Figure 1 Triple stimulation technique (TST) principle for recordings from lower limbs. The motor tract is simplified to three spinal motor neurones; horizontal lines represent three motor units of the abductor hallucis muscle. Black arrowheads depict action potentials that cause a trace deflection, white arrowheads those that do not. The trace recording is given below at each time point. (A) TST_test. (A1) A submaximal transcranial stimulus excites two spinal motor neurones of three (white arrowheads). (A2) On two of three neurones, TMS‐induced action potentials descend. Desynchronisation of the two action potentials has occurred (possibly at spinal cell level). After a delay, a maximal stimulus is applied to the tibial nerve at the ankle. This gives rise to a first negative deflection of the recording trace. The antidromic action potentials collide with the descending action potentials on motor neurones 1 and 2. The action potential on neurone 3 continues to ascend. (A3) After a second delay a maximal stimulus is applied to the sciatic nerve at the gluteal fold. On motor neurone 3, the descending action potential collides with the ascending action potential. On neurones 1 and 2, no collision occurs, and action potentials continue to descend on both neurones. During their descent, only a minor degree of desynchronisation occurs, as is typical for peripheral nerves. (A4) Action potentials on motor neurones 1 and 2 evoke a well synchronised muscle response, giving rise to the second negative deflection in the recording trace. Note that motor neurones 1 and 2 were those initially excited by TMS. (B) TST_control. (B1) A maximal stimulus is applied to the sciatic nerve at the gluteal fold. (B2) After a delay, a maximal stimulus applied to the tibial nerve at the ankle is recorded as the first deflection of the TST control trace. (B3) After a delay, a maximal stimulus is applied to the sciatic nerve, evoking action potentials on all neurones. During their descent, a minor degree of peripheral desynchronisation occurs, matching (and calibrating) the desynchronisation that occurred during the TST test procedure. (B4) A well synchronised response from the three motor neurones is recorded as the second deflection of the TST control trace. The test response is quantified as the ratio of TST_test to TST_control curves.