MR techniques have proven their ability to investigate skeletal muscle function in situ. Their benefit in terms of noninvasiveness is, however, lost in animal research, given that muscle stimulation and force output measurements are usually achieved using invasive surgical procedures, thereby excluding repeated investigations in the same animal. This study describes a new setup allowing strictly noninvasive investigations of mouse gastrocnemius muscle function using 1 H-MRI and 31 P-MR spectroscopy. Its originality is to integrate noninvasive systems for inducing muscle contraction through transcutaneous stimulation and for measuring mechanical performance with a dedicated ergometer. In order to test the setup, muscle function was investigated using a fatiguing stimulation protocol (6 min of repeated isometric contractions at 1.7 Hz). T 2 -weighted imaging demonstrated that transcutaneous stimulation mainly activated the gastrocnemius. Moreover, investigations repeated twice with a 7-day interval between bouts did show a high reproducibility in measurements with regard to changes in isometric force and energy metabolism. In conclusion, this setup enables us for the first time to access mechanical performance, energy metabolism, anatomy, and physiology strictly noninvasively in contracting mouse skeletal muscle. The possibility for implementing longitudinal studies opens up new perspectives in many research areas, including ageing, pharmaceutical research, and gene and cell therapy. Key words: phosphorus MR spectroscopy; functional magnetic resonance imaging; skeletal muscle fatigue; ergometer; in vivo muscle stimulation; muscle contraction During the past two decades, transgenic mouse models have been used in order to identify the roles of genes in skeletal muscle development, physiology, and disease, thereby improving our knowledge in biology and medicine. The potential of these experimental models can be enhanced by the utilization of noninvasive techniques such as MR spectroscopy (MRS) and MRI in order to assess muscle function and metabolism in vivo (1-3). These techniques have indeed made possible the transition from in vitro biology to the integrative investigation of skeletal muscle function (4). In that respect, 31 P-MRS allows characterization of muscular energy metabolism through the measurement of intracellular pH and the concentration of the major phosphorylated compounds in contracting muscle. These measurements can also be used in order to quantify the relative aerobic and anaerobic contributions to energy production (5,6

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