Changes in Muscle Hardness and Electromyographic Response for Quadriceps Muscle during Repetitive Maximal Isokinetic Knee Extension Exercise

Gang Sun, Shumpei Miyakawa, Hiroaki Kinoshita, Hitoshi Shiraki,
Naoki Mukai, Masahiro Takemura and Hajime Kato

[Received September 24, 2008 ; Accepted February 12, 2009] 

There are many cases of Osgood-Schlatter disease (OSD) in soccer players. It's disturbs their activity in playing soccer. It is necessary to protect them from OSD. It's known that quadriceps muscle tightness and hardness are increasing before OSD would occur. Because muscle fatigue makes it's muscle harden. Measuring muscle hardness, we can detect the symptom of OSD. The purpose of this study was to investigate changes in muscle hardness and mean frequency (MNF) of surface electromyography (EMG) during 100 repetitions of maximal isokinetic knee extension. Nineteen healthy subjects performed 100 isokinetic knee extensions at 90°/s using a calibrated isokinetic device. Peak Torque (PT) was determined for each extension (90°-0°), and MNF were recorded from the vastus lateralis (VL), rectus femoris (RF) and vastus medialis (VM) of the right thigh at the same time. Quadriceps muscle hardness of the anterior thigh was measured with knee extended both before- and after-exercise. MNF of each muscle and PT decreased with increasing repetitions. MNF showed higher correlation coefficients with PT for RF than for VL or VM (r=0.59, p<0.01). Muscle hardness for each muscle was increased after exercise (VM, 49.6±3.9; RF, 54.9±3.6; VL, 62.6±5.4 ) compared with before exercise (VM, 47.1±4; RF, 47.1±4.2; VL, 56.7±5.4) (p<0.01 each). Reductions in MNF and PT showed the characteristics of muscle fatigue during repetitive maximal isokinetic knee extension exercise, and a correlation between these values was recognized in RF. Muscle hardness showed consistent increases after exercise in all muscles and can thus be used as a marker of muscle fatigue in the RF.

Keywords: muscle hardness, mean frequency (MNF), muscle fatigue, Osgood-Schlatter Disease (OSD)

[Football Science Vol.6, 17-23, 2009]