Tight Bibs pro's wear in preseason

[p793]

Can somebody explain their purpose as opposed to normal bibs they wear.

Example:

http://www.whoateallthepies.tv/wp-conte ... 149547.jpg

[Zlatan5]

GPS tracking systems. Trainers use them to monitor their heartrates during sprints and runs, it also monitors how many km/miles they are running during practice.

http://gpsports.com/gpsports_website/?c ... ch&item=17