Resistance training increases in vivo quadriceps femoris muscle specific tension in young men R. M. Erskine, 1 D. A. Jones, 1,2 A. G. Williams, 3 C. E. Stewart 1 and H. Degens 1 1 Faculty of Science and Engineering, Institute for Biomedical Research into Human Movement and Health, Manchester Metropolitan University, Manchester, UK 2 School of Exercise and Sport Sciences, University of Birmingham, Edgbaston, UK 3 Department of Exercise and Sport Science, Manchester Metropolitan University, Alsager, UK Received 1 September 2009, revision requested 13 November 2009, revision received 30 November 2009, accepted 11 January 2010 Correspondence: R. M. Erskine, Faculty of Science and Engineering, Institute for Biomedical Research into Human Movement and Health, Manchester Metropolitan University, John Dalton Building, Oxford Road, Manchester M1 5GD, UK. E-mail r.m.erskine@mmu.ac.uk Abstract Aim: The present study investigated whether in vivo human quadriceps femoris (QF) muscle specific tension changed following strength training by systematically determining QF maximal force and physiological cross- sectional area (PCSA). Methods: Seventeen untrained men (20 Æ 2 years) performed high-intensity leg-extension training three times a week for 9 weeks. Maximum tendon force (F t ) was calculated from maximum voluntary contraction (MVC) torque, corrected for agonist and antagonist muscle activation, and moment arm length (d PT ) before and after training. QF PCSA was calculated as the sum of the four component muscle volumes, each divided by its fascicle length. Dividing F t by the sum of the component muscle PCSAs, each multiplied by the cosine of the respective fascicle pennation angle, provided QF specific tension. Results: MVC torque and QF activation increased by 31% (P < 0.01) and 3% (P < 0.05), respectively, but there was no change in antagonist co-acti- vation or d PT . Subsequently, F t increased by 27% (P < 0.01). QF volume increased by 6% but fascicle length did not change in any of the component muscles, leading to a 6% increase in QF PCSA (P < 0.05). Fascicle pennation angle increased by 5% (P < 0.01) but only in the vastus lateralis muscle. Consequently, QF specific tension increased by 20% (P < 0.01). Conclusion: An increase in human muscle specific tension appears to be a real consequence of resistance training rather than being an artefact of measuring errors but the underlying cause of this phenomenon remains to be determined. Keywords muscle architecture, PCSA, quadriceps femoris, specific tension, strength training. The maximum force generated by skeletal muscle is primarily determined by the number of sarcomeres arranged in parallel, which is represented by the physiological cross-sectional area (PCSA) of the muscle (Close 1972). Consequently, an increase in PCSA should be accompanied by a proportional increase in force generating capacity as is observed in experiments on animal muscle subjected to chronic overload (Degens et al. 1995, Hornberger & Farrar 2004). In human muscle, however, resistance training is com- monly reported to induce a proportionally larger increase in strength than muscle size (Ikai & Fukunaga 1970, Moritani & deVries 1979, Young et al. 1983, Jones & Rutherford 1987, Davies et al. 1988, Narici et al. 1989, 1996b, Ferri et al. 2003). One possible explanation for this discrepancy between animals and humans may be the difficulty of assessing maximum human muscle force and PCSA in situ, compared to Acta Physiol 2010, 199, 83–89 Ó 2010 The Authors Journal compilation Ó 2010 Scandinavian Physiological Society, doi: 10.1111/j.1748-1716.2010.02085.x 83