Background This study evaluated the time course of recovery of transverse strain in the Achilles and patellar tendons following a bout of resistance exercise.

Methods Seventeen healthy adults underwent sonographic examination of the right patellar (n=9) or Achilles (n=8) tendons immediately prior to and following 90 repetitions of weight–bearing exercise. Quadriceps and gastrocnemius exercise were performed against an effective resistance of 175% and 250% body weight, respectively. Sagittal tendon thickness was determined 20 mm from the tendon enthesis and transverse strain was repeatedly monitored over a 24 hour recovery period.

Results Resistance exercise resulted in an immediate decrease in Achilles (t7=10.6, P<.01) and patellar (t8=8.9, P<.01) tendon thickness, resulting in an average transverse strain of 0.14±0.04 and 0.18±0.05. While the average strain was not significantly different between tendons, older age was associated with a reduced transverse strain response (r=0.63, P<.01). Recovery of transverse strain, in contrast, was prolonged compared with the duration of loading and exponential in nature. The mean primary recovery time was not significantly different between Achilles (6.5±3.2 hours) and patellar (7.1±3.2 hours) tendons and body weight accounted for 62% and 64% of the variation in recovery time, respectively.

Discussion Despite structural and biochemical differences between the Achilles and patellar tendons,¹ the mechanisms underlying transverse creep–recovery in vivo appear similar and are highly time dependent. Primary recovery required about 7 hours in healthy tendons, with full recovery requiring up to 24 hours. These in vivo recovery times are similar to those reported for axial creep recovery of the vertebral disc in vitro,² and may be used clinically to guide physical activity to rest ratios in healthy adults. Optimal ratios for high–stress tendons in clinical populations, however, remain unknown and require further attention in light of the knowledge gained in this study.