Muscle length is a major determinant of eccentric exercise damage.

Rat vastus intermedius experiments

• VI is postural knee extensor, damaged during downhill treadmill running.
• Muscle remained attached to bones to avoid slippage of attachments. Length is replaced by knee angle, tension by torque.
• Animals were trained with incline or decline training
  • 30mins/day for 5 days.
• Stretches began either from 90° knee angle or from optimum length for each muscle.
  • 20 contractions including a stretch of 27° in 33ms.

• Damage correlated strongly with the difference between optimum angle and the angle where the stretches began, independent of training.

•  When plotted against absolute knee angle, dependence on angle remained, but training was also a significant factor.

• This supports the idea that eccentric training works by increasing the number of sarcomeres connected in series in fibres, so preventing activity on the descending limb of the length-tension curve.