Nature of the coupling between neural drive and force-generating capacity in the human quadriceps muscle

Abstract

The force produced by a muscle depends on both the neural drive it receives and several biomechanical factors. When multiple muscles act on a single joint, the nature of the relationship between the neural drive and forcegenerating capacity of the synergistic muscles is largely unknown. This study aimed to determine the relationship between the ratio of neural drive and the ratio of muscle force-generating capacity between two synergist muscles (vastus lateralis (VL) and vastus medialis (VM)) in humans. Twenty-one participants performed isometric knee extensions at 20 and 50% of maximal voluntary contractions (MVC). Myoelectric activity (surface electromyography (EMG)) provided an index of neural drive. Physiological cross-sectional area (PCSA) was estimated from measurements of muscle volume (magnetic resonance imaging) and muscle fascicle length (threedimensional ultrasound imaging) to represent the muscles' force-generating capacities. Neither PCSA nor neural drive was balanced between VL and VM. There was a large (r ¼ 0.68) and moderate (r ¼ 0.43) correlation between the ratio of VL/VM EMG amplitude and the ratio of VL/VM PCSA at 20 and 50% of MVC, respectively. This study provides evidence that neural drive is biased by muscle force-generating capacity, the greater the force-generating capacity of VL compared with VM, the stronger bias of drive to the VL.

Ultrasound-derived Biceps Femoris Long Head Fascicle Length: Extrapolation Pitfalls

“…The main issues regarding BFlh Lf assessment at rest are that the BFlh has: i) a non-homogeneous fascicle arrangement along its muscle belly and ii) its fascicles are non-linear and may present a "S" shape (Fig 1), which has also been described by Pimenta and colleagues (7). A very similar fascicle arrangement has also been observed for the vastus medialis using freehand three-dimensional ultrasound imaging (16) and perhaps unsurprisingly (1,3,7,10,(22)(23)(24)(25)(26)37) and that we also found this method resulted in the longest estimated Lf on average. Moreover, following Nordic hamstring exercise interventions, mean Lf estimates can be found up to 13.4 cm (1), which is 3.66 cm greater than the largest reported cadaveric value (34).…”

Surface electromyographic amplitude does not identify differences in neural drive to synergistic muscles

“…Since a vast number of studies apply normalization of the surface EMG before comparing levels of muscle activations (4,17), we analyzed the effect of normalization of both EMG amplitude and MUAP size with respect to MVC. Even though normalization decreased the VM/VL activation ratio and cancelled out the differences in MUAP size between muscles, normalized EMG amplitude was greater for VL compared with VM, contrary to the result without normalization.…”

“…Changes in the relative activations of synergistic muscles are believed to be associated with the development of musculoskeletal disorders (19). For example, researchers argue that pathologies such as patellofemoral joint pain and Achilles tendinopathy might occur due to misbalanced activation of the vasti and calf muscles, respectively (17,19). For patellofemoral joint pain, it is assumed that a greater activation of the vastus lateralis (VL) compared with the vastus medialis (VM) muscle induces a lateral shift of the patella, leading to misalignment of the patellofemoral joint (17,19).…”

“…For example, researchers argue that pathologies such as patellofemoral joint pain and Achilles tendinopathy might occur due to misbalanced activation of the vasti and calf muscles, respectively (17,19). For patellofemoral joint pain, it is assumed that a greater activation of the vastus lateralis (VL) compared with the vastus medialis (VM) muscle induces a lateral shift of the patella, leading to misalignment of the patellofemoral joint (17,19). Although these explanations seem plausible, there is still no consensus in the literature (7,31), mainly because of limitations of surface EMG amplitude in measuring the neural drive (6).…”

The Effects of Different Vibration Frequencies, Amplitudes and Contraction Levels on Lower Limb Muscles during Graded Isometric Contractions Superimposed on Whole Body Vibration Stimulation

“…The VM has been shown also to display a nonlinear EMG/force relationship during isometric leg press exercise (Alkner et al 2000). Further, recent investigation which looked at the ratio of VL/VM contraction during knee extension, concluded that the neural drive may be biased towards the VL compared to the VM and seems to be dependent on the force level (Hug et al 2015). The authors suggested that the higher the force generation capacity of an individual 's VL, the higher the bias of neural drive towards VL over the VM.…”