Effect of loading and unloading on skeletal muscle

It is well established that the amount of loading placed on a muscle significantly alters its size and composition. If loading placed on a muscle is reduced, for example during longterm bed rest or during low gravity conditions, muscle mass will decrease and its fibre type composition changes leading to significant functional limitations. On the other hand, increasing the mechanical load leads to increases in muscle size, as, for example, can clearly be seen in bodybuilders. How a muscle ‘feels’ the mechanical loading placed upon it, and how it translates this mechanical signal into the phenotypical adaptations is still poorly understood. In this chapter we will discuss in detail the models which are available to study various muscle loading conditions, i.e. muscle unloading, muscle overloading, extreme muscle loading which leads to muscle damage, and how these conditions eventually change the phenotype observed. We discuss in detail the various intra- and extramuscular sensors which respond to changes in loading conditions. The extramuscular proprioceptive system is briefly discussed and an up to date review is given on the possible intracellular sensors present. Finally, some important signaling pathways involved in leading to the changes in gene transcription are summarized briefly. This chapter therefore gives an overview of the current state of knowledge and of the questions still open regarding the mechanical sensing-and signal transduction in skeletal muscle and the models which can be used to study this intriguing phenomena.