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  • Author: Randy J. Schmitz x
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Justin P. Waxman, Randy J. Schmitz and Sandra J. Shultz

Hamstring stiffness (KHAM) and leg stiffness (KLEG) are commonly examined relative to athletic performance and injury risk. Given these may be modifiable, it is important to understand day-to-day variations inherent in these measures before use in training studies. In addition, the extent to which KHAM and KLEG measure similar active stiffness characteristics has not been established. We investigated the interday measurement consistency of KHAM and KLEG, and examined the extent to which KLEG predicted KHAM in 6 males and 9 females. KHAM was moderately consistent day-to-day (ICC2,5 = .71; SEM = 76.3 N·m–1), and 95% limits of agreement (95% LOA) revealed a systematic bias with considerable absolute measurement error (95% LOA = 89.6 ± 224.8 N·m–1). Day-to-day differences in procedural factors explained 59.4% of the variance in day-to-day differences in KHAM. Bilateral and unilateral KLEG was more consistent (ICC2,3 range = .87–.94; SEM range = 1.0–2.91 kN·m–1) with lower absolute error (95% LOA bilateral= –2.0 ± 10.3; left leg = –0.36 ± 3.82; right leg = –1.05 ± 3.61 kN·m–1). KLEG explained 44% of the variance in KHAM (P < .01). Findings suggest that procedural factors must be carefully controlled to yield consistent and precise KHAM measures. The ease and consistency of KLEG, and moderate correlation with KHAM, may steer clinicians toward KLEG when measuring lower-extremity stiffness for screening studies and monitoring the effectiveness of training interventions over time.

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Jennifer A. Hogg, Randy J. Schmitz and Sandra J. Shultz

Clinical femoral anteversion (Craig test) and hip range of motion (ROM) have been associated with valgus collapse, but their clinical usefulness in predicting biomechanics is unknown. Our purpose was to determine the individual and combined predictive power of femoral anteversion and passive hip ROM on 3-dimensional valgus collapse (hip internal rotation and adduction, knee rotation, and abduction) during a single-leg forward landing in females. Femoral anteversion and passive hip ROM were measured on 20 females (24.9 [4.1] y, 168.7 [8.0] cm, 63.8 [11.6] kg). Three-dimensional kinematics and kinetics were collected over 5 trials of the task. Each variable was averaged across trials. Backward, stepwise regressions determined the extent to which our independent variables were associated with valgus collapse. The combination of greater hip internal and external rotation ROM (partial r = .52 and .56) predicted greater peak knee internal rotation moment (R 2 = .38, P = .02). Less hip internal rotation ROM (partial r = −.44) predicted greater peak knee abduction moments (R 2 = .20, P = .05). Greater total hip ROM (internal and external rotation ROM) was not consistently associated with combined motions of valgus collapse but was indicative of isolated knee moments. Passive hip ROM is more associated with knee moments than is femoral anteversion as measured with Craig test.

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Anthony S. Kulas, Thomas C. Windley and Randy J. Schmitz

Context:

Functional implications of clinically relevant abdominal postures have been sparsely examined.

Objectives:

To evaluate the reliability of sustaining abdominal postures during single-leg landings and the effects of abdominal postures on lower extremity kinetics and energetics.

Design:

One-way ANOVA tested effects of leg-spring stiffness and lower extremity energetics across groups (control, abdominal hollowing [AH], and pelvic tilting [PT]).

Participants:

12 male (24.0 ± 3.4 years) and 12 female (21.9 ± 2.3 years) healthy, recreationally active subjects.

Main Outcome Measures:

Leg-spring stiffness and relative joint-energy absorption from control, AH, and PT groups.

Results:

AH and PT ICCs2,k and standard error of measurements (AH = 0.53 ± 0.4 cm, PT = 0.9° ± 0.8°) were moderate to high. Relative knee-energy-absorption effect sizes comparing the control and treatment groups revealed moderate treatment effects (AH = 0.66%, PT = 0.41%).

Conclusions:

Abdominal postures can be reliably performed during a single-leg-landing task. Energy-absorption effect sizes suggest a link between the trunk and lower extremity.

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Randy J. Schmitz, Bryan L. Riemann and Timothy Thompson

Objective:

To determine whether gluteus medius (GM) activity increases in response to isometric closed-chain external hip rotation.

Design:

Subjects performed single-leg stances in 3 different conditions: 0° knee flexion, 0° hip flexion (C1); 0° knee flexion, 20° hip flexion (C2); and knee flexed 20–30°, 20° hip flexion (C3). Posteriorly directed forces of 8.9 N (F1), 17.8 N (F2), and 26.7 N (F3) were applied at the lateral pelvis of the nonstance side during each condition.

Subjects:

20 college students.

Measurements:

Surface EMG RMS amplitude from the GM and kinematic data from the trunk, hip, and knee.

Results:

Statistical analyses revealed a significant Condition 3 Force interaction and significant increases of EMG activity from C1F1 and C1F2 to C1F3 and from C3F1 to C3F2 and C3F3. F2 and F3 of C2 were significantly less than F2 and F3 of both C1 and C3.

Conclusions:

GM activity increases in response to isometric, closed-chain, external hip-rotation forces, and forward movement of the upper body with respect to the base of support decreases GM activity.

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Randy J. Schmitz, John C. Cone, Timothy J. Copple, Robert A. Henson and Sandra J. Shultz

Context:

Potential biomechanical compensations allowing for maintenance of maximal explosive performance during prolonged intermittent exercise, with respect to the corresponding rise in injury rates during the later stages of exercise or competition, are relatively unknown.

Objective:

To identify lower-extremity countermovement-jump (CMJ) biomechanical factors using a principal-components approach and then examine how these factors changed during a 90-min intermittent-exercise protocol (IEP) while maintaining maximal jump height.

Design:

Mixed-model design.

Setting:

Laboratory.

Participants:

Fifty-nine intermittent-sport athletes (30 male, 29 female) participated in experimental and control conditions.

Interventions:

Before and after a dynamic warm-up and every 15 min during the 1st and 2nd halves of an individually prescribed 90-min IEP, participants were assessed on rating of perceived exertion, sprint/cut speed, and 3-dimensional CMJ biomechanics (experimental). On a separate day, the same measures were obtained every 15 min during 90 min of quiet rest (control).

Main Outcome Measures:

Univariate piecewise growth models analyzed progressive changes in CMJ performance and biomechanical factors extracted from a principal-components analysis of the individual biomechanical dependent variables.

Results:

While CMJ height was maintained during the 1st and 2nd halves, the body descended less and knee kinetic and energetic magnitudes decreased as the IEP progressed.

Conclusions:

The results indicate that vertical-jump performance is maintained along with progressive biomechanical changes commonly associated with decreased performance. A better understanding of lower-extremity biomechanics during explosive actions in response to IEP allows us to further develop and individualize performance training programs.

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Randy J. Schmitz, David E. Martin, David H. Perrin, Ali Iranmanesh and Alan D. Rogol

The purpose of this study was to assess the effect of interferential current (IFC) on perceived pain and serum Cortisol levels in subjects with delayed onset muscle soreness (DOMS). DOMS was induced in 10 subjects through repeated eccentric contractions of the elbow flexors. Forty-eight hours later subjects were evaluated. Starting at t = 0:00, blood samples were withdrawn from a superficial vein every 5 min for 65 min. At t = 0:05, subjects received IFC of 10 bps or IFC of 100 bps. Perceived pain levels were evaluated prior to catheter insertion and at t = 0:35, 0:50, and 0:65. Two mixed-model analyses of variance revealed a significant decrease in perceived pain scores across time for both treatment groups but no significant difference in serum Cortisol for the two groups. It was concluded that IFC of high and low beat frequency is effective in controlling the pain of DOMS but does not elicit a generalized stress response as indexed by increasing serum Cortisol levels.

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Anthony S. Kulas, Randy J. Schmitz, Sandra J. Shultz, Mary Allen Watson and David H. Perrin

Although leg spring stiffness represents active muscular recruitment of the lower extremity during dynamic tasks such as hopping and running, the joint-specific characteristics comprising the damping portion of this measure, leg impedance, are uncertain. The purpose of this investigation was to assess the relationship between leg impedance and energy absorption at the ankle, knee, and hip during early (impact) and late (stabilization) phases of landing. Twenty highly trained female dancers (age = 20.3 ± 1.4 years, height = 163.7 ± 6.0 cm, mass = 62.1 ± 8.1 kg) were instrumented for biomechanical analysis. Subjects performed three sets of double-leg landings from under preferred, stiff, and soft landing conditions. A stepwise linear regression analysis revealed that ankle and knee energy absorption at impact, and knee and hip energy absorption during the stabilization phases of landing explained 75.5% of the variance in leg impedance. The primary predictor of leg impedance was knee energy absorption during the stabilization phase, independently accounting for 55% of the variance. Future validation studies applying this regression model to other groups of individuals are warranted.