Under MORE>APP SETTINGS there are two workout locking options:
Workout Auto-Lock - Automatically locks the Start/Pause/Stop buttons during an active workout. You will need to swipe the screen to unlock these options.
Screen Auto-Lock - Allows the device’s screen to automatically lock/turn off during an active workout. Disable this setting to keep your screen active for the duration of your workout, or enable this setting to increase iPhone battery life.