Can measures of muscle–tendon interaction improve our understanding of the superiority of Kenyan endurance runners?

Leg muscle activation profiles and muscle–tendon interaction were studied with eleven internationally high-level Kenyan and eleven national level Japanese distance runners.

Ultrasonography and kinematics were applied together with surface electromyography (EMG) recordings of leg muscles when subjects ran on treadmill at 9.0 (SLOW) and 13.9 km h⁻¹ (MEDIUM).

At each speed, both groups presented similar contact and flight times. The kinematic and ultrasound analyses revealed that, in contrast to the Japanese runners, the Kenyans demonstrated during contact smaller stretching and shortening amplitudes (p < 0.01) of the tendinous tissue of medial gastrocnemius (MG), but greater tendon contribution to the muscle–tendon unit shortening (p < 0.05). The MG fascicles of the Kenyans were shorter not only at the resting standing position, but also during the contact phase at both running speeds (p < 0.01). The EMG profiles of the Kenyans showed lower braking/preactivation ratio in both MG and tibialis anterior (p < 0.05) muscles. They were also characterized by negative relationships between the Achilles tendon moment arm and the MG fascicle shortening during contact (r = −0.54, p < 0.01). In contrast, the Japanese presented the classical stretch–shortening cycle muscle activation profile of relatively high MG EMG activity during the braking phase.

These findings provide new suggestions that the Kenyans have unique structural characteristics which can result in the reduction of muscle and tendinous stretch–shortening loading together with smaller muscle activation during contact at submaximal running speed.