County jails are generally expected to provide some form of out-of-cell time and access to recreation, though the specific legal requirements vary by state and the standards are enforced inconsistently. Most facilities have a designated recreation area, either an outdoor space with walls and a covered top where inmates can get fresh air and exercise, or an indoor gym that opens two to three times per week when outdoor access is not possible. That access is considered part of basic conditions of confinement.
Complete lockdown with no out-of-cell time at all is typically only applied as a disciplinary measure or during a facility-wide lockdown due to a specific incident. If it is being applied routinely without a specific justification, that raises legitimate concerns.
If your person is being kept in their cell continuously without any recreation or movement opportunities and there is no stated disciplinary reason for it, that is worth documenting and raising formally. The inmate can submit a grievance through the facility's process, and you can contact the Missaukee County Jail administrator or the Michigan Department of Corrections oversight office to inquire about the policy. For persistent issues, a civil rights attorney familiar with jail conditions can evaluate whether the situation rises to a constitutional violation.