Subjects
The sample size required for the study was pre-estimated using G*Power3.1 software (Dusseldorf, Germany), effect size, alpha and test efficacy (1-β) were set at 0.40, 0.05 and 0.80, respectively, in reference to the findings of Seitz L B et al18. and Faul F et al19. The results of the estimation showed that the minimum sample size required to carry out the present study is 12 individuals. Considering a potential sample dropout rate of 20%, a minimum of 15 participants were recruited for this study. Accordingly, 18 male sprinters were recruited and one withdrew due to an ankle sprain during the intervention. The final 17 participants [age (22.88 ± 1.13) years, height (178.46 ± 3.24) cm, weight (72.12 ± 3.74) kg, thigh circumference (56.67 ± 1.77) cm, blood flow restriction pressure values (271.26 ± 28.85) mmHg, years of training (4.31 ± 1.29), and 1RM hip thrust ( 196.41 ± 35.28) kg, 1RM barbell half squat (150.19 ± 19.92) kg] completed all the tests. All of the above participants were free of injury, sleep disorders, and non-smokers, volunteered to participate, and signed an informed consent form after being informed of the testing process and potential risks. Research have been performed in accordance with the Declaration of Helsinki. Approval for this study was obtained from the Ethics Committee of Beijing Sport University(2023215H) and informed consent was obtained from all participants, all of them signed a consent form.
Procedures
This study utilizes a randomized crossover control trial design. During the familiarization session, participants are required to familiarize themselves with the evaluation process of all test tasks and indicators to mitigate the influence of learning effects and physical discomfort on the formal experiment. During the familiarization session, participants underwent the following: 1) Introduction to testing procedures, encompassing methods for 20m sprint run, explosive vertical jump, feather angle, and lower limb muscle-tendon stiffness, during which testing tools could be briefly examined; 2) Collection and recording of subjects' age, height, weight, thigh circumference, years of training, and best performance in the 100m sprint; 3) Assessment of maximal force in barbell hip thrust and barbell half-squat, and establishment of baseline kinematic and kinetic parameters for 20m sprint and vertical jump on the force platform. Additionally, participants were briefed on test precautions: 1) Avoid high-intensity physical exercise, consumption of caffeine- or alcohol-containing beverages for 24 hours prior to the experiment, and ensure at least 8 hours of sleep20; 2) Hydrate appropriately and refrain from eating for 2 hours before the test 21; 3) Attempt to schedule tests at the same time each time, ideally within a deviation of 1 hour; 4) Maintain consistent or similar training attire; 5) Ensure uniformity in dressing and attire; 6) Refrain from engaging in high-intensity training activities during the washout period.
Participants followed the same procedure during the formal experimental session. Participants completed interventions in a randomized order, including 90% 1RM HT, 90% 1RM HS, 30% 1RM HT + BFRT, and 30% 1RM HS + BFRT. For the formal experimental intervention, participants engaged in an 8–10 minute standardized warm-up, involving jogging, glute activation, dynamic stretching, and marching movement integration22. Following a 3–5 minute rest at the end of the warm-up, participants underwent task interventions. These interventions included a low-intensity resistance combined with blood flow restriction training protocol of 30% 1RM (HT/HS) × 15 repetitions/set × 3 sets with a 30-second interval between sets, as referenced in studies by Abe et al. (2005)23, Patterson et al.(2019)24. Additionally, a high-intensity resistance exercise protocol of 90% 1RM (HT/HS) × 3 repetitions25. It is noteworthy that studies have confirmed, for collegiate male sprinters, the optimal window for enhancing sprint running and jumping performance is between minutes 4 and 8 post-intervention, with the majority of studies indicating peak efficacy at the fifth minute26,27. Therefore, the 20-m sprint run, the force platform vertical jump test (including CMJ, SJ, and DJ completed in one session), the rectus femoris pennation angle, and the lower extremity muscle-tendon stiffness test were all conducted at minute 5 post-intervention, with only one of these tests being completed in each experiment. To minimize the potential for interaction between interventions, participants were explicitly instructed to refrain from engaging in high-intensity training activities during the washout period and to ensure their readiness for each training session (ensuring physiological recovery from fatigue and maintaining psychological anticipation of the training). Additionally, based on the studies of Mina et al. (2019)28 and Dello et al. (2016)29, a 72-hour interval was maintained between each pair of training interventions. The room temperature during the testing period ranged from 23.2°C to 26.3°C, with humidity between 67% and 85%.
Selection of blood flow restriction training equipment model, operation method and training pressure
The blood flow restriction training equipment used is the B STRONG pressurized training belt (B STRONG, Utah, USA), featuring an adjustable design and employing a distributed airbag pressure filling method. This design effectively alleviates compression pain and ensures safe, convenient operation. Specifically, participants assumed a standing position and wrapped the pressurized training belt around the vertical longitudinal axis of the thigh, targeting the gluteus transversus muscle on both sides. Considering gradual adaptation of blood vessels to changes in pressure induced by blood flow restriction, inflation pressure was incrementally increased until reaching the target pressure value. In this study, we employed personalized blood flow restriction pressure, determined by the relative pressure value selected according to the individual subject's thigh circumference. Research by Loenneke et al. (2012) 30, and Natsume et al. (2015) 31supports the superiority of personalized pressure over fixed values. Specifically, pressure selection was based on thigh circumference: 200 mmHg for < 45–50 cm, 250 mmHg for 51–55 cm, 300 mmHg for 56–59 cm, and 350 mmHg for > 60 cm. The mean blood flow restriction training pressure for subjects in this study was 271.26 ± 28.85 mmHg.
Barbell hip thrust and barbell half squat 1RM test
The 1RM test included the Smith barbell half squat and barbell hip thrust tests, adhering to the 1RM test requirements outlined by the American Physical Fitness Association. During the Smith barbell half squat 1RM test, subjects positioned their feet slightly wider than shoulder width, rotated their toes outward, and descended until their thighs were parallel to the floor32. During the hip thrust test, a soft cushion was positioned at the subject's anterior superior iliac spine, and the upper back was placed on a training bench. The feet were positioned slightly wider than shoulder width, with toes externally rotated. The movement involved lowering the barbell until it touched the ground, while maintaining a neutral spine and pelvis position on the ascent20. During the formal test, subjects initially attempted a weight they could easily lift for 5–10 repetitions, followed by a 2-minute rest. Subsequent attempts increased the weight by 10–20% each time, with a 2-4-minute rest between sets. The subject's 1RM was determined within 3–5 attempts.
20m sprint test
Participants conducted the 20m sprint run test on the track using portable Smart Speed timing gates (Smart Speed Pro, Fusion Sport, Australia), positioned at 0m, 10m, and 20m, and configured in running application mode. To prevent interference with the timing system, athletes assumed a three-point pre-sprint position 30 cm behind the starting line, ensuring their bodies did not cross the starting infrared beam 20. The test was conducted three times with 2–3 minute intervals between attempts, and the best performance was selected for statistical analysis.
Vertical jump test
Jump tests comprised the counter movement jump (CMJ), squat jump (SJ), and drop jump (DJ), all conducted on the KISTLER Quattro Jump (2822A1-1, Winterthur, Switzerland). Participants sequentially completed three attempts of CMJ, SJ, and DJ, with 1–2 minute intervals between each attempt33. The best score from each test was recorded. During CMJ jumps, subjects stood on a force platform with hands on hips, maintaining an upright position, squatted to 90 degrees of knee flexion, and exerted maximal effort to jump vertically upwards.SJ jumps involve initiating a vertical jump from a half-squat position, followed by a forceful vertical upward movement to maintain continuity. The SJ also assesses peak force, peak power, and peak rate of force development (RFD= \(\frac{\text{Fmax}-\text{Fstart}}{\varDelta \text{T}}\)) ) at the optimal jump height34.Weight normalization was conducted to derive relative peak force, relative peak power, and relative peak rate of force generation. During DJ jumps, subjects stood on a 30 cm high jump box35, with hands placed at the waist, took a small step forward, dropped vertically with feet together, then quickly jumped upward upon contacting the force measuring platform. The reactive strength index (RSI) was calculated using the time in the air and the time of ground contact during the DJ. Reactive Strength Index (\(\text{R}\text{S}\text{I}=\frac{\text{Flight time}}{\text{C}\text{o}\text{n}\text{t}\text{a}\text{c}\text{t} \text{t}\text{i}\text{m}\text{e}}\))34.
Rectus femoris pennation angle test
The pennation angle of the rectus femoris muscle in athletes was measured using a GE-LOGIQ-E9 color ultrasound diagnostic device (GE LOGIQ-E9, Wauwatosa, WI, USA), known for its high clarity, resolution, and absence of radiation exposure36. Following the guidelines of the American Institute of Ultrasound Medicine, athletes were instructed to wear shorts, relax their legs, and assume a supine position with the femur neutrally positioned for the pennation angle ultrasound examination. An experienced evaluator obtained all images using consistent techniques, applied ultrasound gel uniformly to the ultrasound probe, and positioned it along the long axis of the anterior thigh. The pennation angle measurements were taken in the muscle bellies of the lateral femoral and rectus femoris muscles of the athlete's dominant leg. A custom-made ultrasound probe fixation device ensured the probe was aligned parallel to the muscle fibers' direction. Markers were drawn on the subject's legs to ensure reproducibility of image locations during subsequent ultrasound assessments. A 12 MHz linear probe scanning head was selected to enhance spatial resolution. Water-soluble transmission gel was applied to the location of the rectus femoris muscle belly for longitudinal imaging of the pennation angle. Subjects underwent pre-testing of the rectus femoris pennation angle 3 minutes before the formal warm-up and post-testing at the 5th minute after the various exercise-induced interventions. Ultrasound testing was conducted by a physician experienced in operating GE ultrasound equipment.
Lower extremity muscle-tendon stiffness test
Higher stiffness facilitates the stretch-shortening cycle (SSC) and higher velocity explosive movements. Therefore, testing the stiffness of lower limb muscles and tendons is crucial for assessing the effectiveness of preparatory activities. The Myoton PRO (Myoton AS, Tallinn, Estonia) portable diagnostic equipment for muscle function utilizes parameters and measurements referenced from the study conducted by Papla M et al.(2023)37 The device's accelerometer was set to 3200 Hz, and the average value was calculated from 3 consecutive measurements, each monitored with a 3% error margin. Pre-measurements were conducted 3 minutes before the official warm-up, During testing, the Myoton PRO probe was perpendicular to the surface of each measurement point (duration: 15 ms; force: 0.58 N)38.while post-measurements were taken at the 5th minute following the warm-up. Muscle measurements were taken at the muscle belly of each muscle, while Achilles tendon measurements were positioned 5 cm directly above the calcaneus. Subjects were instructed to fully relax their muscles during the measurement process to standardize muscle activity.
Statistical analysis
Data were analyzed using SPSS 22.0. Normal distribution was confirmed via the Shapiro-Wilk method, and results were uniformly reported as mean ± standard deviation (M ± SD). Repeated measures ANOVA was utilized to examine differences in sprint ability, explosive jumping performance, rectus femoris pennation angle, and lower limb muscle-tendon stiffness among male sprinters following the task intervention. For ANOVA results not meeting the assumption of sphericity by Mauchly's test, corrections were applied using the Greenhouse-Geisser method. Post-hoc comparisons were conducted using the Bonferroni method. Cohen's d-value was employed to assess Effect Size (ES), with effect size interpreted as medium (0.50) or large (0.80) 39. The significance level was set at P < 0.05.