The inter-individual variability in human muscle strength and in the response to resistance training

Erskine, Robert M. (2010) The inter-individual variability in human muscle strength and in the response to resistance training. Doctoral thesis (PhD), Manchester Metropolitan University.

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Abstract

Inter-individual differences in strength are not fully explained by muscle size, suggesting that muscle specific tension [force per unit physiological cross-sectional area (PCSA)] varies between untrained individuals. Furthermore, many reports demonstrate greater gains in muscle strength than size following resistance training, thus indicating an increase in specific tension. Moreover, there is considerable variation in the response to training that may have a genetic origin. The aims of the work described in this thesis were i) to examine the degree of variability in muscle specific tension; ii) to investigate whether specific tension changed following resistance training; iii) to quantify the variability in the response of human muscle to resistance training; iv) to identify gene variants that may be associated with the variable training responses. In a group of untrained young men and before and after 9 weeks of resistance training, the quadriceps femoris (QF) muscle specific tension was calculated from the maximum isometric voluntary contraction (MVC) torque, taking into account the contribution of voluntary activation, antagonist muscle co-activation, moment arm length, QF volume, muscle fascicle length and pennation angle. Correcting for these factors made little difference to the between subjects variance of MVC torque, thus demonstrating that muscle specific tension varies considerably between individuals. Resistance training increased QF muscle force much more than PCSA, indicating that most of the increase in force was a result of increased specific tension. This increase was not associated with a change in myosin heavy chain expression and was not accompanied by an increase in single fibre specific tension, or an increase in power per unit muscle volume measured during isokinetic cycling. The results are consistent with an increase in lateral force transmission in the muscle. Substantial variation in the training responses was observed and the final part of this thesis is concerned with linking this variation with specific gene variants.