Muscle as a molecular machine for protecting joints and bones by absorbing mechanical impacts

Abstract

We hypothesize that dissipation of mechanical energy of external impact to absorb mechanical shock is a fundamental function of skeletal muscle in addition to its primary function to convert chemical energy into mechanical energy. In physical systems, the common mechanism for absorbing mechanical shock is achieved with the use of both elastic and viscous elements and we hypothesize that the viscosity of the skeletal muscle is a variable parameter which can be voluntarily controlled by changing the tension of the contracting muscle. We further hypothesize that an ability of muscle to absorb shock has been an important factor in biological evolution, allowing the life to move from the ocean to land, from hydrodynamic to aerodynamic environment with dramatically different loading conditions for musculoskeletal system. The ability of muscle to redistribute the energy of mechanical shock in time and space and unload skeletal joints is of key importance in physical activities. We developed a mathematical model explaining the absorption of mechanical shock energy due to the increased viscosity of contracting skeletal muscles. The developed model, based on the classical theory of sliding filaments, demonstrates that the increased muscle viscosity is a result of the time delay (or phase shift) between the mechanical impact and the attachment/detachment of myosin heads to binding sites on the actin filaments. The increase in the contracted muscle's viscosity is time dependent. Since the forward and backward rate constants for binding the myosin heads to the actin filaments are on the order of 100 s-1, the viscosity of the contracted muscle starts to significantly increase with an impact time greater than 0.01 s. The impact time is one of the key parameters in generating destructive stress in the colliding objects. In order to successfully dampen a short high power impact, muscles must first slow it down to engage the molecular mechanism of muscle viscosity. Muscle carries out two functions, acting first as a nonlinear spring to slow down impact and second as a viscous damper to absorb the impact. Exploring the ability of muscle to absorb mechanical shock may shed light to many problems of medical biomechanics and sports medicine. Currently there are no clinical devices for real-time quantitative assessment of viscoelastic properties of contracting muscles in vivo. Such assessment may be important for diagnosis and monitoring of treatment of various muscle disorders such as muscle dystrophy, motor neuron diseases, inflammatory and metabolic myopathies and many more.

How muscle stiffness affects human body model behavior

“…Both can cause high associated costs for the social sector. Further, this study was motivated by the hypothesis that one of the main function of muscles is the dissipation of mechanical energy after external impacts [ 40 ].…”

Tennis Racket Vibrations and Shock Transmission to the Wrist during Forehand Drive

“…A reduced perception of the racket vibration by the tennis player might be related with the attenuation capacity of the human soft-tissue. It would seem that the attenuation of the vibration might be linked to the viscoelastic properties of the soft-tissues [ 17 ]. The dynamic viscosity would be the most efficient mechanism to attenuate vibration, but would mainly act on the low frequencies of the vibration signal [ 17 ].…”

“…It would seem that the attenuation of the vibration might be linked to the viscoelastic properties of the soft-tissues [ 17 ]. The dynamic viscosity would be the most efficient mechanism to attenuate vibration, but would mainly act on the low frequencies of the vibration signal [ 17 ]. The increased frequency of vibrations may decrease the soft-tissue dynamic viscosity, hence limiting the absorption capacities of the soft-tissues in response to off-centered impact during forehand drive.…”

Stress Injuries of the Spine in Sports

“…Second, muscles absorb impact by acting as a viscous damper. 20 However, with repetitive use, muscles may fatigue and become less efficient in shock absorption, therefore allowing the force to be transferred to the bone, rather than being dissipated by the muscles.…”