In these trials, each participant performed a series of forward descents which resembled the downward portion of a push-up using a customized apparatus (see Appendix E). Each hand contacted a force plate (OR6-7, AMTI, Watertown, MA) that was mounted to a
rigid, adjustable frame (Figure 2). The descent task was similar to the eccentric lowering phase of a push up and was designed to replicate the post impact phase of a simulated forward fall. The descent task was first described to the participants, and then demonstrated by the researcher and practiced three times against the wall. The task involved positioning the individual at a given body lean angle, and using a repetitive auditory stimulus to cue the onset and speed of
their descent. This resulted in an approximate interval of one second between then start and end of the descent. Participants were told to start each trial with elbows in full extension without locking, shoulders flexed to 90° and hands shoulder width apart. They were encouraged to maintain a neutral spine and fully extend the knees without locking with feet maintained in the neutral positon and touching the foot plate. From the starting position, participants were told to lower themselves to 90° of elbow flexion while maintaining 30° of shoulder abduction (matching the strength testing protocol). If participants descended past 90° of elbow flexion the safety harness would stop the forward movement of their body to eliminate the chance of head
impact. Participants began at the easiest level of difficulty (60° body lean) and then progressed to 45° body lean and the most difficult, 30° body lean. This sequence was chosen rather than a randomized order to ensure safety of not attempting a more difficult descent that would risk
injury. The protocol consisted of one practice trial, followed by 3 repeated descents at each body lean angle, with one minute of rest between each repetition and a ten minute rest period between angles.
Before commencing the trials, measures were acquired of body height, weight and
limb length. Arm length was measured from acromion to wall while the participant was standing with the shoulders flexed to 90° with hands flat on the wall. The foot length was measured from the lateral malleolus to the end of the longest toe. Shoulder height was measured from the acromion to the floor when the participant was standing in their usual posture. The height and angles of the force plates and the standing platform were then adjusted based on these
measurements to ensure each participant’s torso was parallel to the force plates with ankles in a neutral position on the standing platform, hands touching the force plate and arms
fully extended for body lean angles of 30°, 45°, 60° from the horizontal. All participants wore a helmet with a full face guard and a safety harness secured to the ceiling by a tether that prevented contact of the head or torso with the force plates.
During each descent trial, an eight-camera, three-dimensional motion capture measurement system (VICON Nexus, VICON, Centennial, CO) was used to capture the positions of the surface markers at a sampling rate of 100 Hz. Forty-two reflective makers were used which enabled the calculation of 3D arm and body movement. These were located on the sides of the helmet, the front of the helmet, seventh cervical vertebra, tenth thoracic vertebra, fifth lumbar vertebra; and bilaterally at the acromion processes greater trochanters of the femurs, lateral condyles of the femurs, and lateral malleoli. Marker clusters were placed on the lateral distal shaft of the humerus and anterior proximal ulna. Surface markers were also placed over the sternum, and bilaterally over the lateral and medial epicondyles of the humerus and over the
radial and ulnar styloid processes for the purposes of calibration. Kinematic data were low pass filtered at a cut-off of 15 Hz using a 4th order Butterworth filter. Shoulder joint
centres were obtained through functional calibration methods 221 and arm kinematics were expressed using published standards 222. Hand contact force was obtained from the force plates for both the right and left hands throughout the descent at each angle. Both the EMG and force plate data were captured at a sampling rate of 2000 Hz using an analog to digital board
controlled by the motion capture system and were automatically synchronized to the kinematic data within the VICON software.
Electromyographic (EMG) data were recorded with a telemetered surface EMG system (Telemyo GT2400, Noraxon, Scottsdale, AZ). Muscle activity was recorded as mean amplitude value from the initiation to the cessation (peak elbow flexion) of the descent. Electrodes were placed on six muscle sites: anterior deltoid (AntDEL), pectoralis major (PM), triceps brachii (Long Head) (TRI), biceps brachii (BB), external oblique (EO) and internal
oblique/transversus abdominus (IO/TrA). The electrodes were placed unilaterally on the non- dominant side. We chose to measure the non-dominant side only as this is the weaker side in a bilateral closed chain activity 223 and to limit testing burden for the participants. EMG
signals were first recorded during three maximum voluntary contractions (MVC) using standard manual muscle testing positions 224-226. MVC tests were all performed by the same researcher. One minute of rest was given between repetitions and the researcher gave standard verbal encouragement throughout each muscle contraction. Peak MVC EMG amplitude for each muscle was used to normalize EMG amplitudes arising in the subsequent descent trials.
The biomechanical variables evaluated in this study were: peak energy absorption (ENRG), normalized to height and weight; maximum vertical force (VF) in Newtons (N), normalized for
body weight; maximum elbow flexion angle (FA) in degrees; and maximum elbow joint extensor moment (EM) in Nm/kg. ENRG is a measure of the total energy absorbed by upper limbs
and was calculated using the vector sum of the total force applied to the hands (measured by the force plate) and the displacement of the shoulder (mean of right and left sides) 29. VF was the maximum force observed during the descent. EM was calculated using standard inverse dynamics techniques and normalized by body weight. All data were calculated using custom software (Matlab, R2006b, Mathworks, Natick, MA).
A. B.
Figure 3.2 A. Starting (60° body lean) and B. end phase (45° body lean) of the descent on outstretched arms.