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Fig. 2. Isometric forces induced in the middle leg extensor tibiae muscle by electrical stimulation of nerve nl3 with different current amplitudes. In all panels the top trace is a stimulus monitor (note pulse height changes as stimulus amplitude was increased), the second trace is an extracellular recording of nerve nl3, and the third trace is muscle force. (Ai–Aiii) Sequential recruitment of FETi (Ai), FETi and SETi (Aii) and FETi, SETi and CI₁ (Aiii) recorded in extensor leg nerve F2 in response to single stimuli. (Aiv) An enlarged version of the recordings, showing the sequential addition of new units (asterisks). 1 T=0.0023 mA. (Bi–Biv) F2-recordings and forces in response to a 50 Hz pulse train. (Bi) 75% of the pulses excited FETi and 25% FETi and SETi. (Bii) 50% of the stimuli elicited FETi and 50% FETi, SETi and CI₁. (Biii) Recruitment of all three motor units with every pulse. Doubling the current amplitude (Biv) induced no further increase in force. In this experiment the SETi spikes were of larger amplitude than FETi spikes. This is uncommon and likely because nerve F2 was recorded very distally in the femur. In all panels the electrical disturbance in the nerve recording that coincides with the stimulus is a stimulus artifact, not an action potential (arrow in Ai). T, threshold.

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