Author and graduation date: Keith Edward GORDON, May, 1999

Committee members: Peter R. Francis (Chair), Robert Pozos, Denise Wiksten

Thesis title: Motor Control of the Lower Back and Legs During a Two-footed Landing

The effects of free fall height on muscle onset times and EMG amplitudes during a two-footed landing were investigated in 10 male collegiate volleyball players. Subjects performed two-footed landings by stepping off platforms of heights of 30.5 cm, 61 cm, and 91.5 cm. During free fall and landing EMG signals were recorded by surface electrodes from the medial gastrocnemius, soleus, tibialis anterior, biceps femoris, rectus femoris, and erector spinae. Muscle onset times during free fall and EMG amplitude during both free fall and impact were calculated. Muscle activity began immediately at the initiation of take-off and continued throughout free fall and impact. Height of free fall did not have a significant effect (p >0.05) on the time until muscle activation began following the initiation of take-off in any of the muscles examined. All muscles showed a trend for EMG amplitude during free fall to increase with increases in free fall height. During impact no differences were found in EMG amplitude with changes in landing height. From heights of 61 and 91.5 cm EMG amplitude was found to be greater during free fall than impact. These findings indicate that muscle activity was timed to begin relative to take-off time for this population using this technique for the initiation of free fall. This observation is inconsistent with previous findings involving motor control patterns during landing. Previous investigators have suggested that EMG activity during landing is timed to begin relative to the time of ground contact. Changes in landing strategy such as increasing impact times and increasing joint flexion during impact allowed subjects to land successfully from different heights without significant differences in EMG amplitude during impact. Future studies should be performed to determine the effects of population, free fall initiation protocol, and practice on the motor control patterns of landing.

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