Finding the sweet spot via personalised Achilles tendon training: the future is within reach

Musculoskeletal tissues, including tendon, are sensitive to their mechanical environment, with both excessive and insufficient loading resulting in reduced tissue strength. Although further research is required to elucidate the specific loading conditions (ie, magnitude, duration, frequency and rate) that maximise tendon adaptation, in vitro 1 2 and in vivo 3–6 evidence indicates tendon has an optimal range or ‘sweet spot’7 within which mechanical loading promotes positive tissue adaptation or homeostasis, with either too much or too little loading being detrimental (figure 1). In vitro bioreactor studies of rabbit Achilles tendon have shown that continuous 8 hours/day for 6 days of 6% cyclic strain at 0.25 Hz maintains1 and promotes2 tendon health, but the same duration and frequency with strain magnitude of 3% and 9% disrupted the extracellular matrix. Tendons that were load-deprived for 12 days increased their expression of collagen of inferior material properties (ie, type III) and had diminished mechanical performance. Health of these load-deprived tendons, represented by increased expression of type I collagen, was restored following the same loading regime (6 days, 8 hours/day, magnitude 6%, 0.25 Hz). In vivo studies have also systematically modulated Achilles tendon training regime by varying strain magnitude,3 6 8 duration,6 rate6 and frequency.3 6 Training at a frequency of 0.17 Hz and strain magnitude of 6.5% resulted in superior material properties (ie, increased stiffness) when compared with training with lower strain magnitude (~3%),3 higher frequency (0.5 Hz),3 higher rates (ie, one-legged jump)6 or longer durations (12 s).6 Conversely, tendon disuse, observed in elderly and spinal cord injury populations, has been associated with loss of stiffness.4