Contributions of neural tone to in vivo passive muscle--tendon unit biomechanical properties in a rat rotator cuff animal model.

Passive viscoelastic properties of muscle-tendon units are key determinants of intra- and post-operative success. Atrophied, retracted, and stiff muscle-tendon units are technically challenging to manipulate and perform poorly after surgical repair. This study employs botulinum neurotoxin A (BoNT-A)-mediated inhibition of presynaptic acetylcholine release to examine in vivo neural contributions to soft-tissue biomechanical properties. In vivo load-relaxation and active muscle contractile force testing protocols were performed in the rat rotator cuff model. The passive properties were assessed using linear regression analysis and Fung's quasi-linear viscoelastic (QLV) model. BoNT-A injected muscle--tendon units had a significant reduction in force of contraction (p = 0.001). When compared to saline injected controls, the BoNT-A significantly decreased parameter 'A' of the QLV model, which represents the linear elastic response (p = 0.032). The viscous properties in the BoNT-A treatment group were not significantly different from saline injected controls, as determined by comparison of QLV model parameters 'C,' 'τ(1),' and 'τ(2).' In conclusion, neural tone contributes significantly to muscle-tendon unit passive biomechanical properties. Pre-surgical treatment with BoNT-A may improve the rehabilitation of muscle by altering its passive elastic properties. Accordingly, pharmacological modulation of skeletal muscle stiffness with BoNT-A increases flexibility, potentially improving function. Chemical denervation with BoNT-A may also improve the manipulation of stiff and difficult to mobilize muscles during surgical procedures.