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.