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!