Author(s): DE STE CROIX, M., JAMES, D.V., IGA, J., EL NAGAR, Y., Institution: UNIVERSITY OF GLOUCESTERSHIRE, Country: UNITED KINGDOM, Abstract-ID: 431

INTRODUCTION:
Neuromuscular capability is likely compromised when fatigue is present, which likely predisposes the athlete to increased injury risk (1). To date no studies appear to have examined the effect of downhill running on sex differences in the electromechanical delay (EMD) of the hamstrings. This is somewhat surprising given the importance of the hamstrings to work eccentrically to reduce injury risk and keep the joint stable.
METHODS:
One hundred healthy males (n=50) and females (n=50) were recruited from the university population. Each participant performed a 40 min intermittent downhill running protocol consisting of 5 × 8 min bouts on a -10% decline at 80% or heart rate max, with 2 min standing rest between each bout (2). Immediately before and after the downhill running task, Electromyography (EMG) of the semitendinosus, semimembranosus and biceps femoris of the dominant limb (DelSys Myomonitor III) was determined during isokinetic eccentric muscle actions at 60, 120 and 240˚·s–1 (Biodex system 3). The Biodex square wave synchronization pulse was configured with the EMG software so that EMG and torque data were time aligned. Raw EMG data were collected at a sampling frequency of 1024 Hz and included a common mode rejection ratio of <80 dB and an amplifier gain of 1000. Raw EMG data was band pass filtered at 20 – 450 Hz. EMD was determined as the time delay between the onset of muscle activation (change in activation of +15 µV) and onset of torque production (9.6 Nm) (3). A mixed-factorial analysis of variance (ANOVA) was used to explore the effects of time, sex, and muscle on electromechanical delay. Effect sizes were determined using Cohens d with <0.2 considered a small effect, 0.5 a medium effect and >0.8 large effects.
RESULTS:
Irrespective of sex, muscle or velocity, EMD of the hamstrings muscle was significantly longer post-downhill running (F(1, 98) = 709.406, P < 0.01), with large effects observed (d = 1.05-2.80). Significant interactions between sex and time for EMD of hamstring muscles were found (F(3, 98) = 28.738, P < 0.01) with significantly longer EMD post fatigue in females. These differences demonstrated large effects (d = 0.72-1.46). No significant effects of muscle were observed in any analysis.
CONCLUSION:
These data suggest that neuromuscular capability of the hamstrings during eccentric actions is compromised post downhill running irrespective of sex or muscle group examined. This compromise is greater in women and may make them more susceptible to injury risk due to reduced stability of the knee. Effective neuromuscular functioning when fatigue is present is important for mainlining performance and reducing injury risk. Thus, eccentric hamstring training should be proposed to limit the effects of neuromuscular fatigue, especially in female athletes.