The prone leg extension test is commonly used in the evaluation of lumbopelvic function. It has been theorized that the activation of muscles during a prone leg extension (PLE) simulates the muscle recruitment pattern of hip extension during gait. The theory suggests that the temporal activation of the posterior muscle groups should occur in the following order (right PLE exercise): right gluteus maximus, right hamstring, left lumbar erector spinae, right lumbar erector spinae, left thoracolumbar erector spinae and lastly right thoracolumbar erector spinae [1
According to this theory, differences in temporal recruitment patterns decrease the stability of the pelvis during gait and thus hinder the body's mechanical efficiency. It is the clinical belief that a delay in the gluteus maximus recruitment is a dysfunctional pattern of muscular recruitment. The theory concludes that the poorest recruitment pattern during the PLE occurs when the ipsilateral erector spinae and even the shoulder girdle muscles initiate the movement. The gluteus muscle activation is delayed and hip extension is achieved by hamstring muscle activation, forward pelvic tilt and hyperlordosis of the lumbar spine. Sahrman [2
] has suggested that if the hamstrings are dominant because of inhibition of the gluteus maximus an anterior shear of the trochanter can be palpated during the prone leg extension. Poor gluteus maximus strength and activation is postulated to decrease the efficiency of gait [3
Bullock-Saxton et al [5
] and Vogt and Banzer [6
] suggests that there is a consistent pattern of muscle activation during prone leg extension (although they disagree on the order of firing) and imply that there is a fixed motor program in normal subjects. Bullock-Saxton et al concluded that in pain free subjects muscle onset times were "almost simultaneous". A muscle time span (in seconds) between the activation of the first muscle to the muscle to be last activated was calculated. For the control group the average time span was .306 seconds. They stated that the Hamstrings typically were the first recruited. Furthermore, Bullock-Saxton et al found a lack of consistency and a higher degree of variability within subjects who had sustained previous ankle injury [5
Vogt and Banzer [6
] evaluated the muscle onset times during the PLE in pain free subjects finding statistically different activation onset times for the muscles studied. The authors found that the muscles studied fired in the following order: ipsilateral lumbar erector spinae, semitendinosis, contralateral lumbar erector spinae, tensor facia latae and gluteus maximus. However, when the means and standard deviations (expressed as a percentage of the movement cycle) are compared for the onsets of the ipsilateral erector spinae (mean = 13.91, SD = 10.97), contralateral erector spinae (mean = 17.27, SD = 12.86) and hamstrings (mean = 17.61, SD = 13.04) there is a great deal of overlap and they occur very close in time (amount expressed in seconds not known). This proximity in time may be identical to what the Bullock-Saxton study found but described as "almost simultaneous". Again, with the large overlap in the Vogt and Banzer [6
] study, it is possible that some of the muscles came on in a different order even though statistically a significant difference between the muscle onsets was found.
The differences between the conclusions may have been due to collecting similar data, analyzing it slightly differently and subsequently finding a different conclusion. One similarity between the two studies is that the gluteus maximus is consistently the last muscle to become active. A functional and anatomical relationship has also been described between the gluteus maximus and the contralateral latissimus dorsi which is theorized to ensure stability of the SI joint during gait and movement [7
]. Due to this relationship the muscle activation of the contralateral latissimus dorsi was also studied during the prone leg extension in the current study.
The performance of the prone leg extension during a physical exam is based on the belief that a "normal "pattern of muscle activation occurs. If variability in muscle recruitment times occurs across patients then the use of this test in identifying dysfunction may be limited. Due to the different conclusions from two previous studies and the lack of quantified muscle onset times, expressed in absolute time (milliseconds) this study had two objectives.
1. Develop a database of muscle onset times (in milliseconds) for the posterior muscle groups during the prone leg extension, while noting if a consistent order of activation exists and whether a timing relationship also exists between the gluteus maximus and contralateral latissimus dorsi.
2. Express the muscle onset times in relation to the onset of hamstring muscle activity to alleviate the need for timing equipment to determine onset of movement.