Influence of time of day on tendon compliance and estimations of voluntary activation levels.

We investigated the importance of tendon stiffness on estimations of central activation capacity (CAC). Patella tendon-force elongation relationship, quadriceps torque, and response to 50-micros, 100-HZ electrical twitch doublets were studied at approximately 8:00 A.M. and approximately 6:00 P.M. in 8 healthy young men. Results show that in the evening, as compared with the morning, tendon stiffness decreased by 20.2 +/- 9.5% (P = 0.03) and time to twitch peak (Ttp) increased by 55% (P = 0.04). Tendon stiffness and Ttp correlated significantly (r = 0.28, P < 0.05). Whereas the CAC, calculated as the ratio of force preceding the interpolated twitch (IT) to the total force at the peak of the IT, showed no change with time of day, calculations that used twitch ratios suggested CAC decrements (-10.3%, P = 0.03, unpotentiated; -10%, P = 0.02, potentiated ratio) in the evening. This study shows that CAC estimations are invalid when tendon properties are not taken into account, an error that would have important implications in a clinical setting.