Skeletal muscle may provide a totally implantable power source for cardiac assistance devices. Continuous power from electrically conditioned human latissimus dorsi muscle has been obtained in the clinical cardiomyoplasty procedure (Chachques et al., 1997). Maximum sustained power output during repetitive work-producing contractions is required for cardiac assistance (Jarvis, 1993). Previous work in our laboratory has focused on the biomechanical optimization of skeletal muscle cycle work and power (Reichenbach and Farrar, 1994; Reichenbach et al., 1995, 1999). Biomechanical power optimization and the concomitant operating conditions should be constrained by the muscle metabolic factors that limit sustained power. This constraint may be identified on the basis of relative relationships; therefore, an absolute measurement technique is not necessarily required. Metabolic utilization is reflected in the muscle heat generation during contraction and is dependent on a number of variables, including the specific contraction variables, species and muscle fiber type (Constable et al., 1997). Although myothermal relationships and the biochemical basis of skeletal muscle contraction have been extensively studied in in vitro preparations of amphibian muscle fibers at reduced temperatures, the energetic characteristics of work performance by mammalian muscle are unclear (Barclay et al., 1993a; Curtin and Woledge, 1993). The large in situ mammalian muscles, short contraction durations and cyclic operating conditions inherent in cardiac assistance applications may not be directly comparable with previous work. The purpose of the present study was (i) to develop an in vivo measurement technique to provide an estimate of metabolic energy utilization in whole mammalian muscle during repetitive contractions; (ii) to identify experimentally the significant contraction variables involved in metabolic energy utilization and their relationships; and (iii) to develop a model to predict metabolic energy utilization over a range of contraction conditions. Materials and methods Surgical preparation Seven New Zealand white rabbits were anesthetized with Ketamine (60 mg kg -1 ) and Xylazine (5 mg kg -1 ), and a tracheostomy was performed. The animals were supported on a volume-controlled mechanical ventilator and monitored with a pulse oximeter, and anesthesia was maintained with intermittent intravenous boluses of sodium pentobarbital. All 3667 The Journal of Experimental Biology 203, 3667–3674 (2000) Printed in Great Britain © The Company of Biologists Limited 2000 JEB2997 Electrically conditioned skeletal muscle can provide the continuous power source for cardiac assistance devices. Optimization of the available sustained power from in vivo skeletal muscle requires knowledge of its metabolic utilization and constraints. A thermistor-based technique has been developed to measure temperature changes and to provide a relative estimate for metabolic utilization of in situ rabbit soleus muscle. The relative thermistor response, active tension and muscle displacement were measured during cyclic isometric and isotonic contractions across a range of muscle tensions and contraction durations. The thermistor response demonstrated linear relationships versus both contraction duration at a fixed muscle length and active tension at a fixed contraction duration (r 2 =0.90±0.14 and 0.70±0.21, respectively; means ± S.D.). A multiple linear regression model was developed to predict normalized thermistor response, ∆T, across a range of conditions. Significant model variables were identified using a backward stepwise regression procedure. The relationships for the in situ muscles were qualitatively similar to those reported for mammalian in vitro muscle fiber preparations. The model had the form ∆T=C+at c F+bW, where the constant C, and coefficients for the contraction duration t c (ms), normalized active tension F and normalized net work W were C=-1.00 (P<0.001), a=5.97 (P<0.001) and b=2.12 (P<0.001). Key words: skeletal muscle, model, myothermic measurement, muscle contraction, contraction duration, force, work, thermistor, metabolic utilization, rabbit. Summary Introduction RELATIVE METABOLIC UTILIZATION OF IN SITU RABBIT SOLEUS MUSCLE: THERMISTOR-BASED MEASUREMENTS AND MODEL KENNETH J. GUSTAFSON, GLENN D. EGRIE AND STEVEN H. REICHENBACH* Department of Cardiac Surgery Research, California Pacific Medical Center, 2351 Clay Street Room S-637, San Francisco, CA 94115-7999, USA *Author for correspondence (e-mail: reichen@thoratec.com) Accepted 24 August; published on WWW 2 November 2000