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Fain, AC, Semore, KD, Lobb, NJ, and Brown, TN. Lower-limb biomechanics differ between sexes during maximal loaded countermovement jumps. J Strength Cond Res 35(2): 325–331, 2021—To improve military personnel's operational performance, this study determined the impact of heavy, military body–borne load on vertical jump performance. Twenty men and 17 women had lower-limb work and power quantified during a series of countermovement jumps with 4 body-borne loads (20, 25, 30, and 35 kg). For each jump, subjects stood in athletic position with feet shoulder-width apart, then squatted down and immediately performed a maximal-effort vertical jump. Subjects performed 3 successful jumps with each load. During each jump, limb and hip, knee and ankle work and power, each joint's contribution to limb work, as well as jump height and center of mass velocity were quantified. Each dependent measure was submitted to a 2-way repeated-meausres analysis of variance, with alpha level 0.05. Body-borne load reduced jump height (p = 0.001) but increased ankle work (p < 0.001). To jump higher (p < 0.001) with a greater center of mass velocity (p = 0.001), men produced more limb work (p < 0.001), hip (p = 0.001; p < 0.001), knee (p < 0.001; p < 0.001), and ankle (p < 0.001; p < 0.001) joint power and work. But, women produced a greater percentage of work at the ankle (p = 0.020) than men. Military practitioners may target different training adaptations to improve male and female personnel operational performance because lower-limb biomechanics differ between sexes during loaded vertical jumps.

This study determined changes in lower limb joint stiffness when running with body-borne load, and whether they differ with stride or sex. Twenty males and 16 females had joint stiffness quantified when running (4.0 m/s) with body-borne load (20, 25, 30, and 35 kg) and 3 stride lengths (preferred or 15% longer and shorter). Lower limb joint stiffness, flexion range of motion (RoM), and peak flexion moment were submitted to a mixed-model analysis of variance. Knee and ankle stiffness increased 19% and 6% with load (P < .001, P = .049), but decreased 8% and 6% as stride lengthened (P = .004, P < .001). Decreased knee RoM (P < .001, 0.9°–2.7°) and increased knee (P = .007, up to 0.12 N.m/kg.m) and ankle (P = .013, up to 0.03 N.m/kg.m) flexion moment may stiffen joints with load. Greater knee (P < .001, 4.7°–5.4°) and ankle (P < .001, 2.6°–7.2°) flexion RoM may increase joint compliance with longer strides. Females exhibited 15% stiffer knee (P = .025) from larger reductions in knee RoM (4.3°–5.4°) with load than males (P < .004). Stiffer lower limb joints may elevate injury risk while running with load, especially for females.

Increasing lower limb flexion may reduce risk of musculoskeletal injury for military personnel during landing. This study compared lower limb biomechanics between sexes and limbs when using normal and greater lower limb flexion to land with body borne load. Thirty-three participants (21 male, 12 female, age: 21.6±2.5 years, height: 1.7±0.1 m, weight: 74.5±9.0 kg) performed normal and flexed lower limb landings with four body borne loads: 20, 25, 30 and 35 kg. Hip and knee biomechanics, peak vertical ground reaction force (GRF), and the magnitude and direction of the GRF vector in frontal plane were submitted to two separate repeated measures ANOVAs to test the main and interaction effects of sex, load, and landing, as well as limb, load, and landing. Participants increased GRFs (between 5 and 10%) and hip and knee flexion moments when landing with body borne load, but decreased vertical GRF 19% and hip adduction and knee abduction joint range of motion and moments during the flexed landings. Both females and the non-dominant limb presented greater risk of musculoskeletal injury during landing. Females exhibited larger GRFs, increased hip adduction range of motion, and greater knee abduction moments compared to males. Whereas, the non-dominant limb increased knee abduction moments and exhibited a more laterally-directed frontal plane GRF vector compared to the dominant limb during the loaded landings. Yet, increasing lower limb flexion during landing does not appear to produce similar reductions in lower limb biomechanics related to injury risk for both females and the non-dominant limb during landing.

Background: This study determined whether the knee and ankle muscle extensor forces increase when running with a body-borne load and whether these forces differ between the sexes. Methods: Thirty-six (twenty male and sixteen female) adults had the knee and ankle extensor force quantified when running 4.0 m/s with four body-borne loads (20, 25, 30, and 35 kg). Peak normalized (BW) and unnormalized (N) extensor muscle force, relative effort, and joint angle and angular velocity at peak muscle force for both the ankle and the knee were submitted to a mixed model ANOVA. Results: Significant load by sex interactions for knee unnormalized extensor force (p = 0.025) and relative effort (p = 0.040) were observed, as males exhibited greater knee muscle force and effort than females and increased their muscle force and effort with additional load. Males also exhibited greater ankle normalized and unnormalized extensor force (p = 0.004, p < 0.001) and knee unnormalized force than females (p = 0.005). The load increased the normalized ankle and knee muscle force (p < 0.001, p = 0.030) and relative effort (p < 0.001, p = 0.044) and the unnormalized knee muscle force (p = 0.009). Conclusion: Running with a load requires greater knee and ankle extensor force, but males exhibited greater increases in muscle force, particularly at the knee, than females.