Twenty-six volunteers were recruited for participation in this study. Fourteen females (age, 22 ± 2 years, height, 1.68 m ± .05 m, weight, 61.95 kg ± 10 kg) and 12 males (age, 25 ± 3 years, height, 1.79 m ± .05 m, weight, 78.11 kg ± 8.8 kg) all signed an informed consent that was approved by the Institutional Review Board. Written informed consent was obtained from the patient for publication of this research and accompanying images. A copy of the written consent is available for review by the Editor-in-Chief of this journal. Volunteers were free from injury to the lower extremity at least three months prior to the study, did not exceed normal limits of hamstring and quadriceps flexibility, and were physically active, partaking in regular physical activity three times per week for at least 20 minutes a session. Subjects were randomly assigned to an intervention group (n = 13) or a control group (n = 13). One subject dropped out of the study due to an injury of the ankle two days prior to final testing. Two other subjects were dropped due to instrumentation errors.
The examiner lightly abraded and cleaned areas on the dominant leg for EMG electrode placement on the vastus medialis, vastus lateralis, biceps femoris, and medial hamstrings. Electrodes were placed 2 cm center to center and in line with the longitudinal axis of the muscle. A ground electrode was placed on the head of the fibula. Electrode placement was visually verified inspecting the signal during an isometric contraction. Electrodes were outlined with permanent marker for replacement during subsequent data collection sessions. EMG data were recorded using a Biopac MP150 system (BIOPAC Systems Inc., Santa Barbara, CA). Signals were amplified (TEL100M, BIOPAC Systems Inc., Santa Barbara, CA) from disposable, pre-gelled Ag-AgCl electrodes. The input impedance of the amplifier was 1.0 megaohm, with a common mode rejection ratio of 110 dB, high and low pass filters of 10 and 500 Hz, a signal to noise ratio of 70 dB, and a gain of 1000. EMG data were collected at 2000 Hz using Acknowledge 3.73 software package (BIOPAC Systems Inc., Santa Barbara, CA). EMG data were processed using a root mean square (RMS) algorithm with a 10 ms moving window.
With each testing session, the subjects maintained an isometric reference position (IRP) lasting 15 seconds in order to serve as a normalization contraction for EMG amplitude measurements. Athletic tape was used on the floor to mark shoulder width foot position. A 45-pound bar was balanced across the posterior neck and shoulders, very similar to the typical technique for squatting exercises. A goniometer was utilized to ensure both knees were positioned with a joint angle of 30 degrees of flexion. Subjects were instructed to balance in this position and keep the weight of the bar equally distributed across both feet. Subjects performed three repetitions prior to baseline and final testing. These data were filtered and processed as previously described and a mean value from a 150 msec window between 7 and 8 seconds was taken for normalization of each landing trial.
Subjects performed 3-5 practice drop landings from a platform (47 cm) onto the force plate (AMTI OR6, Newton, MA). The subjects were instructed to hang the dominant leg off the platform so they were able to "fall" onto the force plate, landing on the dominant leg. The subjects were instructed not to jump off the platform, and to land as if they were landing from a jump. Testing sessions included 7 drops with 30 seconds rest observed between each trial. Sessions were completed prior to any flexibility intervention, immediately following the first flexibility session, and following 14 days of flexibility intervention. Data from the force plate provided the onset of landing for data reduction. These data were input into the EMG capture for a time reference.
The hamstring flexibility measurement technique used in this study was the lying passive knee-extension test (LPKE)[23]. A brief description of this method is as follows:
When performing the LPKE, the subjects lay supine on an examination table. A mark (made with a permanent marker) was made on the lateral leg (about the level of the knee), which corresponded to a mark on the table. This mark was to assure positioning of the subjects when performing the follow-up measurement. The non-dominant leg stayed positioned against the table, while the dominant leg, which was measured in each test, was positioned at 90 degrees of hip flexion so that the thigh rested against a cross-bar that was attached to the table to allow consistent positioning of each repeated hip flexion. The subject was asked to "relax" as much as possible and the examiner then passively moved the tibia into terminal knee extension, which was operationally defined as the point at which the subject began to complain of "mild discomfort" in the hamstrings. At this point the knee-extension value (as measured using and inclinometer) was recorded. The inclinometer was placed just below the tibial tuberosity.
Subjects in the control group were not required to do anything but continue activities of daily living and test at the same time as subjects in the stretching group. Subjects in the stretching group, were instructed to perform five thirty-second bouts of static stretching of the hamstrings and quadriceps. Stretching of each muscle group lasted no longer than five minutes. The entire stretching treatment lasted a total of ten minutes each day for 14 continuous days. All weekday sessions were supervised to ensure consistency. On weekends, the tester notified the subjects by phone to remind them to stretch on their own. In reply the subjects notified the tester once the regimen was completed.
The dominant leg of each subject was stretched from a standing position. The subject positioned him/herself near a table for aid in stabilization during stretching. Each subject positioned him/herself with the treatment leg approximately two feet in front of the opposite leg, and brought the chest toward the knee, while keeping their back straight and using the table for balance and to minimize stabilizing contraction of the hamstrings (Figure 1). In order to ensure a sufficient stretch, the subject kept the leg being stretched in a straight position and bent forward taking each stretch to a point of slight discomfort and holding that position.
The quadriceps stretch required the subject to stand with the aid of a table or chair for stability. The subject grasped the ankle of the leg being stretched with the ipsilateral hand, and the knee was flexed so that the heel of the foot approached the buttocks (Figure 2)[24]. This stretch of the knee extensors was taken to a point of slight discomfort and held in that position. All stretches were repeated five times for 30 seconds each. Five to ten seconds rest were observed between each stretch.
Prior to testing, hamstring flexibility was measured for all subjects. A total of three testing sessions were completed for the entire experiment. For the first testing session, every subject, regardless of group, performed seven pretreatment drop landing trials to attain baseline measurements on the first day. A period of approximately 30 seconds elapsed between each trial while the subject was reset. Pre and postlanding EMG amplitudes were measured for the vastus lateralis, vastus medialis, medial hamstrings, and lateral hamstrings. All testing sessions were completed in this fashion. Subjects in the treatment group began their static stretching regimen immediately following the pre-treatment trials. Subjects in the control group rested for 10 minutes. The second testing session took place immediately after completion of either the 10-minute stretching regimen for the stretching group or ten minutes of rest for the control group. After these acute posttreatment measurements were recorded, the subjects in the treatment group continued their static stretching regimen for a total of 14 days, while the subjects in the control group continued their normal activities for the duration of the experiment. On day fourteen, all subjects were again tested in the same fashion.
EMG amplitude values prior to (150 ms) and following (150 ms) landing from the vastus lateralis, vastus medialis, medial hamstrings, and biceps femoris were collected from 26 subjects for analysis. Each value was normalized to the mean IRP for that measurement session. High and low values for each session were dropped and means of the remaining 5 trials were analyzed using a 2 × 3 ANOVA with repeated measures on time to detect differences between groups over time. A 2 × 2 ANOVA with repeated measures on time was used to determine if knee extension with 90° of hip flexion measures were different between pretreatment and final posttreatment between groups. Statistical significance was set at p = 0.05.