Comparative study of the biomechanical performance of trained and untrained skeletal muscle.

UNLABELLED Changes in contraction and relaxation parameters during chronic electrical stimulation can exert profound effects on diastolic augmentation during skeletal muscle assistance (SMA) of the circulation, in both short and long term. The physiological properties of latissimus dorsi muscle (LD) performance in a system that mimics the clinical setting has not been adequately studied. OBJECTIVE To quantify changes in the biomechanical performance of trained and untrained skeletal muscle in relation to circulatory assistance using an ex-vivo Windkessel mock circulation. METHODS Twelve Welsh Mountain sheep were divided into 2 groups: Group A (n = 6) underwent implantation of intramuscular electrodes into the left LD connected to a myostimulator (Telectronics Pacing Systems, 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 microseconds per pulse); Group B (n = 6) were the untrained controls. At the end of 12 weeks, the LD was mobilised on its neurovascular pedicle and wrapped around a latex rubber aorta connected to two Windkessel chambers pressurised to 70 mmHg and stimulated to contract 40 times per minute continuously for 60 min. Pressure change per contraction (augmentation, delta P), volume displacement, contraction (Ct) and relaxation to 90% (Rt90) times, and the standardised rate of change of pressure generation (+dP/dt: delta P) and decay (-dP/dt: delta P) were determined and assessed for potential clinical efficacy. RESULTS In Group A, the LD was fatigue-resistant in all 6 animals with a mean pressure augmentation of 13.7 (s.e.m. 1.3) mmHg and mean stroke volume of 12.5 (s.e.m. 1.0) ml. These muscles were slow with a mean Ct and +dP/dt: delta max of 243.2 (s.e.m. 6.1) ms and + 6.5s-1, respectively, and Rt90 and -dP/dt: delta max of 261.0 (s.e.m. 4.8) ms and -7.8s-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 Group B coincided with a prolongation in the Rt90 by 101% compared to the Ct which decreased by less than 5% (P < 0.001). The Ct/Rt90 ratio did not significantly change during performance testing in Group A (fatigue-resistant animals) (P > 0.1). CONCLUSIONS Using a mock circulation system, we have identified significant differences in biomechanical properties of trained and untrained skeletal muscle. Optimisation of these parameters during and after electrical training may alter the clinical efficacy of SMA.