Ulnar and tibial bending stiffness as an index of bone strength in synchronized swimmers and gymnasts

The purpose of this study is to compare a mechanical property of bone in world-class female athletes with different loading histories. Bone bending stiffness or EI (E is the modulus of elasticity and I, the moment of inertia) was measured noninvasively with the mechanical response tissue analyzer, that analyzes the response of bone to a vibratory stimulus. We evaluated the ulna, ulnar width, wrist density and tibia in 13 synchronized swimmers (SYN), eight gymnasts (GYM) and 16 untrained women (UNT) of similar age. Muscle strength in the flexors and extensors at elbows and knees was measured in the athletes. SYN were taller than GYM or UNT (168±0.7 vs. 152±1.1 or 157±1.2 cm, P< 0.01). Ulnar EI, Nm², was similar in SYN and GYM (41±5.4 vs. 42±4.2, NS) and 50% higher than in UNT (27±2.1, P<0.05). Ulnar EI, Nm² was related to ulnar width (r=0.497, P<0.002, n=37) but not to wrist density. Tibial EI, Nm², in SYN and GYM (270±42 vs. 285±49, NS) was similar and more than twice as high as in UNT (119±6; p<0.05). Knee flexor strength measured at 60° s⁻¹ and elbow extensor strength at 200° s⁻¹ correlated with tibial EI (r=0.44 and 0.41, P<0.05). In spite of different loading histories, the tibiae and ulnas of world-class athletes showed similar high values for bending stiffness that exceeded values in untrained women. EI in the ulna could be related to bone width and in the tibia, to muscle strength.