Stretch-shortening cycle during plantar flexion in young and elderly women and men.

The stretch-shortening cycle (SSC) is a combination of eccentric and concentric muscle actions. The purpose of the study was to compare the SSC of four different groups comprising a total of 29 women and 30 men, divided according to sex and age (i.e. 20-40 years and 70-85 years). A KIN-COM dynamometer was used for strength measurements of the plantar flexion of the right foot. An electromyogram (EMG) from the gastrocnemius muscle was recorded simultaneously. Maximal voluntary concentric muscle actions at 120 degrees.s-1 and 240 degrees.s-1 with and without prior eccentric muscle actions were performed. Average torque values of the range of motion between 90 degrees and 99 degrees of the ankle joint were extracted. All four groups were significantly stronger at 120 degrees.s-1 than at 240 degrees.s-1 for pure concentric actions. The average torque values of the concentric phases in the SSC movement were significantly higher than the torque values for pure concentric actions in all four groups and at both velocities. The EMG was significantly lower or unchanged in the SSC movement compared to a pure concentric action in all groups. A larger percentage increase in torque with prior eccentric action was found in young women compared to young men at both velocities. Our results suggested that the enhanced performance was even more marked when a concentric action was preceded by an eccentric action in the young women than in the young men, probably due to better utilization of elastic forces, but we could not demonstrate any age-related differences in enhanced performance with SSC.