Bone rigidity to neuromuscular performance ratio in young and elderly men.

Given the adaptation of bone to prevalent loading, bone loss should follow, but lag behind, the decline in physical performance during aging. Furthermore, bone responsiveness to load-induced strains is believed to decrease with aging. However, the relationship between bone and lean body ( approximately muscle) mass appears to remain rather constant throughout adulthood. The purpose of this study was to examine the association between age and bone to neuromuscular performance ratio. Young (N=20, age 24 SD+/-2 years, body mass 77+/-11 kg, height 178+/-6 cm) and elderly (N=25, 72+/-4 years, 75+/-9 kg, 172+/-5 cm) men served as subjects. Bone structural traits were measured at the right distal tibia and tibial mid-shaft with peripheral quantitative computed tomography (pQCT). Maximal section modulus (Z(max50)), total area (ToA(d)), cortical area (CoA(50)), total density (ToD(d)) and cortical density (CoD(50)) were determined from the pQCT images. Neuromuscular performance was measured by recording vertical ground reaction force (GRF) in maximal bilateral hopping. Load-induced strains were estimated by calculating appropriate indices for compressive and tensile loading that took into account both the bone structure and apparent biomechanics of the given bone site. Young subjects had significantly higher maximal GRF compared to older men (4260+/-800 N vs. 3080+/-600 N, P<0.001). They also had smaller ToA(d) (1100+/-170 mm(2) vs. 1200+/-100 mm(2), P=0.028) while their ToD(d) was higher (370+/-46 g/cm(3) vs. 330+/-22 g/cm(3), P=0.002). The Z(max50) did not differ significantly between young (1660+/-320 mm(3)) and elderly men (1750+/-320 mm(3)) (P=0.224). Compressive (0.484+/-0.102 vs. 0.399+/-0.078, P=0.016) and tensile (0.107+/-0.016 vs. 0.071+/-0.018, P<0.001) strain indices were significantly higher in the younger group. In conclusion, the difference in bone to loading ratio at the tibial mid-shaft is bigger than expected from the delay in bone adaptation alone. Potential candidates to explain this phenomenon include a decrease in mechanosensitivity with aging, inability of maximal physical performance to adequately represent the bone loading environment, or the need to maintain constant safety factors to functional strains.