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Active contraction materials may be used to impose a (typically) contractile stress within a solid material, by incorporating the contraction material within a solid mixture. The total stress in the solid mixture is simply $\boldsymbol{\sigma}=\boldsymbol{\sigma}^{s}+\boldsymbol{\sigma}^{a}$ , where $\boldsymbol{\sigma}^{s}$ is the solid stress (due to strain and strain history), and $\boldsymbol{\sigma}^{a}$ is the active contractile stress. The calculation of the contractile stress is the same, wether the solid mixture consists of uncoupled or unconstrained materials.