The Effect of Stirrup Length on Impact Attenuation and Its Association With Muscle Strength.

ABSTRACT: Keener, MM, Critchley, ML, Layer, JS, Johnson, EC, Barrett, SF, and Dai, B. The effect of stirrup length on impact attenuation and its association with muscle strength. J Strength Cond Res 35(11): 3056-3062, 2021-Horseback-riders have a high prevalence of low back injuries, which may be related to the repetitive low back impacts experienced in riding. The purposes of this study were to quantify the effect of 3 stirrup lengths and 2 riding styles on the peak acceleration experienced by the rider and the association between the peak acceleration and the rider's different elements of muscle strength. Thirteen female riders performed a sitting or rising trot at each of the 3 stirrup lengths (2-point length, mid-seat length, or dressage length), while the acceleration of the tibia, sacrum, seventh cervical vertebra (C7), and head were collected. Subjects completed a push-up, a vertical jump, and 4 core exercises to assess upper-body strength, lower-body strength, and core endurance, respectively. Peak acceleration of the sacrum, C7, and head were generally lower in the standing phase of the rising trot compared with the sitting phase of either the sitting or rising trot, particularly at the shortest stirrup length. Peak acceleration of the sacrum, C7, and head decreased as the stirrup length was shortened in the standing phase of the rising trot. Canonical correlations showed nonsignificant correlations between strength measurements and peak acceleration. Riding with more weight supported through the legs with a short stirrup length may decrease low back impacts and their associated injury risk. Technique training is likely needed to encourage riders to use lower-body and core strength for impact attenuation.

Longitudinal assessments of balance and jump-landing performance before and after anterior cruciate ligament injuries in collegiate athletes.

The purpose was to quantify the effect of an anterior cruciate ligament (ACL) injury on balance and jump-landing performance and bilateral asymmetries. Among 500 collegiate athletes who performed a reaching test and a double-leg counter-movement jump-landing test at baseline, 8 male and 6 female athletes suffered ACL injuries. In the follow-up, they performed the reaching test 3 and 6 months after ACL reconstruction (ACLR) and the jump-landing test 6 months after ACLR. Less reaching distances for the injured leg and increased reaching distance asymmetries were observed 3 and 6 months after ACLR compared to baseline. Less peak jumping and landing forces for the injured leg and increased jumping and landing force asymmetries were found 6 months after ACLR compared to baseline. The decreased performance of the injured leg and increased asymmetries may contribute to the high ACL re-injury rates. Baseline assessments would be useful for establishing an individual's pre-injury performance.

Mid-flight lateral trunk bending increased ipsilateral leg loading during landing: a center of mass analysis.

Increased lateral trunk bending to the injured side has been observed when ACL injuries occur. The purpose was to quantify the effect of mid-flight lateral trunk bending on center of mass (COM) positions and subsequent landing mechanics during a jump-landing task. Forty-one recreational athletes performed a jump-landing task with or without mid-flight lateral trunk bending. When the left and right trunk bending conditions were compared with the no trunk bending condition, participants moved the COM of the upper body to the bending direction, while the COM of the pelvis, ipsilateral leg, and contralateral leg moved away from the bending direction relative to the whole body COM. Participants demonstrated increased peak vertical ground reaction forces (VGRF) and knee valgus and internal rotation angles at peak VGRF for the ipsilateral leg, but decreased peak VGRF and knee internal rotation angles at peak VGRF and increased knee varus angles at peak VGRF for the contralateral leg. Mid-flight lateral trunk resulted in an asymmetric landing pattern associated with increased ACL loading for the ipsilateral leg. The findings may help to understand altered trunk motion during ACL injury events and the discrepancy in ACL injuries related to limb dominance in badminton and volleyball.

Kinetic Analysis of Isometric Back Squats and Isometric Belt Squats.

Layer, JS, Grenz, C, Hinshaw, TJ, Smith, DT, Barrett, SF, and Dai, B. Kinetic analysis of isometric back squats and isometric belt squats. J Strength Cond Res 32(12): 3301-3309, 2018-Belt squats seem to provide an alternative to back squats. However, it is not clear how musculoskeletal loading differs between the two. This study compared lower extremity and low-back kinetics during isometric back squats and isometric belt squats. Sixteen men (age: 22.6 ± 3.4 years; height: 1.74 ± 0.11 m; mass: 82.0 ± 5.6 kg) and 10 women (age: 21.5 ± 2.5 years; height: 1.64 ± 0.10 m; mass: 68.9 ± 7.1 kg) performed isometric back squats and belt squats at 4 squat depths. Joint resultant moments were calculated from kinematic and ground reaction force data. Linear interpolation was used to estimate peak vertical forces and joint moments at a 45° thigh segment angle. Subjects increased peak forces, ankle moments, and knee moments but decreased low-back moments from back to belt squats (p ≤ 0.023). Hip moments did not significantly change between 2 squats. Subjects demonstrating smaller ankle and knee moments during back squats showed greater increases in these moments from back to belt squats (p ≤ 0.012, R ≤ 0.24). Subjects whose back squats were characterized by greater low-back moments displayed greater decreases in low-back moments from back to belt squats (p < 0.001, R = 0.98). Compared with isometric back squats, isometric belt squats may provide a similar or greater external loading for the musculoskeletal system of the lower extremities while reducing external spinal loading. Belt squats may be considered by individuals with upper-body or spinal injuries and those displaying excessive external back moments.

Lower-Extremity Kinematics Differed Between a Controlled Drop-Jump and Volleyball-Takeoffs.

Previous studies utilizing jump-landing biomechanics to predict anterior cruciate ligament injuries have shown inconsistent findings. The purpose of this study was to quantify the differences and correlations in jump-landing kinematics between a drop-jump, a controlled volleyball-takeoff, and a simulated-game volleyball-takeoff. Seventeen female volleyball players performed these 3 tasks on a volleyball court, while 3-dimensional kinematic data were collected by 3 calibrated camcorders. Participants demonstrated significantly increased jump height, shorter stance time, increased time differences in initial contact between 2 feet, increased knee and hip flexion at initial contact and decreased peak knee and hip flexion for both left and right legs, and decreased knee-ankle distance ratio at the lowest height of midhip for the 2 volleyball-takeoffs compared with the drop-jump (P < .05, Cohen's dz ≥ 0.8). Significant correlations were observed for all variables between the 2 volleyball-takeoffs (P < .05, ρ ≥ .6) but were not observed for most variables between the drop-jump and 2 volleyball-takeoffs. Controlled drop-jump kinematics may not represent jump-landing kinematics exhibited during volleyball competition. Jump-landing mechanics during sports-specific tasks may better represent those exhibited during sports competition and their associated risk of anterior cruciate ligament injury compared with the drop-jump.

Longitudinal assessments of strength and dynamic balance from pre-injury baseline to 3 and 4 months after labrum repairs in collegiate athletes.

BACKGROUND: There is a lack of quantitative assessments of athletes' functional strength and dynamic balance following labrum repairs. PURPOSE: To compare the upper extremity strength and dynamic balance among pre-injury baseline and approximately 3 and 4 months after labrum surgeries in collegiate athletes to identify critical values to inform rehabilitation. METHODS: Fifteen male and one female collegiate athletes between 18 and 22 years old were tested at pre-injury baseline (n = 14) and 2.7 (n = 16) and 3.8 months (n = 12) after labrum surgeries. Strength was assessed using the peak forces produced in a maximal push-up test. Dynamic balance was assessed using the reaching distances in a reaching test. RESULTS: The injured side's peak forces significantly decreased from the baseline to the 3-month post-surgery and then significantly increased between the 3-month and 4-month post-surgery assessments but remained significantly less at the 4-month post-surgery compared to the baseline (p ≤ 0.024; Cohen's dz ≥ 0.75). Peak force asymmetries were greater at the 3-month and 4-month post-surgery assessments than the baseline (p ≤ 0.005; Cohen's dz ≥ 1.02). CONCLUSION: With a relatively small sample size, the results support the use of objective functional assessments for rehabilitation and return-to-play decisions among collegiate athletes following labrum repairs.

Kinematic Analyses of Parkour Landings from as High as 2.7 Meters.

Developing effective landing strategies has implications for both injury prevention and performance training. The purpose was to quantify the kinematics of Parkour practitioners' landings from three heights utilizing four techniques. Seventeen male and three female Parkour practitioners landed from 0.9, 1.8, and 2.7 m utilizing the squat, forward, roll, and stiff landing techniques when three-dimensional kinematics were collected. The stiff landing demonstrated the shortest landing time, and the roll landing showed the longest landing time for 1.8 and 2.7 m. Roll landings demonstrated the greatest forward velocities at initial contact and at the end of the landing. Stiff landings showed the greatest changes in vertical velocity during the early landing, while roll landings showed the least changes for 0.9 and 1.8 m. Both roll and stiff landings generally resulted in decreased changes in horizontal velocity during the early landing compared to squat and forward landings. The four landing techniques also demonstrated different lower extremity joint angles. Stiff landings may increase injury risk because of the quick decrease of vertical velocities. Roll landings allow individuals to decrease vertical and horizontal velocities over a longer time, which is likely to decrease the peak loading imposed on the lower extremities.

Biomechanical comparisons of back and front squats with a straight bar and four squats with a transformer bar.

The purpose was to quantify trunk and lower extremity biomechanics among back and front squats with a straight bar and four squats with different anterior-posterior load placements imposed by a transformer bar. Ten males and eight females performed six squat conditions: back and front squats with a straight bar, back and front squats with a transformer bar, and squats with more posteriorly or anteriorly placed loads with a transformer bar. A constant load of 70% of the participant's one-repetition maximum in the straight-bar front squat was used. Kinematic and kinetic data were collected to quantify joint biomechanics at an estimated parallel squat position in the descending and ascending phases. Squats with more anteriorly placed load significantly decreased trunk flexion and pelvis anterior tilt angles with large effect sizes but increased low-back extension moments with medium to large effect sizes. Hip, knee, and ankle extension moments were generally similar among most conditions. Participants adjusted their trunk and pelvis to mediate the effects of load placements on low-back and lower extremity moments. While lower extremity loading was similar among different squats, the different trunk and pelvis angles and low-back moments should be taken into consideration for people with low-back impairment.

The effects of mid-flight whole-body and trunk rotation on landing mechanics: implications for anterior cruciate ligament injuries.

The purpose was to quantify the effects of mid-flight whole-body and trunk rotation on knee mechanics in a double-leg landing. Eighteen male and 20 female participants completed a jump-landing-jump task in five conditions: no rotation, testing leg ipsilateral or contralateral (WBRC) to the whole-body rotation direction, and testing leg ipsilateral (TRI) or contralateral to the trunk rotation direction. The WBRC and TRI conditions demonstrated decreased knee flexion and increased knee abduction angles at initial contact (2.6 > Cohen's dz > 0.3) and increased peak vertical ground reaction forces and knee adduction moments during the 100 ms after landing (1.7 > Cohen's dz > 0.3). The TRI condition also showed the greatest knee internal rotation angles at initial contact and peak knee abduction and internal rotation angles and peak knee extension moments during the 100 ms after landing (2.0 > Cohen's dz > 0.5). Whole-body rotation increased contralateral knee loading because of its primary role in decelerating medial-lateral velocities. Trunk rotation resulted in the greatest knee loading for the ipsilateral knee due to weight shifting and mechanical coupling between the trunk and lower extremities. These findings may help understand altered trunk motion in anterior cruciate ligament injuries.