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Chen Deng, Jason C. Gillette and Timothy R. Derrick

A detailed understanding of the hip loading environment is needed to help prevent hip fractures, minimize hip pain, rehabilitate hip injuries, and design osteogenic exercises for the hip. The purpose of this study was to compare femoral neck stress during stair ascent and descent and to identify the contribution of muscles and reaction forces to the stress environment in mature adult subjects (n = 17; age: 50–65 y). Motion analysis and inverse dynamics were combined with musculoskeletal modeling and optimization, then used as input to an elliptical femoral neck cross-sectional model to estimate femoral neck stress. Peak stress values at the 2 peaks of the bimodal stress curves (stress vs time plot) were compared between stair ascent and descent. Stair ascent had greater compressive stress than descent during the first peak at the anterior (ascent: −18.0 [7.9] MPa, descent: −12.9 [5.4] MPa, P < .001) and posterior (ascent: −34.4 [10.9] MPa, descent: −27.8 [10.1] MPa, P < .001) aspects of the femoral neck cross section. Stair descent had greater tensile stress during both peaks at the superior aspect (ascent: 1.3 [7.0] MPa, descent: 24.8 [9.7] MPa, peak 1: P < .001; ascent: 15.7 [6.1] MPa, descent: 18.0 [8.4] MPa, peak 2: P = .03) and greater compressive stress during the second peak at the inferior aspect (ascent: −43.8 [9.7] MPa, descent: −51.1 [14.3] MPa, P = .004). Understanding this information can provide a more comprehensive view of bone loading at the femoral neck for older population.

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John J. McMahon, Paul A. Jones, Timothy J. Suchomel, Jason Lake and Paul Comfort

Purpose: The Reactive Strength Index Modified (RSImod) has been recently identified and validated as a method of monitoring countermovement-jump (CMJ) performance. The kinetic and kinematic mechanisms that optimize a higher RSImod score are, however, currently unknown. The purpose of this study, therefore, was to compare entire CMJ force–, power–, velocity–, and displacement–time curves (termed temporal-phase analysis) of athletes who achieve high versus low RSImod scores. Methods: Fifty-three professional male rugby league players performed 3 maximal-effort CMJs on a force platform, and variables of interest were calculated via forward dynamics. The top (high RSImod group) and bottom (low RSImod group) of 20 athletes’ CMJ kinetic- and kinematic-time curves were compared. Results: The high-RSImod group (0.53 ± 0.05 vs 0.36 ± 0.03) jumped higher (37.7 ± 3.9 vs 31.8 ± 3.2 cm) with a shorter time to takeoff (TTT) (0.707 ± 0.043 vs 0.881 ± 0.122 s). This was achieved by a more rapid unweighting phase followed by greater eccentric and concentric force, velocity, and power for large portions (including peak values) of the jump, but a similar countermovement displacement. The attainment of a high RSImod score therefore required a taller, but thinner, active impulse. Conclusions: Athletes who perform the CMJ with a high RSImod, as achieved by high jumps with a short TTT, demonstrate superior force, power, velocity, and impulse during both the eccentric and concentric phases of the jump. Practitioners who include the RSImod calculation in their testing batteries may assume that greater RSImod values are attributed to an increase in these underpinning kinetic and kinematic parameters.

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Jason Lake, Peter Mundy, Paul Comfort, John J. McMahon, Timothy J. Suchomel and Patrick Carden

This study examined concurrent validity of countermovement vertical jump reactive strength index modified and force–time characteristics recorded using a 1-dimensional portable and laboratory force plate system. Twenty-eight men performed bilateral countermovement vertical jumps on 2 portable force plates placed on top of 2 in-ground force plates, both recording vertical ground reaction force at 1000 Hz. Time to takeoff; jump height; reactive strength index modified; and braking and propulsion impulse, mean net force, and duration were calculated from the vertical force from both force plate systems. Results from both systems were highly correlated (r ≥ .99). There were small (d < 0.12) but significant differences between their respective braking impulse, braking mean net force, propulsion impulse, and propulsion mean net force (P < .001). However, limits of agreement yielded a mean value of 1.7% relative to the laboratory force plate system (95% confidence limits, 0.9%–2.5%), indicating very good agreement across all of the dependent variables. The largest limits of agreement were for jump height (2.1%), time to takeoff (3.4%), and reactive strength index modified (3.8%). The portable force plate system provides a valid method of obtaining reactive strength measures, and several underpinning force–time variables, from unloaded countermovement vertical jump. Thus, practitioners can use both force plates interchangeably.

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Travis Saunders, Nerissa Campbell, Timothy Jason, Gail Dechman, Paul Hernandez, Kara Thompson and Chris M. Blanchard

Background:

Although individual studies have reported on the number of steps/day taken by individuals with chronic obstructive pulmonary disease (COPD), this evidence has not been systematically reviewed or synthesized.

Methods:

MEDLINE and PsycINFO were searched for studies reporting objectively-measured steps/day and percent predicted forced expiratory volume in 1 second (FEV1%) in patients with COPD. Meta-analyses were used to estimate steps/day across studies, while metaregression was used to estimate between-study variance based on clinical and demographic factors (year and location of study, activity monitor brand, number of days wearing the monitor, whether participants were about to enter pulmonary rehabilitation, 6-minute walk distance (6MWD), FEV1%, age, and sex).

Results:

38 studies including 2621 participants met inclusion criteria. The pooled mean estimate was 4579 steps/day (95% CI:4310 to 5208) for individuals with COPD. Only 6MWD, FEV1% and whether patients were about to undergo pulmonary rehabilitation explained a significant portion of the variance (P < 0.1) in univariate meta-regression. In a multivariate model including the above risk factors, only FEV1% was associated with steps/day after adjustment for other covariates.

Conclusions:

These results indicate that patients with COPD achieve extremely low levels of physical activity as assessed by steps/day, and that severity of airflow obstruction is associated with activity level.

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Boyi Dai, Christopher J. Sorensen, Timothy R. Derrick and Jason C. Gillette

The effects of training on biomechanical risk factors for anterior cruciate ligament (ACL) injuries have been investigated, but the effects of detraining have received little attention. The purpose of this study was to evaluate the effects of a one-month postseason break on knee biomechanics and lower extremity electromyography (EMG) during a stop-jump task. A postseason break is the phase between two seasons when no regular training routines are performed. Twelve NCAA female volleyball players participated in two stop-jump tests before and after the postseason break. Knee kinematics, kinetics, quadriceps EMG, and hamstring EMG were assessed. After one month of postseason break, the players demonstrated significantly decreased jump height, decreased initial knee flexion angle, decreased knee flexion angle at peak anterior tibial resultant force, decreased prelanding vastus lateralis EMG, and decreased prelanding biceps femoris EMG as compared with prebreak. No significant differences were observed for frontal plane biomechanics and quadriceps and hamstring landing EMG between prebreak and postbreak. Although it is still unknown whether internal ACL loading changes after a postseason break, the more extended knee movement pattern may present an increased risk factor for ACL injuries.

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Jason C. Gillette, Catherine A. Stevermer, Stacey A. Meardon, Timothy R. Derrick and Charles V. Schwab

Farm youth commonly perform animal care tasks such as feeding and watering. The purpose of this study was to determine the effects of age, bucket size, loading symmetry, and amount of load on upper body moments during carrying tasks. Fifty-four male and female participants in four age groups (8–10 years, 12–14 years, 15–17 years, and adults, 20–26 years) participated in the study. Conditions included combinations of large or small bucket sizes, unilateral or bilateral loading, and load levels of 10% or 20% of body weight (BW). During bucket carrying, elbow flexion, shoulder flexion, shoulder abduction, shoulder external rotation, L5/S1 extension, L5/S1 lateral bending, and L5/S1 axial rotation moments were estimated using video data. The 8–10 year-old group did not display higher proportional joint moments as compared with adults. Decreasing the load from 20% BW to 10% BW significantly decreased maximum normalized elbow flexion, shoulder flexion, shoulder abduction, shoulder external rotation, L5/S1 lateral bending, and L5/S1 axial rotation moments. Carrying the load bilaterally instead of unilaterally also significantly reduced these six maximum normalized joint moments. In addition, modifying the carrying task by using smaller one-gallon buckets produced significant reductions in maximum L5/S1 lateral bending moments.

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Ross H. Miller, Stacey A. Meardon, Timothy R. Derrick and Jason C. Gillette

Previous research has proposed that a lack of variability in lower extremity coupling during running is associated with pathology. The purpose of the study was to evaluate lower extremity coupling variability in runners with and without a history of iliotibial band syndrome (ITBS) during an exhaustive run. Sixteen runners ran to voluntary exhaustion on a motorized treadmill while a motion capture system recorded reflective marker locations. Eight runners had a history of ITBS. At the start and end of the run, continuous relative phase (CRP) angles and CRP variability between strides were calculated for key lower extremity kinematic couplings. The ITBS runners demonstrated less CRP variability than controls in several couplings between segments that have been associated with knee pain and ITBS symptoms, including tibia rotation–rearfoot motion and rearfoot motion–thigh ad/abduction, but more variability in knee flexion/extension–foot ad/abduction. The ITBS runners also demonstrated low variability at heel strike in coupling between rearfoot motion–tibia rotation. The results suggest that runners prone to ITBS use abnormal segmental coordination patterns, particular in couplings involving thigh ad/abduction and tibia internal/external rotation. Implications for variability in injury etiology are suggested.

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Jason R. Karp, Jeanne D. Johnston, Sandra Tecklenburg, Timothy D. Mickleborough, Alyce D. Fly and Joel M. Stager

Nine male, endurance-trained cyclists performed an interval workout followed by 4 h of recovery, and a subsequent endurance trial to exhaustion at 70% VO2max, on three separate days. Immediately following the first exercise bout and 2 h of recovery, subjects drank isovolumic amounts of chocolate milk, fluid replacement drink (FR), or carbohydrate replacement drink (CR), in a single-blind, randomized design. Carbohydrate content was equivalent for chocolate milk and CR. Time to exhaustion (TTE), average heart rate (HR), rating of perceived exertion (RPE), and total work (WT) for the endurance exercise were compared between trials. TTE and WT were significantly greater for chocolate milk and FR trials compared to CR trial. The results of this study suggest that chocolate milk is an effective recovery aid between two exhausting exercise bouts.

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Maria K. Talarico, Robert C. Lynall, Timothy C. Mauntel, Erin B. Wasserman, Darin A. Padua and Jason P. Mihalik

Although single-leg squats are a common dynamic balance clinical assessment, little is known about the relationship between parameters that influence squat movement and postural control performance. The objective of this study was to determine the relationships between squat parameters (speed and depth) and postural control under single task and dual task. A total of 30 healthy college students performed single-leg squats under single task and dual task with Stroop. Random-intercepts generalized linear mixed models determined the effect of squat parameters on center of pressure (CoP) parameters. For each 1-cm·s−1 increase in squat speed, sway range (mediolateral: β = −0.03; anteroposterior: β = −0.05) and area (β = −0.25) decreased, whereas sway speed (mediolateral: β = 0.05; anteroposterior: β = 0.29; total: β = 0.29) increased. For each 1-cm increase in squat depth, sway range (mediolateral: β = 0.05; anteroposterior: β = 0.20) and area (β = 0.72) increased, whereas sway speed (anteroposterior: β = −0.14; total: β = −0.14) decreased. Compared with single task, the association between total and anteroposterior sway speed and squat speed was stronger under dual task. Clinicians and researchers should consider monitoring squat speed and depth when assessing dynamic balance during single-leg squats, as these parameters influence postural control, especially under dual task.