I need help with a trail script, I need it to activate when at a certain speed...?

I need it to be activated when I reach a certain speed, and deactivate if I reach a speed lower than the activation speed. I've tried putting it in while loops, repeats and if statements etc. The code I have so far is below.

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
	trail()
end

Thanks!