Gender, age and midsole hardness effects on lower extremity muscle activity during running

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Abstract
Introduction. The biomechanical knowledge for developing running shoe cushioning guidelines is scarce. This study tested whether changes in midsole hardness, gender and age affect the lower-extremity pre- and post-heel strike muscle activity during heel-toe running. Methods. Surface electromyography (EMG) of the gastrocnemius medialis (GM), biceps femoris (BF) and vastus medialis (VM) was analysed for 54 male and female runners aged 10–75. Participants ran at 12 ± 0.6 km/h on a 30-m long runway with Soft (Asker C-40), Medium (Asker C-52) and Hard (Asker C-65) midsoles. EMG signals were resolved into time-frequency space using wavelet analysis. EMG intensities were extracted from the wavelet transformed signals to quantify frequency and time domain differences. The total energies for the pre- and post-heel strike EMG signals were calculated. Results. Similar muscle patterns and no frequency or time differences were found between the three midsole conditions. Compared to women, male runners had similar or decreased relative EMG intensity at lower frequencies, but greater intensities at higher frequencies for all three muscles. Male runners exhibited less pre- and more post-heel strike BF activity. Older runners had less GM and BF intensities at low frequencies, but greater intensities for higher frequencies. The differences were smaller for the VM and the trend was reversed. The VM pre-heel strike contribution increased with age. Kids showed less pre- versus post-heel strike BF activation while all other groups showed the opposite. Conclusions. The tested midsole hardness changes did not affect lower extremity muscle activity for the general population of runners, but greater hardness variations and/or other construction manipulations might. Should that be the case, those design modifications should take into account functional differences with gender and age.

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