Estimation of persistent inward currents in the human ankle flexor and extensor muscles


Movement is achieved by combining synaptic inputs from various sources and activating motor unit populations. Motor units are the quantal elements of motor control which act as a neuromechanical transducer that converts sensory inputs into motor output. Because of the tight neuromuscular junctions between motoneuron axon terminals and a large number of muscle fibers each motoneuron innervates, synaptic inputs to the motoneuron pool can be routinely observed as motor unit firing patters via electromyography (EMG) signals. Persistent inward currents (PICs) are modulated by monoaminergic inputs from the brainstem and increase the motoneuron excitability. Animal models have shown that PICs play an important role in normal movement such as balance and postural control. However, the role of PICs in human motor control and how they affect motor unit firing patterns are not well understood. Through recent advances in EMG recording technics, researchers can now simultaneously record many motor units in humans using non-invasive high-density surface EMG arrays. Combined with a well-established delta-F (F) technique, this dissertation investigated PICs in human soleus and tibialis anterior muscles (TA). The results showed F was significantly higher in the TA than the soleus and the trend continued across various effort levels. Surprisingly, changing effort levels did not affect F’s in either of the muscles. The effects of muscle contraction speed were also studied, and the results showed that it also did not change F. Lastly, the biases of the EMG decomposition algorithm were discussed, and the results showed for lower contraction levels and speeds, surface EMG decomposition discriminated a large number of low-threshold units. However, during at higher contraction speeds and torque levels the proportion of low threshold motor units decomposed was reduced, resulting in a relatively uniform distribution of recruitment thresholds. These studies provided valuable insights on PICs in humans but also revealed how little we understand human motor control. Continuing efforts are needed to further understand the functional role of PICs in human movement.

Alternate Identifier
Date created
Resource type
Rights statement