Muscle is regarded as a kind of thermodynamic open system that exchanges matter and energy with its surroundings. In this paper we attempt to give a thermodynamic description of muscular disorders as well as considering the thermodynamic meaning of their physical treatment. Muscular contracture dU=dQ-dW (2-1) This is the first law of thermodynamics. If a muscle performs in a cyclic process as in fig. 1→fig. 2→fig. 3, the change of internal energy through the whole process will be as follows. fig. 1→fig. 2 U2-U1=-ΔQ-ΔW<0 (2-2) fig. 2→fig. 3 U3-U2=ΔW>0 (2-3) from (2-2) and (2-3) U1>U3>U2 (2-4) The internal energy level of contracted muscle U2 is less than U1 and U3, which is in a relaxed state. If the muscle remains in this shortened state, we call this state muscular contracture. It has been proved above that this is the state of reduction of internal energy within the muscle. Muscular atrophy All the extensive properties of the thermodynamic system are homogeneous functions of the first order in the mass of the system. For example, internal energy, U (m1, m2....mγ) may be doubled by doubling the mass of the system without changing its composition, so that U(2m1, 2m2, ....2mγ)=2U (m1, m2....mγ) Here m1 indicates the mass of the composition. If muscle becomes atrophic, the mass of this muscular composition will be reduced. Therefore, if a muscle with internal energy level U1 reaches an atrophic state, in which internal energy level is U2, we will obtain the next equation between U1 and U2. U2=U1 (λAm1, λm2, ...., λmγ) =λU1 (m1, m2, ...., mγ) =λU1(0<λ<1) U2/U1<1 ∴ U2<U1 (2-5) We have proved that muscular atrophy is a state with a lower internal energy level than normal muslce. Meanings of physical therapy Almost all neuromuscular diseases involve shortening of muscle, contracture, and muscular atrophy, in which the internal energy level becomes low. Therefore, physical therapy for these diseases must at least be directed towards increasing the internal energy level of the muscles. Since the first law of thermodynamics is ΔU=ΔQ-ΔW, to increase the ΔU of the malfunctioning muscles, we must provide moderate heat ΔQ or work ΔW to the muscles. This is the thermodynamic principle of physical therapy. Various kinds of heat therapy and the stretching of shortened muscle are therefore reasonable. From the thermodynamic point of view, it is very interesting that isotonic exercise is more useful in increasing muscle power than isometric exercise, which does not perform work. Further studies will be attempted in the future.