Patellar tendon buckling is altered with age.

Recent evidence has revealed that the patellar tendon exhibits buckling during passive knee extension, wherein the tendon folds back onto itself. The clinical relevance of such buckling is unclear, but it has been suggested that it serves to protect the patellar tendon from rupture when subjected to a sudden extreme contraction. Although prior evidence suggests buckling occurs universally, it is poorly understood, and may be influenced by age and sex. Healthy adults (n = 41, aged 21-80 years) were recruited to assess age- and sex-based differences in patellar tendon buckling during passive knee extension. 93% of subjects exhibited buckling in extension, with buckling more prominent in the distal tendon. No age- or sex-based differences in buckling magnitude were observed, but a significant age-based difference in buckling angle was found, with the tendon unbuckling later in flexion in younger adults compared with middle-aged (p = 0.025) and older (p = 0.014) adults. Intrinsic factors were also linked with buckling; for example, smaller maximum knee extension (i.e. less flexibility) correlated with smaller buckling magnitude (p = 0.037, R2  = 0.116), suggesting a link between patellar tendon buckling and joint-level mechanics. These results suggest that buckling is an inherent component of normal knee function that older adults may be failing to take advantage of, predisposing them to injury. Further study will be critical to elucidate the clinical implications of patellar tendon buckling.