The group participated in a circuit outside and seemed to have a great time getting sweaty while working out on machines and with weights.
Derek shared pics and videos of the session on his Instagram Story, dubbing the event, "Brooks Birthday Blast Workout."
Once the workout ended, the group headed inside to reward themselves with some celebratory treats. Derek's girlfriend, Hayley Erbert, helped to make lemon meringue pie and homemade cinnamon rolls in honor of the birthday boy.
In a video on his Instagram Story, Brooks called the cinnamon rolls his "weakness."
"Dear God that looks good. Happy birthday to me," he remarked.
The birthday celebration comes the same week that Brooks was spotted without his wedding ring while house hunting. This followed his and Julianne's May announcement that they had decided to separate after nearly three years of marriage.
"We have lovingly and carefully taken the time we have needed to arrive at our decision to separate," the pair told People. "We share an abundance of love and respect for one another and will continue to lead with our hearts from that place. We kindly request your compassion and respect for our privacy moving forward."
Prior to their announcement, a source told ET that Brooks and Julianne were struggling with quarantining separately, with the former in Idaho at the time and the latter in Los Angeles.
"They talk often but this is a very hard time," the source said. "They've been working on their marriage and being apart is not easy."