  Skeletal muscle modeling has a vital role in movement studies and the
development of therapeutic approaches. In the current study, a Huxley-based
model for skeletal muscle is proposed, which demonstrates the impact of
impairments in muscle characteristics. This model focuses on three identified
ions: H + , inorganic phosphate Pi and Ca 2+. Modifications are made to
actin-myosin attachment and detachment rates to study the effects of H + and
Pi. Additionally, an activation coefficient is included to represent the role
of calcium ions interacting with troponin, highlighting the importance of Ca
2+. It is found that maximum isometric muscle force decreases by 9.5% due to a
reduction in pH from 7.4 to 6.5 and by 47.5% in case of the combination of a
reduction in pH and an increase of Pi concentration up to 30 mM, respectively.
Then the force decline caused by a fall in the active calcium ions is studied.
When only 15% of the total calcium in the myofibrillar space is able to
interact with troponin, up to 80% force drop is anticipated by the model. The
proposed fatigued-injured muscle model is useful to study the effect of various
shortening velocities and initial muscletendon lengths on muscle force; in
addition, the benefits of the model go beyond predicting the force in different
conditions as it can also predict muscle stiffness and power. The power and
stiffness decrease by 40% and 6.5%, respectively, due to the pH reduction, and
the simultaneous accumulation of H + and Pi leads to a 50% and 18% drop in
power and stiffness.
