BACKGROUND:Patella-patellar tendon junction (PPTJ) injuries are a common disease in athletes and vulnerable to re-injury. Post-injury training is a double-edged sword. OBJECTIVE:To examine the effect of post-injury resting days on the quality of healing in PPTJ in view of histomorphology, expression levels of growth factor and collagen, and muscle tension. METHODS:Forty New Zealand white rabbits were randomly divided into control, and five post-injury training groups representing different time to restart training after acute injury:the rabbits restarted training at 24 (PIT24), 48 (PIT48), 72 (PIT72), 96 (PIT96) and 120 (PIT120) hours, respectively. All post-injury training groups received acupuncture at the PPTJ and then underwent 4-week low-intensity training. Afterwards, all rabbits were killed to take specimens for hematoxylin-eosin, safranin O and immuohistochemical staining. The muscle tension and hardness were detected using Myoton-3. The thickness of fibrous cartilage zones, cell density, as well as the expression levels of vascular endothelial growth factor and collagen were measured on Metamorph software. RESULTS AND CONCLUSION:Compared with the control group, in the PIT groups, the collagens arranged in disorder, and decreased cell density, increased mucopolysaccharide andrising tidephenomenon appeared. The fibrocartilage in the PIT24 and PIT120 groups was significantly thicker than that in the control group (P<0.05). The ratio of collagen type Ⅰ/Ⅲ was significantly decreased in the PIT24 and PIT48 groups compared with the control group (P<0.05), and the highest in the PIT72 group. The expression level of vascular endothelial growth factor in the PIT48, PIT72 and PIT96 groups was significantly higher than that in the control group (P<0.05). The muscle tension was negatively correlated with the collagen ratio (r=-0.538;P<0.01) and chondrocyte density (r=-0.570;P<0.01), while muscle hardness was positively correlated with the expression level of vascular endothelial growth factor (r=0.613;P<0.01). Our results suggest that different resting time makes significant differences in the PPTJ healing and the 72-hour resting is the best choice. Moreover, muscular properties are related to the cel density, collagen synthesis and growth factor expression, which can be used as a non-invasive indicator to guide post-injury training and prevent recurrence.