LShift to Run with Animation - Roblox

script.lua

--// Services
local Players = game:GetService("Players")
local UIS = game:GetService("UserInputService")

--// Local Variables
local player = Players.LocalPlayer
local animation = Instance.new("Animation")
local trackAnimation = nil

--// Setters
animation.AnimationId = "http://www.roblox.com/Asset?ID=707925354"

--// Function
function playAnimation(Anim)
	trackAnimation = player.Character.Humanoid:LoadAnimation(animation)
	trackAnimation:Play()
	trackAnimation:AdjustSpeed(4)

	--// Debug speed
	player.Character.Humanoid.Running:connect(function(speed)
    		if speed > 0 then
        		print("Speed: " .. speed)
    		else
        		print("Player has stopped")
    		end
	end)
end

--// Input Began
UIS.InputBegan:Connect(function(input)
	if(input.KeyCode == Enum.KeyCode.LeftShift)then
		playAnimation(animation)

		while(player.Character.Humanoid.WalkSpeed < 40)do
			player.Character.Humanoid.WalkSpeed = player.Character.Humanoid.WalkSpeed + 5
			wait(0.5)
		end
	end
end)

--// Input Ended
UIS.InputEnded:Connect(function(input)
	if(input.KeyCode == Enum.KeyCode.LeftShift)then
		while(player.Character.Humanoid.WalkSpeed > 20)do
			player.Character.Humanoid.WalkSpeed = player.Character.Humanoid.WalkSpeed - 5
			wait(0.2)
		end
		trackAnimation:Stop()
	end
end)