What makes long-term resistance-trained individuals so strong? A comparison of skeletal muscle morphology, architecture, and joint mechanics.

The greater muscular strength of long-term resistance-trained (LTT) individuals is often attributed to hypertrophy but the role of other factors, notably maximum voluntary specific tension (ST), muscle architecture and any differences in joint mechanics (moment arm) have not been documented. The aim of the present study was to examine the musculoskeletal factors that might explain the greater Quadriceps strength and size of LTT vs untrained (UT) individuals. LTT (n = 16, age 21.6 ± 2.0 years) had 4.0 ± 0.8 years of systematic knee extensor heavy-resistance training experience, whereas UT (n = 52; age 25.1 ± 2.3 years) had no lower-body resistance training experience for > 18 months. Knee extension dynamometry, T1-weighted magnetic resonance images of the thigh and knee and ultrasonography of the Quadriceps muscle group at 10 locations were used to determine Quadriceps: isometric maximal voluntary torque (MVT), muscle volume (QVOL), patella tendon moment arm (PTMA), pennation angle (QΘP) and fascicle length (QFL), physiological cross-sectional area (QPCSA) and ST. LTT had substantially greater MVT (+60% vs UT, P<0.001) and QVOL (+56%, P<0.001) and QPCSA (+41%, P<0.001) but smaller differences in ST (+9%, P<0.05) and moment arm (+4%, P<0.05), and thus muscle size was the primary explanation for the greater strength of LTT. The greater muscle size (volume) of LTT was primarily attributable to the greater QPCSA (+41%; indicating more sarcomeres in parallel) rather than the more modest difference in FL (+11%; indicating more sarcomeres in series). There was no evidence for regional hypertrophy after LTT.