PURPOSE: The discovery of musculoskeletal tissues, including muscle, tendons, and cartilage, as peripheral circadian clocks strongly implicates their role in tissue-specific homeostasis. Age-related dampening and misalignment of the tendon circadian rhythm and its outputs may be responsible for the decline in tendon homeostasis. It is unknown which entrainment signals are responsible for the synchronization of the tendon clock to the light-dark cycle.

METHODS: We sought to examine any changes in the expression levels of core clock genes (BMAL1, CLOCK, PER2, CRY1, and NR1D1) in healthy human patellar tendon biopsies obtained from three different intervention studies: increased physical activity (leg kicks for 1 h) in young, reduced activity (2 weeks immobilization of one leg) in young, and in old tendons.

RESULTS: The expression level of clock genes in human tendon in vivo was very low and a high variation between individuals was found. We were thus unable to detect any differences in core clock gene expression neither after acute exercise nor immobilization.

CONCLUSIONS: We are unable to find evidence for an effect of exercise or immobilization on circadian clock gene expression in human tendon samples.