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, m₂mγ) may be doubled by doubling the mass of the system without changing its composition, so that
U(2m₁, 2m₂, 2mγ)=2U (m₁, m₂mγ)
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
∴ U₂1 (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.