Muscle activity in tight hip and loose hip subjects during two different hip extension tasks
Purpose. The purpose of the study was to investigate: 1) whether motor programming as a role in perpetuating muscle imbalance and 2) if subjects with tight hip f lexors demonstrate hyperactivity of the erector spinae musculature. Subjects. From 254 potential subjects, 16 subjects who met the criteria for "loose hip" and 16 for "tight hip" flexors were selected. Subjects were selected who varied 1.5 sd from the mean on measures standardized for this study using an inclinometer. Methods. Subjects performed two movements: 1) voluntary lift; and 2) isometric hold, while in a hip extended or hip flexed prone position. Surface EMG was taken at the right and left erector spinae, right gluteus maximus and right hamstring. Dependent variables taken from the EMG were: 1) duration of muscle onset sequence (DMOS) - the time between the onsets of the first and fourth muscles; 2) latency of the erector spinae muscles (LES latency and RES latency) -the time between muscle onset and movement onset, and 3) early amplitude of the erector spinae (EAC) - average percent MVC during 0.4 sec after muscle onset minus average percent MVC during 0.1 sec prior to muscle onset. Data Analysis. The research design was a 2X2X2 repeated measures. Therefore, a three-way multi-variate analysis (MANOVA) was used . Results. A three-way interaction was found for DMOS. The difference was between the loose and tight hip subjects for the lifting movement in the flexed hip position (p < 0.0004). This suggests that the muscle activation of the back and hip extensors is different in these two populations when hip tension is altered, but only for the lift movement. A main effect of movement (p < 0.01) was found for LES latency. There was no main effect for hip type. For EAC there was a significant movement by position 2-way interaction (p < 0.016), and no main effect for hip type. Conclusion. It does not appear that motor programming has a role in perpetuating this particular muscle imbalance and that subjects with tight hip flexors do not demonstrate hyperactivity of the erector spinae musculature. Clinical Relevance. This research seems to support Kendall's suggestion that muscle behaviors respond more to immediate external mechanical changes rather than constant internal mechanical differences.