Effect of knee position on quadriceps muscle force steadiness and activation strategies

Effects of practice on variability of muscle force

The motor skill required to decrease the variability in muscle force steadiness can be challenging. The purposes of this study were to determine whether muscle force steadiness improved following repeated trials and whether the number of trials varied for healthy younger adults, healthy older adults, and older adults who have fallen to obtain stable muscle force steadiness measures. Sixty participants performed 30 concentric and eccentric contractions of the knee extensors on an isokinetic dynamometer. Each group had significant improvements in muscle force steadiness and obtained stable measures within six to nine trials. Healthy younger and older adults, and older adults who have fallen, can improve muscle force steadiness. These findings provide a framework for methodological approaches when testing steadiness in varying populations.

Diminished sub-maximal quadriceps force control in anterior cruciate ligament reconstructed patients is related to quadriceps and hamstring muscle dyskinesia

The aim of this study was to determine the effects of anterior cruciate ligament reconstruction (ACLR) on sub-maximal quadriceps force control with respect to quadriceps and hamstring muscle activity. Thirty ACLR individuals together with 30 healthy individuals participated. With real-time visual feedback of muscle force output and electromyographic electrodes attached to the quadriceps and hamstring muscles, subjects performed an isometric knee extension task where they increased and decreased their muscle force output at 0.128Hz within a range of 5-30% maximum voluntary capacity. The ACLR group completed the task with more error and increased medial hamstring and vastus medialis activation (p<0.05). Moderate negative correlations (p<0.05) were observed between quadriceps force control and medial (Spearman's rho=-0.448, p=0.022) and lateral (Spearman's rho=-0.401, p=0.034) hamstring activation in the ACLR group. Diminished quadriceps sub-maximal force control in ACLR subjects was reflective of medial quadriceps and hamstring dyskinesia (i.e., altered muscle activity patterns and coordination deficits). Within the ACLR group however, augmented hamstring co-activation was associated with better quadriceps force control. Future studies should explore the convergent validity of quadriceps force control in ACLR patients.

Anodal transcranial direct current stimulation alters elbow flexor muscle recruitment strategies

BACKGROUND

Transcranial direct current stimulation (tDCS) is known to reliably alter motor cortical excitability in a polarity dependent fashion such that anodal stimulation increases cortical excitability and cathodal stimulation inhibits cortical excitability. However, the effect of tDCS on agonist and antagonist volitional muscle activation is currently not known.

OBJECTIVE

This study investigated the effect of motor cortical anodal tDCS on EMG/force relationships of biceps brachii (agonist) and triceps brachii (antagonist) using surface electromyography (EMG).

METHODS

Eighteen neurologically intact adults (9 tDCS and 9 controls) participated in this study. EMG/force relationships were established by having subjects perform submaximal isometric contractions at several force levels (12.5%, 25%, 37.5%, and 50% of maximum).

RESULTS

Results showed that anodal tDCS significantly affected the EMG/force relationship of the biceps brachii muscle. Specifically, anodal tDCS increased the magnitude of biceps brachii activation at 37.5% and 50% of maximum. Anodal tDCS also resulted in an increase in the peak force and EMG values during maximal contractions as compared to the control condition. EMG analyses of other elbow muscles indicated that the increase in biceps brachii activation after anodal tDCS was not related to alterations in synergistic or antagonistic muscle activity.

CONCLUSIONS

Our results indicate that anodal tDCS significantly affects the voluntary EMG/force relationship of the agonist muscles without altering the coactivation of the antagonistic muscles. The most likely explanation for the observed greater EMG per unit force after anodal tDCS appears to be related to alterations in motor unit recruitment strategies.