("Left Leg").Position.Y-0,script.Parent.Parent.Character:FindFirstChild("Left Leg").Position.Z)--Edited
	game.Debris:AddItem(fll,0.8)
	local frl = Instance.new("Part")
	frl.TopSurface = "Smooth"
	frl.BottomSurface = "Smooth"
	frl.Anchored = true
	frl.FormFactor = "Custom"
	frl.Size = Vector3.new(0.98, 1.98, 0.98)
	frl.Transparency = 0.7
	frl.Parent = game.workspace
	frl.CanCollide = false
	frl.BrickColor = BrickColor.new("Bright yellow")
	frl.CFrame = CFrame.new(script.Parent.Parent.Character:FindFirstChild("Right Leg").Position.X,script.Parent.Parent.Character:FindFirstChild("Right Leg").Position.Y-0,script.Parent.Parent.Character:FindFirstChild("Right Leg").Position.Z)--Edited
	game.Debris:AddItem(frl,0.8)
	local fh = Instance.new("Part")
	fh.TopSurface = "Smooth"
	fh.BottomSurface = "Smooth"
	fh.Anchored = true
	fh.FormFactor = "Custom"
	fh.Size = Vector3.new(0.98, 0.98, 0.98)
	fh.Transparency = 0.7
	fh.Parent = game.workspace
	fh.CanCollide = false
	fh.BrickColor = BrickColor.new("Bright yellow")
	fh.CFrame = CFrame.new(script.Parent.Parent.Character:FindFirstChild("Head").Position.X,script.Parent.Parent.Character:FindFirstChild("Head").Position.Y-0,script.Parent.Parent.Character:FindFirstChild("Head").Position.Z)--Edited
	game.Debris:AddItem(fh,0.8)
end

while sp.WalkSpeed >= activate do
	repeat
		trail()
	until sp.WalkSpeed <= deactivate
end

The function basically creates a trail that will follow the player. The script is in the starterpack, and works without all the loops and stuff. When the player reaches the walk speed of 50 or above, I want the function to be repeated, but for some reason it won't work. When the player has come back down from the walk speed of 50 or above, and goes below or equal to 49, I need the repeat to stop, and then clear all the debris with it.