Was this your longest ride so far this year? What is your normal length ride? It is always possible your back is hurting because the ride was longer than usual and because you were working harder to keep up with the group.
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“One morning you wake up, the girl is gone, the bikes are gone, all that's left behind is a pair of old tires and a tube of tubular glue, all squeezed out"
Sugar "Kane" Kowalczyk