Oh dear, I am so sorry...I hope Emerson is OK...Perhaps he just sprained it...? You will wake up tomorrow and he will be running around like normal...
Hang in there! Let us know how Emerson is tomorrow am if you can. I'm sure it's nothing...Maybe the grass was HOT? Duncan walks on 3 legs when his paws are cold and sometime when it's too hot? Maybe he hit a rock running that startled him...
Kisses to Emerson and hugs to you.