For iPhone running iOS8.2 and above, can be used with your Apple Watch.
This app enables you to you add custom exercises, and access them by muscle group. Select a list of exercises and it will automatically walk you through the sequence and log your start and stop times for each set. Set goal weights and reps for each set and modify them on the fly as needed for each set. A log of your workout will be displayed in the Log tab. Use standalone on your iPhone or together with your paired Apple Watch.