COMPARATIVE-STUDY OF THE BIOMECHANICAL PERFORMANCE OF TRAINED AND UNTRAINED SKELETAL-MUSCLE

Changes in contraction and relaxation parameters during chronic electr ical stimulation can exert profound effects on diastolic augmentation during skeletal muscle assistance (SMA) of the circulation, in both sh ort aid long term. The physiological properties of latissimus dorsi mu scle (LD) performance in a system that mimics the clinical setting has not been adequately studied. Objective: To quantify changes in the bi omechanical performance of trained and untrained skeletal muscle in re lation to circulatory assistance using an ex-vivo Windkessel mock circ ulation. Methods: Twelve Welsh Mountain sheep were divided into 2 grou ps: Group A (n = 6) underwent implantation of intramuscular electrodes into the left LD connected to a myostimulator (Telectronics Pacing Sy stems, Inc., Colorado) and progressively trained by burst stimulation over a 12-week period using standard stimulation parameters (2.5-5 V, 35 Hz, 6 pulses per burst, 240 mu s per pulse); Group B (n = 6) were t he untrained controls. At the end of 12 weeks, the LD was mobilised on its neurovascular pedicle and wrapped around a latex rubber aorta con nected to two Windkessel chambers pressurised to 70 mmHg and stimulate d to contract 40 times per minute continuously for 60 min. Pressure ch ange per contraction (augmentation, Delta P), volume displacement, con traction (C-t) and relaxation to 90% (R(t90)) times, and the standardi sed rate of change of pressure generation (+dP/dt:Delta P) and decay ( -dP/dt:Delta P) were determined and assessed for potential clinical ef ficacy. Results: In Group A, the LD was fatigue-resistant in all 6 ani mals with a mean pressure augmentation of 13.7 (s.e.m. 1.3) mmHg and m ean stroke volume of 12.5 (s.e.m. 1.0) ml. These muscles were slow wit h a mean C-t and +dP/dt:Delta max of 243.2 (s.e.m. 6.1) ms and +6.5 s( -1), respectively, and R(t90) and -dP/dt:Delta max of 261.0 (s.e.m. 4. 8) ms and -7.8 s(-1), respectively. In contrast, the LD in Group B was fatiguable with a mean pressure augmentation and stroke volume of 24. 6 (s.e.m. 0.9) mmHg and 21.1 (s.e.m. 0.7) ml at 1 min and only 5.4 (s. e.m. 0.3) mmHg and 5.2 (s.e.m. 0.3) ml, respectively, at 30 min (P < 0 .001). These muscles were faster at all time points compared to group A (P < 0.02). Acute diminution of power output per contraction in Grou p B coincided with a prolongation in the R(t90) by 101% compared to th e C-t which decreased by less than 5% (P < 0.001). The C-t/R(t90) rati o did not significantly change during performance testing in Group A ( fatigue-resistant animals) (P > 0.1). Conclusion: Using a mock circula tion system, we have identified significant differences in biomechanic al properties of trained and untrained skeletal muscle. Optimisation o f these parameters during and after electrical training may alter the clinical efficacy of SMA.