If any convict committed to the county jail is physically unable to do any kind of manual labor, then, upon the certificate of the county health officer or physician designated by the board of supervisors of the county, to this effect, such convict shall not be required, during the period of such physical disability, to perform manual labor on the convict farm. But all convicts shall be required each day to do and perform such work as they are physically able to do and perform and which will not impair the health of such convict, or as is not inhumane to require of him.