Assessment of Contractile and Kinetic Properties of Skeletal and Cardiac Multicellular Preparations in Mouse Models: A Comprehensive Methodological Guide.

The mechanical functions of skeletal and cardiac muscles are fundamentally defined by their contractile and passive properties, reflecting their unique physiological roles and operational dynamics. Contractile properties, which include the generation of force during muscle contraction, the duration required to reach peak force, and the subsequent relaxation period, are pivotal in assessing muscle performance, strength, injury, fatigue resistance, and overall health. These properties are crucial for understanding muscle function across various physiological and pathological conditions, including muscular dystrophies, myopathies, and during recovery post-injury or therapy. Muscular disorders, such as Duchenne muscular dystrophy (DMD), significantly impair several of these mechanical properties, causing a progressive decline in muscle strength, loss of mobility, respiratory muscle weakness, and early mortality most commonly due to respiratory failure. DMD also adversely affects cardiac muscle, leading to progressive deterioration and complications like dilated cardiomyopathy with congestive heart failure or arrhythmias, significant contributors to mortality in affected individuals.Here we detail the methodologies and experimental techniques used for ex vivo assessment of contractile and kinetic properties in multicellular preparations from mouse models, specifically focusing on the extensor digitorum longus (EDL) muscle, the diaphragm, and cardiac trabeculae and papillary muscles. We describe protocols for measuring EDL muscle force output and twitch characteristics, tetanic contractions and eccentric contractions, and the force-frequency and length-tension relationships, as well as temperature effects. We elaborate on protocols assessing diaphragm muscle strength, including fatigue resistance evaluations. Additionally, we detail the experimental techniques used to investigate the three main mechanisms regulating cardiac contractility: the Frank-Starling mechanism, the force-frequency relationship, and ß-adrenergic stimulation through the use of isolated trabeculae and papillary muscles from mouse hearts. These techniques enable researchers and clinicians to quantify the effects of diseases, drugs, training regimens, and genetic modifications on muscle function, contributing to the development of treatments and interventions for muscle-related conditions, offering hope for improved patient outcomes in conditions like DMD.