Abstract
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 yr) had 4.0 ± 0.8 yr of systematic knee extensor heavy-resistance training experience, whereas UT (n = 52; age 25.1 ± 2.3 yr) had no lower-body resistance training experience for >18 mo. 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 in the present study for regional hypertrophy after LTT.NEW & NOTEWORTHY Here we demonstrate that the larger muscle strength (+60%) of a long-term (4+ yr) resistance-trained group compared with untrained controls was due to their similarly larger muscle volume (+56%), primarily due to a larger physiological cross-sectional area and modest differences in fascicle length, as well as modest differences in maximum voluntary specific tension and patella tendon moment arm. In addition, the present study refutes the possibility of regional hypertrophy, despite large differences in muscle volume.
Highlights
Muscular strength is integral to athletic performance [21], helps to reduce injury risk [19] and the likelihood of developing musculoskeletal disorders such as osteoarthritis [84], and facilitates independence and functional mobility [18, 53] with aging
quadriceps volume (QVOL) was 56% greater in long-term resistance-trained (LTT) than untrained controls (UT) (P Ͻ 0.001; Effect size (ES) ϭ 3.7), QACSAMAX was 50% greater (P Ͻ 0.001, ES ϭ 3.3) and QEFFPCSA 41% greater in LTT compared with UT (P Ͻ 0.001, ES ϭ 4.1)
The proportional volume, and ACSAMAX, of the individual constituent muscles were similar for LTT and UT (P ϭ 0.56 – 0.94; volume data shown in Table 1) and the percentage of femur length where ACSAMAX of each constituent muscle occurred was similar for both groups (VM: 28% vs. 29%; vastus intermedius (VI): 58% vs. 58%; VL: 57% vs. 56% and rectus femoris (RF): 68% vs. 68% femur length for UT and LTT, respectively; P ϭ 0.26 – 0.80; Fig. 2)
Summary
Muscular strength is integral to athletic performance [21], helps to reduce injury risk [19] and the likelihood of developing musculoskeletal disorders such as osteoarthritis [84], and facilitates independence and functional mobility [18, 53] with aging. Long-term RT individuals are known to be substantially stronger than untrained controls (UT) [9, 56], a functional difference that is often attributed to their larger muscle size [i.e., greater volume or cross-sectional area (CSA) due to hypertrophy]. Long-term systematic RT (i.e., multiple years) has been shown to result in substantially greater muscle size compared with untrained controls {ϩ70 –76% greater biceps brachii anatomical CSA [ACSA [9, 52]]; ϩ85% greater quadriceps volume [38]}, but whether an increase in muscle size is accompanied by similar, smaller, or no changes in maximum voluntary specific tension (ST) remains unknown. The extent to which increases in overall muscle size (volume) after long-term RT are due to increases in either sarcomeres in parallel (i.e., increased physiological CSA; PCSA) and/or in series (i.e., fiber/fascicle length) has not been examined. A rigorous assessment of muscle size (ACSA, PCSA, and volume), ST, and architectural contributions to enhanced strength after longterm RT appears warranted
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