Introduction Although quadriceps rectus femoris anatomical cross-sectional area (RFACSA) has been shown to correlate with both volitional and non-volitional measures of quadriceps strength, this only incorporates the cross-sectional muscle mass and disregards the contribution of fibre orientation to the force generating capacity of the muscle. We therefore hypothesised that quadriceps rectus femoris physiological cross-sectional area (RFPCSA), which incorporates both RFACSA and rectus femoris pennation angle (RFPA) would demonstrate a stronger relationship with, and be more representative of, quadriceps strength.
Method 21 healthy adults were recruited, 9 of whom were males, median (IQR) age 31 (25–37) years. RFACSA and RFPA were determined using real-time B-mode ultrasonography using an 8MHz 5.6 cm linear transducer (PLM805, Toshiba Medical Systems Ltd, Crawley, UK) at a point three-fifths distance from the anterior superior iliac spine to the superior patellar border. Values for RFPCSA were calculated from RFACSA and RFPA. QMVC was assessed using the technique of isometric maximum voluntary contraction and twitch tension (TwQ) following magnetic stimulation of the femoral nerve.
Results Males had significantly greater QMVC (55.2±7.1 kg vs 36.3±7.5 kg; p<0.0001), TwQ (12.6±3.6 kg vs 7.4±2.5 kg; p=0.0002), RFACSA (8.9±1.7 cm2 vs 5.9±1.3 cm2; p=0.0001) and RFPCSA (8.8±1.7 cm2 vs 5.8±1.2 cm2; p=0.0001). There was no gender difference evident for RFPA (10.3 (9.7–10.7)° vs 10.8 (9.8–12.1)°; p=0.4) and percent muscle activation during QMVC (83.9±9.3% vs 86.9±9.8%; p=0.5). Identical correlations between RFACSA and RFPCSA and both QMVC and TwQ were observed (r=0.7, p=0.001). There were no significant correlations evident between RFPA and anthropomorphic measures of age, height, weight, body-mass index, fat-free mass or thigh length.
Conclusion The pennation angle of the rectus femoris muscle was observed to be independent of anthropomorphic variables. Furthermore, and contrary to our original hypothesis, RFPCSA did not demonstrate a stronger relationship with quadriceps strength than RFACSA. This is an important finding for the clinician as the additional step of measuring the pennation angle of the muscle adds a complexity to this simple bedside test that would reduce its widespread clinical applicability.
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