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Simultaneous Knee Extensor Muscle Action Induces an Increase in Voluntary Force Generation of Plantar Flexor Muscles

Suzuki, Takahito; Shioda, Kohei; Kinugasa, Ryuta; Fukashiro, Senshi

The Journal of Strength & Conditioning Research: February 2017 - Volume 31 - Issue 2 - p 365–371
doi: 10.1519/JSC.0000000000001513
Original Research

Suzuki, T, Shioda, K, Kinugasa, R, and Fukashiro, S. Simultaneous knee extensor muscle action induces an increase in voluntary force generation of plantar flexor muscles. J Strength Cond Res 31(2): 365–371, 2017—Maximum activation of the plantar flexor muscles is required for various sporting activities that involve simultaneous plantar flexion and knee extension. During a multi-joint movement, activation of the plantar flexor muscles is affected by the activity of the knee extensor muscles. We hypothesized that coactivation of the plantar flexor muscles and knee extensor muscles would result in a higher plantar flexion torque. To test this hypothesis, 8 male volunteers performed maximum voluntary isometric action of the plantar flexor muscles with and without isometric action of the knee extensor muscles. Surface electromyographic data were collected from 8 muscles of the right lower limb. Voluntary activation of the triceps surae muscles, evaluated using the interpolated twitch technique, significantly increased by 6.4 percentage points with intentional knee extensor action (p = 0.0491). This finding is in line with a significant increase in the average rectified value of the electromyographic activity of the vastus lateralis, fibularis longus, and soleus muscles (p = 0.013, 0.010, and 0.045, respectively). The resultant plantar flexion torque also significantly increased by 11.5% of the predetermined maximum (p = 0.031). These results suggest that higher plantar flexor activation coupled with knee extensor activation facilitates force generation during a multi-joint task.

1Department of Human Sciences, Kanagawa University, Kanagawa, Japan; and

2Department of Life Sciences, Graduate School of Arts and Sciences, The University of Tokyo, Tokyo, Japan

Address correspondence to Dr. Takahito Suzuki,

Copyright © 2017 by the National Strength & Conditioning Association.