Task-Dependent Intermuscular Motor Unit Synchronization between Medial and Lateral Vastii Muscles during Dynamic and Isometric Squats

Abstract

PurposeMotor unit activity is coordinated between many synergistic muscle pairs but the functional role of this coordination for the motor output is unclear. The purpose of this study was to investigate the short-term modality of coordinated motor unit activity–the synchronized discharge of individual motor units across muscles within time intervals of 5ms–for the Vastus Medialis (VM) and Lateralis (VL). Furthermore, we studied the task-dependency of intermuscular motor unit synchronization between VM and VL during static and dynamic squatting tasks to provide insight into its functional role.MethodsSixteen healthy male and female participants completed four tasks: Bipedal squats, single-leg squats, an isometric squat, and single-leg balance. Monopolar surface electromyography (EMG) was used to record motor unit activity of VM and VL. For each task, intermuscular motor unit synchronization was determined using a coherence analysis between the raw EMG signals of VM and VL and compared to a reference coherence calculated from two desynchronized EMG signals. The time shift between VM and VL EMG signals was estimated according to the slope of the coherence phase angle spectrum.ResultsFor all tasks, except for singe-leg balance, coherence between 15–80Hz significantly exceeded the reference. The corresponding time shift between VM and VL was estimated as 4ms. Coherence between 30–60Hz was highest for the bipedal squat, followed by the single-leg squat and the isometric squat.ConclusionThere is substantial short-term motor unit synchronization between VM and VL. Intermuscular motor unit synchronization is enhanced for contractions during dynamic activities, possibly to facilitate a more accurate control of the joint torque, and reduced during single-leg tasks that require balance control and thus, a more independent muscle function. It is proposed that the central nervous system scales the degree of intermuscular motor unit synchronization according to the requirements of the movement task at hand.

A wavelet based time frequency analysis of electromyograms to group steps of runners into clusters that contain similar muscle activation patterns

“…The difference is on average 0.019 ± 0.002 and thus supports the hypothesis that there is synchronization in the sense that the events occur at the same time and frequency in the two vastii muscles at the level of their EMG intensity. This was expected because of the high coherence observed previously for the vastii muscles during squatting [ 16 ]. The result is significant although much lower than one would expect when considering how highly coherent the individual MUs were between the vastii muscles.…”

“…Hypothesis 1) might be a consequence of the clustering of individual MUs observed in our previous work [ 16 ] and that substantial force can only be produced if MUs action potentials cluster because of a common synaptic input to motor neurons [ 17 ]. Clustering means that the motor unit action potentials (MUAP) arrive at almost the same time irrespective of the reason for it.…”

Coordination amongst quadriceps muscles suggests neural regulation of internal joint stresses, not simplification of task performance

“…Although previous studies have demonstrated a strong correlation between the activity of VM and VL [10,[29][30][31], the functional role of this correlation has not been clear. Since VM and VL have the same contribution to task variables and produce opposing mediolateral patellar forces [11], their strong correlation is consistent with both simplification of task performance [16,19] and regulation of internal joint stresses [29,32,33]. Similarly, the observation that RF could be controlled more independently than the vasti [31,34] is also consistent with both interpretations, as RF has a different task action from VM and VL [35,36] and produces minimal mediolateral patellar force (Fig.…”

“…For example, the quadriceps muscles vastus lateralis (VL) and vastus medialis (VM) have very similar contributions to task performance, with each muscle producing a similar extension torque at the knee [9][10][11]. Consistent with the proposal that co-variation patterns reflect muscles' contribution to task performance [7,[12][13][14][15], the activations of VM and VL are strongly correlated [15][16][17][18], suggesting that the CNS might control VM and VL as a single functional unit to simplify the achievement of task goals [19].…”

High-Frequency Intermuscular Coherence between Arm Muscles during Robot-Mediated Motor Adaptation

“…Moreover, stronger high-frequency synchronization at motor unit level during dynamic in comparison to isometric contractions has been recently shown and linked to a neural optimization strategy for the musculoskeletal system during complex tasks (Mohr et al, 2015). Although we did not measure single motor unit activation in this study, we believe that the reported increase of high–frequency IMC in later adaptation in our work is a plausible mechanism underpinning the gradation of optimal intermuscular coupling for a reduced or maintained level of co-contraction and for the previously observed gradual reduction of metabolic cost during the motor adaptation process (Huang and Ahmed, 2014).…”