Tolerance of the skull to blunt ballistic temporo-parietal impact

Less-lethal ballistic projectiles are used by police personnel to temporarily incapacitate suspects. While the frequency of these impacts to the head is low, they account for more serious injuries than impacts to any other body region. As a result, there is an urgent need to assess the tolerance of the head to such impacts. The focus of this study was to investigate the tolerance of the temporo-parietal skull to blunt ballistic impact and establish injury criteria for risk assessment. Seven unembalmed isolated cadaver heads were subjected to fourteen impacts. Specimens were instrumented with a nine-accelerometer array as well as strain gages surrounding the impact site. Impacts were performed with a 38 mm instrumented projectile at velocities ranging from 18 to 37 m/s. CT images and autopsies were performed to document resulting fractures. Peak fracture force for the seven resulting fractures was 5633±2095 N. Peak deformation for fracture-producing impacts was 7.8±3.2 mm. The blunt criterion (BC), peak force and principal strain were determined to be the best predictors of depressed comminuted fractures. Temporo-parietal tolerance levels were consistent with previous studies. An initial force tolerance level of 2346 N is established for the temporo-parietal region for blunt ballistic impact with a 38 mm diameter impactor.     

The effects of the extraocular muscles on eye impact force–deflection and globe rupture response

There are over 1.9 million eye injuries per year in the United States, with blunt impacts the cause of approximately one-half of all civilian eye injuries. No previous experimental studies have investigated the effects of the extraocular muscles on the impact response of the eye. A spring-powered blunt impactor was used to determine the effects that the extraocular muscles have on the force–deflection and injury response of the eye to blunt trauma. A total of 10 dynamic impact tests were performed at 8.2±0.1 m/s on five human cadaver heads. With the extraocular muscles left intact, the average peak force was found to be 271±51 N at 7.5±0.9 mm posterior translation; with the muscles transected, the average peak force was 268±26 N at 7.6±1.3 mm of posterior translation. From the data available from this study, the peak impact force and overall amount of translation during the impact are not affected by the extraocular muscles. Additionally, from the data presented in this study, the eyes with the extraocular muscles left intact do not rupture with a different injury pattern or display an increased risk for rupture than the eyes with the extraocular muscles transected. Therefore, it is believed that the effect of the extraocular muscles is not sufficient to drastically alter the response of the eye under dynamic impact. This information is useful to characterize the boundary conditions that dictate the eye response from blunt impact and can be used to define the biofidelity requirements for the impact response of synthetic eyes.     

Rapid force recovery in contracting skeletal muscle after brief ischemia is dependent on O(2) availability.

We tested the hypothesis that contracting skeletal muscle can rapidly restore force development during reperfusion after brief total ischemia and that this rapid recovery depends on O(2) availability and not an alternate factor related to blood flow. Isolated canine gastrocnemius muscle (n = 5) was stimulated to contract tetanically (isometric contraction elicited by 8 V, 0.2-ms duration, 200-ms trains, at 50-Hz stimulation) every 2 s until steady-state conditions of muscle blood flow (controlled by pump perfusion) and developed force were attained (3 min). While maintaining the same stimulation pattern, muscle blood flow was then reduced to zero (complete ischemia) for 2 min. Normal blood flow was then restored to the contracting muscle; however, two distinct conditions of oxygenation (at the same blood flow) were sequentially imposed: deoxygenated blood (30 s), blood with normal arterial O(2) content (30 s), a return to deoxygenated blood (30 s), and finally a return to normal arterial O(2) content (90 s). During the ischemic period, force development fell to 39 +/- 6 (SE)% of normal (from 460 +/- 40 to 170 +/- 20 N/100 g). When muscle blood flow was restored to normal by perfusion with deoxygenated blood, developed force continued to decline to 140 +/- 20 N/100 g. Muscle force rapidly recovered to 310 +/- 30 N/100 g (P < 0.05) during the 30 s in which the contracting muscle was perfused with oxygenated blood and then fell again to 180 +/- 30 N/100 g when perfused with blood with low PO(2). These findings demonstrate that contracting skeletal muscle has the capacity for rapid recovery of force development during reperfusion after a short period of complete ischemia and that this recovery depends on O(2) availability and not an alternate factor related to blood flow restoration.     

The damaging punch.

The mechanical properties of a boxing punch have been determined using several techniques. The results are consistent with the medical consequences of boxing discussed in the report of the Board of Science and Education Working Party on boxing. Data were gathered from a world ranked British professional heavyweight, Frank Bruno, as he punched an instrumented, padded target mass suspended as a ballistic pendulum. Within 0.1 s of the start the punch had travelled 0.49 m and attained a velocity on impact of 8.9 m/s. The peak force on impact of 4096N (0.4 ton), attained within 14 ms of contact, represents a blow to the human head of up to 6320N (0.63 ton). The transmitted impulse generated an acceleration of 520 m/s2 (53 g) in the target head. For comparison an equivalent blow would be delivered by a padded wooden mallet with a mass of 6 kg (13 lbs) if swung at 20 mph.     

Computational and experimental models of the human torso for non-penetrating ballistic impact

Both computational finite element and experimental models of the human torso have been developed for ballistic impact testing. The human torso finite element model (HTFEM), including the thoracic skeletal structure and organs, was created in the finite element code LS-DYNA. The skeletal structure was assumed to be linear-elastic while all internal organs were modeled as viscoelastic. A physical human surrogate torso model (HSTM) was developed using biosimulant materials and the same anthropometry as the HTFEM. The HSTM response to impact was recorded with piezoresistive pressure sensors molded into the heart, liver and stomach and an accelerometer attached to the sternum. For experimentation, the HSTM was outfitted with National Institute of Justice (NIJ) Level I, IIa, II and IIIa soft armor vests. Twenty-six ballistic tests targeting the HSTM heart and liver were conducted with 22 caliber ammunition at a velocity of 329 m/s and 9 mm ammunition at velocities of 332, 358 and 430 m/s. The HSTM pressure response repeatability was found to vary by less than 10% for similar impact conditions. A comparison of the HSTM and HTFEM response showed similar pressure profiles and less than 35% peak pressure difference for organs near the ballistic impact point. Furthermore, the peak sternum accelerations of the HSTM and HTFEM varied by less than 10% for impacts over the sternum. These models provide comparative tools for determining the thoracic response to ballistic impact and could be used to evaluate soft body armor design and efficacy, determine thoracic injury mechanisms and assist with injury prevention.     

Strain-rate sensitivity of the rabbit MCL diminishes at traumatic loading rates

This study was performed to determine whether the viscoelastic behavior of ligaments persists at high rates of loading, such as those associated with sports-related trauma or motor vehicle accidents. Medial collateral ligaments (MCLs) from 22 skeletally mature New Zealand White rabbits were tensile tested quasi-statically and via two impact conditions at displacement rates of 0.17 mm/s (n=22), 640+/-160 mm/s (n=10) and 2500+/-270 mm/s (n=12) (corresponding to strain rates of approximately 1.0%/s, 3660%/s and 14,000%/s, respectively). Despite dramatic increases in displacement rate, only a modest strain-rate effect was observed when the specimens tested quasi-statically were compared to those tested via impact (24% and 37% increases in stiffness and failure load, respectively). There were no differences in the structural (e.g. 145+/-30 and 136+/-29 N/mm stiffness values, respectively) or failure properties (e.g. 434+/-91 and 443+/-154 N failure load values, respectively) of the two impact-tested groups. Our findings suggest that the rabbit MCL is not viscoelastic at loading rates approximating those associated with high-energy trauma.     

Strain energy density as a rupture criterion for the kidney: impact tests on porcine organs, finite element simulation, and a baseline comparison between human and porcine tissues

High-velocity (up to 25 m/s) impact tests were performed on pig kidneys to characterize failure behavior at deformation rates associated with traumatic injury. Cylindrical tissue samples (n = 45) and whole perfused organs (n = 34) were impacted using both falling weights and a high-velocity pneumatic projectile impactor. Impact energy was incrementally increased until visible rupture occurred. The strain energy density failure threshold fell between 25 and 60 kJ/m3 for excised porcine tissue samples, and between 15 and 30 kJ/m3 for whole, perfused organs. The relationship between localized failure in whole organ impacts and tissue level failure thresholds observed in cylindrical tissue samples was explored using a detailed finite element model of the human kidney. The model showed good correlation between experimentally observed injury patterns and predicted strain energy density distributions within the renal parenchyma. Finally, to facilitate interpretation of the porcine renal impact results with regard to human trauma, quasi-static compression test results of freshly excised human kidney cortex samples (n = 30) were compared against similar tests on pig kidneys. Human tissues failed at Lagrange strain levels similar to porcine tissue (63+/-6.3%), but at 52% lower Lagrange stress (116+/-28 kPa), and 35% lower strain energy density (17.1+/-4.4 kJ/m3). Thus conservative interpretation of porcine test results is recommended.     

Anacrotic "notch" on the upstroke of external carotid curve may indicate a critically high systolic pulmonary arterial pressure value.

Acute respiratory failure is followed by decreased left ventricular performance probably due to the right ventricle dilatation induced by pulmonary hypertension and intraventricular septal shift to the left. An anacrotic notch on the upstroke slope of the carotid curve was detected in 22 of 36 hemodynamic studies with simultaneous ECG, PCG and external pulse carotid curve recording in 7 burned patients with acute respiratory failure. Comparing the values (x +/- SEM) obtained in group with notch and in group without notch, PAPs, PAPm, PVRI were higher (56 +/- 2.30 mmHg; 32 +/- 0.99 mm Hg; 543 +/- 56.8 dyn x s/cm5/m2 versus 32 +/- 1.08 mm Hg; 20 +/- 0.9 mm Hg; 173 +/- 14.7 dyn x s/cm5/m2) and CI and LVSWI were lower (2.6 +/- 0.17 l/min/m2; 25.8 +/- 2.41 g x m/m2; versus 3.8 +/- 0.26 l/min/m2; 38.3 +/- 2.82 g x m/m2) in group with notch. As it is shown by 11 paired measurements where the notch disappeared immediately after starting vasodilator therapy PAPs, PAPm, PVRI decreased (from 54 +/- 3.1, 35 +/- 0.8 mm Hg, 498 +/- 64.1 dyn x s/cm5/m2 to 35 +/- 0.8, 21 +/- 1.1 mmHg, 189 +/- 18.4 dyn x s/cm5/m2 respectively) and heart performance improved. Since the left ventricle contractility (characterized by EF, PCWP, ICT) was normal in both groups, our findings suggest that critically high PAPs values (over 40 mmHg) cause a septal bulging at the beginning of the systole which in turn narrows the left ventricle outflow tract. Regarding to the clinical importance of the deteriorated biventricular function at the critically high PAPs evidenced by notch phenomenon on carotid curve but measurable only by indwelling pulmonary arterial catheterization always being a source of infection, the noninvasive parameters as independent variables were entered into canonical discriminant analysis. The ratio of the correctly classified cases was 89%.     

Stress wave velocities in bovine patellar tendon.

The velocity of longitudinal stress waves in an elastic body is given by the square root of the ratio of its elastic modulus to its density. In tendinous and ligamentous tissue, the elastic modulus increases with strain and with strain rate. Therefore, it was postulated that stress wave velocity would also increase with increasing strain and strain rate. The purpose of this study was to determine the velocity of stress waves in tendinous tissue as a function of strain and to compare these values to those predicted using the elastic modulus derived from quasi-static testing. Five bovine patellar tendons were harvested and potted as bone-tendon-bone specimens. Quasi-static mechanical properties were determined in tension at a deformation rate of 100 mm/s. Impact loading was employed to determine wave velocity at various strain levels, achieved by preloading the tendon. Following impact, there was a measurable delay in force transmission across the specimen and this delay decreased with increasing tendon strain. The wave velocities at tendon strains of 0.0075, 0.015, and 0.0225 were determined to be 260 +/- 52 m/s, 360 +/- 71 m/s, and 461 +/- 94 m/s, respectively. These velocities were significantly (p < 0.01) faster than those predicted using elastic moduli derived from the quasi-static tests by 52, 45, and 41 percent, respectively. This study has documented that stress wave velocity in patellar tendon increases with increasing strain and is underestimated with a modulus estimated from quasi-static testing.     

Comparative study of plantar pressure during step exercise in different floor conditions

Mechanical load has been estimated during step exercise based on ground reaction force (GRF) obtained by force platforms. It is not yet accurately known whether these measures reflect foot contact forces once the latter depend on footwear and are potentially modified by the compliant properties of the step bench. The aim of the study was to compare maximal and mean plantar pressure (PP), and maximal GRF obtained by pressure insoles after performing seven movements both over two metal force platforms and over the step bench. Fifteen step-experienced females performed the movements at the cadences of 130 and 140 beats per minute. PP and GRF (estimated from PP) obtained for each floor condition were compared. Maximal PP ranged from 29.27 +/- 9.94 to 47.07 +/- 12.88 N/cm2 as for metal platforms, and from 28.20 +/- 9.32 to 43.00 +/- 13.80 N/cm2 as for the step bench. Mean PP ranged from 11.09 +/- 1.62 to 14.32 +/- 2.06 N/cm2 (platforms) and from 10.71 +/- 1.54 to 14.22 +/- 1.77 N/cm2 (step bench). GRF (normalized body weight) ranged from 1.43 +/- 0.14 to 2.41 +/- 0.24 BW (platforms) and from 1.38 +/- 0.14 to 2.36 +/- 0.19 BW (step bench). No significant statistical differences were obtained for most of the comparisons between the two conditions tested. The results suggest that metal force platform surfaces are suitable to assess mechanical load during this physical activity. The forces applied to the foot are similar to the softer step bench and the hard force platform surface. This may reflect the ability of the performers to adapt their movement patterns to normalize the impact forces in different floor conditions.     

Relationship between countermovement jump performance and multijoint isometric and dynamic tests of strength

The purpose of this investigation was to determine the relationship between countermovement vertical jump (CMJ) performance and various methods used to assess isometric and dynamic multijoint strength. Twelve NCAA Division I-AA male football and track and field athletes (age, 19.83 +/- 1.40 years; height, 179.10 +/- 4.56 cm; mass, 90.08 +/- 14.81 kg; percentage of body fat, 11.85 +/- 5.47%) participated in 2 testing sessions. The first session involved 1 repetition maximum (1RM) (kg) testing in the squat and power clean. During the second session, peak force (N), relative peak force (N x kg(-1)), peak power (W), relative peak power (W x kg(-1)), peak velocity (m x s(-1)), and jump height (meters) in a CMJ, and peak force and rate of force development (RFD) (N x s(-1)) in a maximal isometric squat (ISO squat) and maximal isometric mid-thigh pull (ISO mid-thigh) were assessed. Significant correlations (P < or = 0.05) were found when comparing relative 1RMs (1RM/body mass), in both the squat and power clean, to relative CMJ peak power, CMJ peak velocity, and CMJ height. No significant correlations existed between the 4 measures of absolute strength, which did not account for body mass (squat 1RM, power clean 1RM, ISO squat peak force, and ISO mid-thigh peak force) when compared to CMJ peak velocity and CMJ height. In conclusion, multijoint dynamic tests of strength (squat 1RM and power clean 1RM), expressed relative to body mass, are most closely correlated with CMJ performance. These results suggest that increasing maximal strength relative to body mass can improve performance in explosive lower body movements. The squat and power clean, used in a concurrent strength and power training program, are recommended for optimizing lower body power.     

Smith machine counterbalance system affects measures of maximal bench press throw performance

Equipment with counterbalance weight systems is commonly used for the assessment of performance in explosive resistance exercise movements, but it is not known if such systems affect performance measures. The purpose of this study was to determine the effect of using a counterbalance weight system on measures of smith machine bench press throw performance. Ten men and 14 women (mean ± SD: age, 25 ± 4 years; height, 173 ± 10 cm; weight, 77.7 ± 18.3 kg) completed maximal smith machine bench press throws under 4 different conditions (2 × 2; counterbalance × load): with or without a counterbalance weight system and using 'light' or 'moderate' net barbell loads. Performance variables (peak force, peak velocity, and peak power) were measured using a linear accelerometer attached to the barbell. The counterbalance weight system resulted in significant (p < 0.001) reductions in measures of peak force (mean difference ± standard error: light: -112 ± 20 N; moderate: -140 ± 23 N), peak velocity (light: -0.49 ± 0.10 m·s; moderate: -0.33 ± 0.07 m·s), and peak power (light: -220 ± 43 W; moderate: -143 ± 28 W) compared with no counterbalance system for both load conditions. Load condition did not affect absolute or percentage reductions from the counterbalance weight system for any variable. In conclusion, the use of a counterbalance weight system reduces accelerometer-based performance measures for the bench press throw exercise at light and moderate loads. This reduction in measures is likely because of an increase in the external resistance during the movement, which results in a discrepancy between the manually input and the actual value for external load. A counterbalance weight system should not be used when measuring performance in explosive resistance exercises with an accelerometer.     

Active drag related to velocity in male and female swimmers

Propulsive arm forces of 32 male and 9 female swimmers were measured during front crawl swimming using arms only, in a velocity range between 1.0 m s-1 and 1.8 m s-1. At constant velocity, the measured mean propulsive force Fp equals the mean active drag force (Fd). It was found that Fd is related to the swimming velocity v raised to the power 2.12 +/- 0.20 (males) or 2.28 +/- 0.35 (females). Although many subjects showed rather constant values of Fd/v2, 12 subjects gave significantly (p less than 0.01) stronger or weaker quadratic relationships. Differences in drag force and coefficient of drag between males and females (drag: 28.9 +/- 5.1 N, 20.4 +/- 1.9 N, drag coefficient: 0.64 +/- 0.09, 0.54 +/- 0.07 respectively) are especially apparent at the lowest swimming velocity (1 m s-1), which become less at higher swimming velocities. Possible explanations for the deviation of the power of the velocity from the ideal quadratic dependency are discussed.     

Force-frequency relationship and potentiation in mammalian skeletal muscle.

Repetitive activation of a skeletal muscle results in potentiation of the twitch contractile response. Incompletely fused tetanic contractions similar to those evoked by voluntary activation may also be potentiated by prior activity. We aimed to investigate the role of stimulation frequency on the enhancement of unfused isometric contractions in rat medial gastrocnemius muscles in situ. Muscles set at optimal length were stimulated via the sciatic nerve with 50-micros duration supramaximal pulses. Trials consisted of 8 s of repetitive trains [5 pulses (quintuplets) 2 times per second or 2 pulses (doublets) 5 times per second] at 20, 40, 50, 60, 70, and 80 Hz. These stimulation frequencies represent a range over which voluntary activation would be expected to occur. When the frequency of stimulation was 20, 50, or 70 Hz, the peak active force (highest tension during a contraction - rest tension) of doublet contractions increased from 2.2 +/- 0.2, 4.1 +/- 0.4, and 4.3 +/- 0.5 to 3.1 +/- 0.3, 5.6 +/- 0.4, and 6.1 +/- 0.7 N, respectively. Corresponding measurements for quintuplet contractions increased from 2.2 +/- 0.2, 6.1 +/- 0.5, and 8.7 +/- 0.7 to 3.2 +/- 0.3, 7.3 +/- 0.6, and 9.0 +/- 0.7 N, respectively. Initial peak active force values were 27 +/- 1 and 61.5 +/- 5% of the maximal (tetanic) force for doublet and quintuplet contractions, respectively, at 80 Hz. With doublets, peak active force increased at all stimulation frequencies. With quintuplets, peak active force increased significantly for frequencies up to 60 Hz. Twitch enhancement at the end of the 8 s of repetitive stimulation was the same regardless of the pattern of stimulation during the 8 s, and twitch peak active force returned to prestimulation values by 5 min. These experiments confirm that activity-dependent potentiation is evident during repeated, incompletely fused tetanic contractions over a broad range of frequencies. This observation suggests that, during voluntary motor unit recruitment, derecruitment or decreased firing frequency would be necessary to achieve a fixed (submaximal) target force during repeated isometric contractions over this time period.     

Optimal shear cushion stiffness at different gait speeds

The present study quantified the effects of different shear cushion stiffness on the time to peak posterior shear force (TPPSF), peak posterior shear force (PPSF), average posterior loading rate (APLR), and maximum posterior loading rate (MPLR) at different locomotion speeds using a custom-made sliding platform, as well as to identify the optimal stiffness of shear cushion. Twelve male collegiate students (heel-strikers) performed walking at 1.5 m/s, jogging at 2.5 m/s, and running at 3.5 m/s. A custom-made sliding platform was used to provide the different shear cushion conditions. The shear cushion conditions were fixed (a fixed platform; control group), stiff (K = 2746 N/m), medium stiff (K = 2256 N/m), medium soft (K = 1667 N/m), and soft (K = 1079 N/m). The results showed that all cushion conditions produced sliding displacement and delayed the TPPSF during walking, jogging, and running compared with fixed condition. The APLR and MPLR were lowest under medium soft condition during walking, while the PPSF was similar between medium soft and soft conditions. For jogging and running, the PPSF as well as APLR and MPLR were the lowest under medium stiff condition except the maximum PLR was similar among stiff, medium stiff, and medium soft conditions during running. In conclusion, shear cushion produces appropriate sliding displacement and effectively delays the TPPSF to provide the musculoskeletal system additional time to absorb the impact and reduce loading. The present study demonstrates optimal stiffness of shear cushion at different traveling speeds and suggests that a shear cushion system can be applied in future designs of cushion structures.     

Force-velocity analysis of strength-training techniques and load: implications for training strategy and research

The purpose of this study was to investigate the force-velocity response of the neuromuscular system to a variety of concentric only, stretch-shorten cycle, and ballistic bench press movements. Twenty-seven men of an athletic background (21.9 +/- 3.1 years, 89.0 +/- 12.5 kg, 86.3 +/- 13.6 kg 1 repetition maximum [1RM]) performed 4 types of bench presses, concentric only, concentric throw, rebound, and rebound throw, across loads of 30-80% 1RM. Average force output was unaffected by the technique used across all loads. Greater force output was recorded using higher loading intensities. The use of rebound was found to produce greater average velocities (12.3% higher mean across loads) and peak forces (14.1% higher mean across loads). Throw or ballistic training generated greater velocities across all loads (4.4% higher average velocity and 6.7% higher peak velocity), and acceleration-deceleration profiles provided greater movement pattern specificity. However, the movement velocities (0.69-1.68 m.s(-1)) associated with the loads used in this study did not approach actual movement velocities associated with functional performance. Suggestions were made as to how these findings may be applied to improve strength, power, and functional performance.     

Eight weeks of ballistic exercise improves power independently of changes in strength and muscle fiber type expression

This study investigated the effects of ballistic resistance training and strength training on muscle fiber composition, peak force (PF), maximal strength, and peak power (PP). Fourteen males (age = 21.3 +/- 2.9, body mass = 77.8 +/- 10.1 kg) with 3 months of resistance training experience completed the study. Subjects were tested pre and post for their squat one-repetition maximum (1RM) and PP in the jump squat (JS). Peak force and rate of force development (RFD) were tested during an isometric midthigh pull. Muscle biopsies were obtained from the vastus lateralis for analysis of muscle fiber type expression. Subjects were matched for strength and then randomly selected into either training (T) or control (C) groups. Group T performed 8 weeks of JS training using a periodized program with loading between 26 and 48% of 1RM, 3 days per week. Group T showed significant improvement in PP from 4088.9 +/- 520.6 to 5737.6 +/- 651.8 W. Rate of force development improved significantly in group T from 12687.5 +/- 4644.0 to 25343.8 +/- 12614.4 N x s(-1). PV improved significantly from 1.59 +/- 0.41 to 2.11 +/- 0.75 m x s(-1). No changes occurred in PF, 1RM, or muscle fiber type expression for group T. No changes occurred in any variables in group C. The results of this study indicate that using ballistic resistance exercise is an effective method for increasing PP and RFD independently of changes in maximum strength (1RM, PF), and those increases are a result of factors other than changes in muscle fiber type expression.     

Ambulation in simulated fractional gravity using lower body positive pressure: cardiovascular safety and gait analyses.

The purpose of this study is to assess cardiovascular responses to lower body positive pressure (LBPP) and to examine the effects of LBPP unloading on gait mechanics during treadmill ambulation. We hypothesized that LBPP allows comfortable unloading of the body with minimal impact on the cardiovascular system and gait parameters. Fifteen healthy male and female subjects (22-55 yr) volunteered for the study. Nine underwent noninvasive cardiovascular studies while standing and ambulating upright in LBPP, and six completed a gait analysis protocol. During stance, heart rate decreased significantly from 83 +/- 3 beats/min in ambient pressure to 73 +/- 3 beats/min at 50 mmHg LBPP (P < 0.05). During ambulation in LBPP at 3 mph (1.34 m/s), heart rate decreased significantly from 99 +/- 4 beats/min in ambient pressure to 84 +/- 2 beats/min at 50 mmHg LBPP (P < 0.009). Blood pressure, brain oxygenation, blood flow velocity through the middle cerebral artery, and head skin microvascular blood flow did not change significantly with LBPP. As allowed by LBPP, ambulating at 60 and 20% body weight decreased ground reaction force (P < 0.05), whereas knee and ankle sagittal ranges of motion remained unaffected. In conclusion, ambulating in LBPP has no adverse impact on the systemic and head cardiovascular parameters while producing significant unweighting and minimal alterations in gait kinematics. Therefore, ambulating within LBPP is potentially a new and safe rehabilitation tool for patients to reduce loads on lower body musculoskeletal structures while preserving gait mechanics.     

Effect of pelvis impact angle on stresses at the femoral neck during falls

Improved understanding is required of how the mechanics of the fall affect hip fracture risk. We used a hip impact simulator to determine how peak stresses at the femoral neck were affected by pelvis impact angle, hip abductor muscle force, and use of a wearable hip protector.We simulated falls from standing (2 m/s impact velocity) involving initial hip abductor muscle forces of 700 or 300 N. Trials were acquired for impact to the lateral aspect of the greater trochanter, and impact to the pelvis rotated 5°, 10° and 15° anteriorly (positive) or posteriorly (negative). Measures were acquired with and without a commercially available hip protector. During trials, we measured three-dimensional forces with a load cell at the femoral neck, and derived peak compressive and tensile stresses.Peak compressive stress increased 37% (5.91 versus 4.31 MPa; p < 0.0005) and peak tensile stress increased 209% (2.31 versus 0.75 MPa; p < 0.0005) when the pelvis impact angle changed from 15° anterior to −15° posterior. For lateral impacts, the peak tensile and compressive stresses averaged 73% and 8% lower, respectively, in the 700 N than 300 N muscle force condition, but the effect was reversed for anteriolateral or posteriolateral impacts. The attenuation in peak compressive stress from the hip protector was greatest for posteriolateral impacts (−15 to −5°; 36–41%), and least for anteriolateral (+15°; 10%).These results clarify the effects on hip fracture risk during a fall of pelvis impact angle and muscle forces, and should inform the design of improved hip protectors.     

Active responses decrease impact forces at the hip and shoulder in falls to the side.

Active responses, such as using the arm to break the fall, may be an effective means of decreasing likelihood of injury in a fall and may help explain why only a small percentage of falls result in a fracture. We quantified the impact force at the hip and shoulder in falls to the side from a kneeling position under three conditions: (1) attempting to break the fall by using an arm; (2) falling with the body relaxed; and (3) falling with the body tensed. Subjects fell from a kneeling position onto a force platform array covered with foam padding and impact force data were recorded. The ground reaction force-time curve was generally bimodal due to sequential impacts of the hip and shoulder. Impact forces at the hip and shoulder were 12 and 16% less for the slap condition (p < 0.05) than for the tensed condition. The impact forces for the relaxed and tensed conditions were not significantly different, although impact forces tended to be less in the relaxed condition. We concluded that active responses reduce the impact forces experienced at the hip and shoulder in falls to the side. Decreased effectiveness of protective responses, due to increases in reaction time and decreases in strength with age, may help explain why so many hip fractures occur in the elderly but so few occur in younger people.