A mechanism for increased contractile strength of human pennate muscle in response to strength training: changes in muscle architecture

1. In human pennate muscle, changes in anatomical cross-sectional area (CSA ) or volume caused by training or inactivity may not necessarily reflect th e, change in physiological CSA, and thereby in maximal contractile force, s ince a simultaneous change in muscle fibre pennation angle could also occur . 2. Eleven male subjects undertook 14 weeks of heavy-resistance strength tra ining of the lower limb muscles. Before and after training anatomical CSA a nd volume of the human quadriceps femoris muscle were assessed by use of ma gnetic resonance imaging (MRI), muscle fibre pennation angle (theta (p)) wa s measured in the vastus lateralis (VL) by use of ultrasonography, and musc le fibre CSA (CSA(fibre)) was obtained by needle biopsy sampling in VL. 3. Anatomical muscle CSA and volume increased with training from 77.5 +/- 3 .0 to 85.0 +/- 2.7 cm(3) and 1676 +/- 63 to 1841 +/- 57 cm(3), respectively Furthermore, VL pennation angle increased 2 from 8.0 +/- 0.4 to 10.7 +/- 0 .6 deg and CSA(fibre) increased from 3754 +/- 271 to 4238 +/- 202 mum(2). I sometric quadriceps strength increased from 282.6 +/- 11.7 to 327.0 +/- 12. 4 Nm. 4. A positive relationship was observed between theta (p) and quadriceps vo lume prior to training (r = 0.622). Multifactor regression analysis reveale d a stronger relationship when theta (p) and CSA(fibre) were combined (R = 0.728). Post-training increases in CSA(fibre) were related to the increase in quadriceps volume (r = 0.749). 5. Myosin heavy chain (MHC) isoform distribution (type I and II) remained u naltered with training. 6. VL muscle fibre pennation angle was observed to increase in response to resistance training. This allowed single muscle fibre CSA and maximal contr actile strength to increase more (+16 %) than anatomical muscle CSA and vol ume (+10 %). 7. Collectively, the present data suggest that the morphology, architecture and contractile capacity of human pennate muscle are interrel ated, in vivo. This interaction seems to include the specific adaptation re sponses evoked by intensive resistance training.