Effect of movement strategy on injury risk in forward falls from standing height: an experimental investigation in young males

Track 3. Musculoskeletal systems and Performance - Joint ISB/ESB Track

7293 Tu, 14:45-15:00 (P21) Effect of movement strategy on injury risk in forward falls from standing height: an experimental investigation in young males J.-H. Lo, A. Mathias, J. Ashton-Miller. Biomechanics Research Laboratory,

University of Michigan, Ann Arbor, Michigan, USA We measured the peak hand and knee impact forces in 12 healthy young males arresting forward falls from standing height to a gymnastic mat overlying instrumented force plates. The effects of four different subject instruction sets: "arrest the fall naturally"; "impact the hands only"; "impact the hands and knees simultaneously"; and "impact the knees before the hands" were studied by measuring body segment kinematics, ground reaction forces, and upperextremity myoelectric activity. The (null) hypothesis was tested that segmental movements during the descent phase of forward falls do not affect peak knee and wrist impact forces. The results showed in their 'Natural' trials young male subjects impacted the ground with knees and wrists simultaneously (within 50ms time difference) in most (58%) of their trials. Only when knee impact occurred earlier than wrist impact by 200 ms ('Knee then hands' trials) was the wrist impact force significantly less (465 N±168 N, at least 28% less, rm-ANOVA, p=0.005) than the other strategies with simultaneous or later knee impacts. However, this earlier knee impact did not reduce the wrist impact speed significantly. Changes in wrist impact velocity were therefore not the major mechanism responsible for this reduction in wrist impact force in the 'Knees then hands' strategy. Instead, the greater knee impact, up to 200 greater elbow flexion angle, and up to 20% less triceps activation upon wrist impact, are among the factors responsible. On the other hand, the increase in knee impact force associated with increasing knee impact velocity, and the time difference between the wrist and knee impacts primarily determined the magnitude of the knee impact force (Knee: R 2 = 0.80, p = 0.0001 ; Wrist: R 2 = 0.30, p < 0.005). These insights can be useful in designing a forward fall arrest strategy aiming at reducing the wrist and knee impact severity. Acknowledgements: NIH P30 AG08808 5223 Tu, 15:00-15:15 (P21) Matching the mechanics to the task: How endpoint stiffness influences arm posture selection E.J. Perreault 1,2, B.-S. Yang 2. 1Northwestern University, Chicago, IL, USA, 2Rehabilitation Institute of Chicago, Chicago, IL, USA The human motor system is highly redundant, having more kinematic degrees of freedom (DOF) than necessary to complete a given task. Upper limb redundancy implies that an infinite set of arm postures can be selected to position the hand in the workspace. Understanding how these kinematic redundancies are utilized across different tasks remains a fundamental question in motor control. Our hypothesis is that postures are selected to match the mechanical properties of the arm to the functional constraints of the task. Specifically, we explored upper limb tasks that differed in their stability requirements, but not in their net force requirements. Two protocols were conducted. The first explored how arm posture influences endpoint tracking in unstable haptic environments. Environments were simulated using a 3DOF robotic manipulator. Each had the characteristics of a negative stiffness (unstable) along one degree of freedom. Subjects were instructed to accurately track a visual target corresponding to small hand movements (± 1 cm) along the direction of instability. Three unstable directions were simulated: horizontal in the sagittal and frontal planes, and vertical. The hand was positioned in front of the sternum during all tasks. Trunk position and orientation were fixed. Tracking performance was assessed at prescribed and self-selected hand positions; subjects self-selected hand and arm orientation. The second experiment involved quantifying 3DOF endpoint stiffness, the relationship between displacements imposed at the hand and forces generated in response, at the self-selected postures used during the tracking tasks. We found that arm posture could significantly affect the ability to track endpoint position. For conditions in which posture influenced tracking performance, all subjects self-selected similar postures. The stiffness measurements at these self-selected postures showed that each oriented the direction of maximal endpoint stiffness towards the direction of the environmental instability. These novel results suggest that when limb posture has an effect on task performance, subjects self-select postures that match the mechanical properties of the limb to the task requirements. 5457 Tu, 15:15-15:30 (P21) Moment-generating capacity of upper limb muscles K.R.S. Holzbaur 1,2, S.L. Delp 1,2, W.M. Murray 2. 1Bioengineering Department,

Stanford University, Stanford, CA, USA, 2Bone and Joint Center, VA Pale Alto HCS, Pale Alto, CA, USA The purpose of this study was to determine the moment-generating capacity of muscles crossing the shoulder, elbow, and wrist, and to assess whether the

3.1. Joints - Upper Extremity Injury

$85

variability in maximum moments produced by different subjects is explained by differences in their muscle volumes. Maximum isometric moments about these joints were measured in 10 young, healthy subjects (5 male, 5 female), ranging in size from a 20 th percentile female to a 97 th percentile male. The volumes of 32 upper limb muscles were determined from magnetic resonance images of these same subjects. Muscles were grouped according to their primary function; the sum of the muscle volumes for a functional group was compared to the corresponding maximum moment using linear regression analysis. The maximum isometric moments produced by shoulder adductors (67.9±28.4 Nm) were largest, and were approximately 6.5 ( ± 1.2) times greater than those produced by wrist extensors (10.2±4.6 Nm), which generated the smallest moments. Across subjects, isometric moment significantly covaried with total muscle volume of the appropriate functional group. Muscle volumes explained between 95% (p < 0.0001; shoulder adductors) and 66% (p = 0.003; wrist flexors) of variation in maximum isometric joint moments among subjects. We conclude that differences in isometric strength among healthy young adults are explained primarily by differences in muscle volume. This study is the first to evaluate the moment-generating capacity of muscles about the shoulder, elbow, and wrist in a group of subjects with known muscle volumes. Applications of these data include improved parameter estimation for musculoskeletal modeling, creation of subject-specific models, and a normative basis for evaluating muscle function differences in other subject groups. 5462 Tu, 16:15-16:30 (P24) Contributions of the rotator cuff muscles to glenohumeral joint mechanics during the belly-press clinical exam T. Yanagawa 1, M.R. Torry 1, K.B. Shelburne 1, M.G. Pandy 2. 1Steadman-

Hawkins Research Foundation, Vail, Colorado, USA, 2Department of Mechanical and Manufacturing Engineering, University of Melbourne, Melbourne, Australia The "belly-press test" is often used to help diagnose a subscapularis muscle tear in the clinic. A patient is asked to place his or her hand on the abdomen region and press that region by rotating the arm internally with the elbow positioned anterior to the coronal plane. If the elbow is moved posteriorly to that plane, the test is considered positive. This study was conducted to investigate the contributions of individual muscles to internal rotation torque at the glenohumeral joint. We hypothesized that no other muscles would be able to compensate for the loss of the subscapularis during this test. An upper extremity computer model (Garner and Pandy, 2001) was used to estimate forces in the muscles spanning the glenohumeral joint during a simulated belly press. In the simulations, the hand was placed on the abdomen and an external force was applied to the hand anteriorly. Static optimization was used to solve for muscle forces by minimizing the sum of the squares of muscle stresses while maintaining either the negative or positive belly-press positions. The clavicular head of pectoralis major had the largest internal moment arm of 39mm. However, the subscapularis muscle produced the largest torque (3.6Nm) with the moment arm of 21 mm in the negative position. In this position, the model could sustain the external forces of up to 93 N and 5 N with and without the subscapularis respectively, indicating that the subscapularis was the major contributor during this test. In the positive position without the subscapularis, the maximum external force was 18 N. In conclusion, the subscapularis muscle is the major contributor to internal rotation torque during the belly-press test. Our results provide theoretical evidence that the belly-press test is an appropriate exam for a subscapularis muscle tear. References Garner and Pandy (2001). CMBBE 4(2): 93-126.

4987 Tu, 16:30-16:45 (P24) Changes in the moment arm of forearm muscles in a disrupted distal radioulnar joint T.L. Lai 1, B.E Pereira 1, S.~ Chong 1, J.K.S. Low 1, ~Bt. Mohamad-Eusope 1, D. Hutmacher 2. 1Musculoskeletal Research Laboratories, Dept of Orthopaedic

Surgery and 2Division of Bioengineering, National University of Singapore, Singapore A rupture of the distal radioulnar ligaments is often not easily diagnosed unless it becomes symptomatic. In which case, it might already result in instability at the distal forearm. More common are the palmar or dorsal subluxation of the radius relative to the ulna, and this affects the axis of rotation, and subsequently the moment arms of the muscles that cross this axis during forearm rotation. This study investigates the moment arm of various forearm muscles during forearm rotation, and assesses the effect of cutting the distal radioulnar ligaments on the moment arm of various forearm muscles during pronation and supination. In six fresh frozen cadaveric upper extremities, excursions of the flexor carpi radialis (FCR), palmaris Iongus (PL), extensor carpi radialis brevis