LATENCY OF SKELETAL-MUSCLE CONTRACTION AFTER PULSE-TRAIN STIMULATION - AN IMPORTANT FACTOR IN CORRECT TIMING OF SKELETAL-MUSCLE CARDIAC ASSIST DEVICES

Various configurations of conditioned skeletal muscle are under invest igation for cardiac assistance in patients with end-stage cardiac fail ure. Optimal timing of conditioned skeletal muscle contraction is esse ntial for effective cardiac augmentation. However, unlike mechanical m ethods of assistance, skeletal muscle requires time to develop peak te nsion. We measured ''time to 50% peak tension'' and ''time to 90% peak tension'' using an electrical strain gauge in 12 canine latissimus do rsi muscles (6 untrained controls and 6 trained with 3 months of elect rical stimulation at 25 Hz with a 15% duty cycle). The ''time to 50% r elaxation'' and the ''time to 90% relaxation'' after discontinuation o f the stimulus were also measured. Conditioned skeletal muscle require d significantly more time to develop peak tension than unconditioned s keletal muscle. Relaxation was also significantly prolonged in conditi oned muscle. Notably, conditioned lattisimus needed, on average, 0.35 sec to develop peak tension and 0.20 sec for 90% relaxation. Thus, 0.5 5 sec of each muscle contraction/relaxation cycle was devoted to devel opment of peak tension and subsequent relaxation. At normal canine hea rt rates of approximately 120 beats per minute (0.50 sec per cardiac c ycle), conditioned skeletal muscle may take up to 70% of each cardiac cycle (0.35 sec) to develop 90% of peak tension. The recognition of th is phenomenon in conditioned skeletal muscle is important for effectiv e contraction timing of both human and animal skeletal muscle assist d evices. Development of proper conditioning regimens for such devices m ay benefit from identification of those training parameters which prod uct a minimal ''time to peak tension.'' (C) 1994 Academic Press, Inc.