<P> Contracting muscles produce vibration and sound . Slow twitch fibers produce 10 to 30 contractions per second (10 to 30 Hz). Fast twitch fibers produce 30 to 70 contractions per second (30 to 70 Hz). The vibration can be witnessed and felt by highly tensing one's muscles, as when making a firm fist . The sound can be heard by pressing a highly tensed muscle against the ear, again a firm fist is a good example . The sound is usually described as a rumbling sound . Some individuals can voluntarily produce this rumbling sound by contracting the tensor tympani muscle of the middle ear . The rumbling sound can also be heard when the neck or jaw muscles are highly tensed . </P> <P> Skeletal muscle fiber - type phenotype in adult animals is regulated by several independent signaling pathways . These include pathways involved with the Ras / mitogen - activated protein kinase (MAPK) pathway, calcineurin, calcium / calmodulin - dependent protein kinase IV, and the peroxisome proliferator γ coactivator 1 (PGC - 1). The Ras / MAPK signaling pathway links the motor neurons and signaling systems, coupling excitation and transcription regulation to promote the nerve - dependent induction of the slow program in regenerating muscle . Calcineurin, a Ca2+ / calmodulin - activated phosphatase implicated in nerve activity - dependent fiber - type specification in skeletal muscle, directly controls the phosphorylation state of the transcription factor NFAT, allowing for its translocation to the nucleus and leading to the activation of slow - type muscle proteins in cooperation with myocyte enhancer factor 2 (MEF2) proteins and other regulatory proteins . Ca2+ / calmodulin - dependent protein kinase activity is also upregulated by slow motor neuron activity, possibly because it amplifies the slow - type calcineurin - generated responses by promoting MEF2 transactivator functions and enhancing oxidative capacity through stimulation of mitochondrial biogenesis . </P> <P> Contraction - induced changes in intracellular calcium or reactive oxygen species provide signals to diverse pathways that include the MAPKs, calcineurin and calcium / calmodulin - dependent protein kinase IV to activate transcription factors that regulate gene expression and enzyme activity in skeletal muscle . </P> <P> PGC1 - α (PPARGC1A), a transcriptional coactivator of nuclear receptors important to the regulation of a number of mitochondrial genes involved in oxidative metabolism, directly interacts with MEF2 to synergistically activate selective slow twitch (ST) muscle genes and also serves as a target for calcineurin signaling . A peroxisome proliferator - activated receptor δ (PPARδ) - mediated transcriptional pathway is involved in the regulation of the skeletal muscle fiber phenotype . Mice that harbor an activated form of PPARd display an "endurance" phenotype, with a coordinated increase in oxidative enzymes and mitochondrial biogenesis and an increased proportion of ST fibers . Thus--through functional genomics--calcineurin, calmodulin - dependent kinase, PGC - 1α, and activated PPARδ form the basis of a signaling network that controls skeletal muscle fiber - type transformation and metabolic profiles that protect against insulin resistance and obesity . </P>

Skeletal muscles of the body are innervated by