Significance of vasti muscle activity during voluntary isometric contraction: Pre-activation of vastus medialis for engaging vastus intermedius

BACKGROUND: A potential relationship may exist between the vastus medialis (VM) and the contractile efficiency of the vastus intermedius (VI) due to their anatomical connection and common innervation by the same branches of the femoral nerve. However, this relationship has not been explored using physiological experiments. OBJECTIVE: This study aimed to use electromyography (EMG) to investigate the functional significance of the VI and VM muscles during isometric contraction. METHODS: Twenty-six healthy male volunteers underwent EMG recordings from the VI, VM, and vastus lateralis (VL) at 90∘ or 30∘ of knee flexion. The EMG pre-motor time (PMT) was measured during rapid knee extension in response to an auditory stimulus. Additionally, the M-wave latency was determined by femoral nerve stimulation and subtracted from the PMT to calculate the descending conduction time (DCT). RESULTS: DCT was shortest at VM, followed by VL and VI. The DCT difference of VI-VM was significantly greater than that of VI-VL during maximal effort at 90∘ of knee flexion. CONCLUSIONS: VM is neurally driven to act as a pre-activator that primes the VI muscle; this tendency is particularly pronounced when explosive high-moment knee extension is required.