Triceps surae short latency stretch reflexes contribute to ankle stiffness regulation during human running

Abstract

During human running, short latency stretch reflexes (SLRs) are elicited in the triceps surae muscles, but the function of these responses is still a matter of controversy. As the SLR is primarily mediated by Ia afferent nerve fibres, various methods have been used to examine SLR function by selectively blocking the Ia pathway in seated, standing and walking paradigms, but stretch reflex function has not been examined in detail during running. The purpose of this study was to examine triceps surae SLR function at different running speeds using Achilles tendon vibration to modify SLR size. Ten healthy participants ran on an instrumented treadmill at speeds between 7 and 15 km/h under 2 Achilles tendon vibration conditions: no vibration and 90 Hz vibration. Surface EMG from the triceps surae and tibialis anterior muscles, and 3D lower limb kinematics and ground reaction forces were simultaneously collected. In response to vibration, the SLR was depressed in the triceps surae muscles at all speeds. This coincided with short-lasting yielding at the ankle joint at speeds between 7 and 12 km/h, suggesting that the SLR contributes to muscle stiffness regulation by minimising ankle yielding during the early contact phase of running. Furthermore, at the fastest speed of 15 km/h, the SLR was still depressed by vibration in all muscles but yielding was no longer evident. This finding suggests that the SLR has greater functional importance at slow to intermediate running speeds than at faster speeds.

Figure 1. Data from a representative participant running at 12 km/h.

Left: GRF, kinematic and EMG data (data averaged from 35–37 steps per condition). Right: EMG traces for all muscles and ankle joint angle shown on an enhanced timescale. For the sake of clarity, ankle angle is also shown on an enhanced scale on the y-axis.

Figure 2. Mean SLR data.

A: Mean SLR amplitude at all speeds in the soleus and gastrocnemius muscles. B: Mean relative changes in SLR amplitude due to Achilles tendon vibration. C: Amplitude and velocity of ankle yielding at all speeds computed as the difference between the no vibration and 90 Hz vibration conditions 55–150 ms after ground contact. Positive values denote yielding in the 90 Hz vibration condition. For all plots, * denotes a significant difference from the immediately preceding running speed, and ^# denotes a significant difference between the no vibration and 90 Hz vibration conditions at a minimum level of p<0.05. Vertical bars represent 1 SD of the mean. n = 10 for all plots.