Two people that meet for the sole purpose of a work out. One partner usually knows enough about fitness and sports to help the other develop physically and meet a personal goal. An unemotional, homogeneous association between two people without a need for friendship.
John and Jane meet at the gym once weekly to work up a good sweat. They are sweat buddies.
I like to be with my sweat buddy when I workout because I hear positive karma about the future outcome of my sweat work.
Being sweat buddies is much less complex than being friends.