Biomechanics Wheelchair Propulsion

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Biomechanics 3 European School of Physiotherapy Semester 7 Movement Analysis - Manual Wheelchair Propulsion Sagittal view Tane Audi 500678472

Transcript of Biomechanics Wheelchair Propulsion

Page 1: Biomechanics Wheelchair Propulsion

Biomechanics 3

European School of Physiotherapy

Semester 7

Movement Analysis - Manual Wheelchair Propulsion

Sagittal view

Tane Audi

500678472

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Table of Contents Introduction _____________________________ 2 Whole Body Analysis _____________________ 4

GCM Calculation __________________ 4

GCM Displacement ________________ 6

Joint Angles ______________________ 7

External Forces ___________________ 8

Joint Analysis ___________________________11 Muscle activity___________________________ 16

Phase 1 __________________________ 16

Phase 2 __________________________ 17

Phase 3 __________________________ 18

Phase 4 __________________________ 19

Phase 5 __________________________ 20

Phase 6 __________________________ 21

Conclusion _____________________________ 24

References _____________________________ 25

Appendix _______________________________ 27

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Introduction For this project, I have decided to work and analyze the propulsion cycle of a wheelchair user. Manual WCP is a cyclic task that requires repetitive generation of propulsive forces on the pushrim of the WC. Generation of these reaction forces (RFs) applied at the pushrim involves coordinated activation of muscles responsible for simultaneously maintaining shoulder joint stability and controlling shoulder rotation. (Russell et al. 2015) Manual wheelchair users are at great risk for the development of upper extremity injury and pain. Any loss of upper limb function due to pain adversely impacts the independence and mobility of manual wheelchair users. (Sosnoff et al. 2015) While the exact relationship between the physical demands of wheelchair use and the development of shoulder pathology is not yet fully understood, ergonomics studies consistently suggest that there is a link between highly repetitive tasks and the occurrence of upper extremity pain and injury. (Santos et al. 2016) As a physical therapist it is important to know and understand the biomechanical reason behind the often seen injuries or complaints amongst wheelchair users, and the effect that proper training and wheelchair seat position has on patients. This movement analysis is very complex due to the number of joints involved, and most importantly, when looking into the shoulder, the number of bones and muscles involved. It is frequently speculated that propulsion biomechanics contributes to the pathogenesis of shoulder pathology. This speculation is based on simple Newtonian mechanics that forces applied to the wheelchair hand rim resulted in reactive forces acting on the shoulder that may over time lead to musculoskeletal damage. (Moon et al. 2013) The wheelchair propulsion cycle can be divided in two major phases:

1. Push phase 2. Recovery phase

(Moon Y et al. 2013)

The Push Phase starts when the hand makes contact with the push rim and then applies a propulsive force. The push phase makes up approximately 25% of the total cycle in a standard wheelchair. In the

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early part of the push phase, the hand is accelerating to the speed of the rim. A propulsive force is then applied through the hand rim creating a torque about the wheel axle. The Recovery Phase starts when the hands disengage from the push rim and return to initiate the next push phase. The recovery phase is about 65-75% of the propulsion cycle. During the recovery phase, the hand is much less constrained than during the push phase and can follow a number of different paths. The resulting hand patterns (i.e., full- cycle hand paths) are frequently classified into four distinct hand pattern types based on the shape of their projection onto the plane of the handrim (Slowik et al. 2016):

(Slowik et al. 2016)

Arcing (AR): The third metacarpophalangeal (MP) follows an arc along the path of the hand rim during the recovery phase.

● Single looping over propulsion (SL): the hands rise above the hand rim during recovery phase. ● Double looping over propulsion (DL): the hands rise above the hand rim, then cross over and

drop below the hand rim during the recovery phase. ● Semicircular (SC): the hands fall below the hand rim during recovery phase.

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Whole Body Analysis GCM Calculation In order to calculate the GCM a decision must be made in terms of what the analyzed body will be, whether I wanted to analyze the person individually, or the person and the chair as one object.

I decided to consider the person and the wheelchair as one, because of the fact that most wheelchair users are bound to them, and therefore consider the wheelchair part of their body; as well as the physical impact this has on the person and the body.

According to Karman et al. the center of gravity of an unoccupied wheelchair lies in the upper frontal quadrant of the wheel, as seen in the picture. In order to calculate the GCM of the occupied wheelchair, I made use of the construction method (Loozen. 2014) and translated the GCM of the individual and the GCM of the unoccupied wheelchair.

To find the GCM, I first had to calculate the PCM of the leg, the torso, and the arm, as well as the wheelchair. For these calculations, some measurements were necessary, including the weight of the individual: 60KG; and the weight of the wheelchair: 20KG.

Phase 1

The picture on the left shows the calculated PCM’s of the leg, torso, and arm; as well as the GCM of the wheelchair alone (red star), and the individual (green star). The yellow star represents the combined GMC, and therefore what will be used as the GCM for this analysis. The GCM lies more cranially and dorsally than that of a motionless wheelchair user due to the position of the arms. As seen in the picture on the right, the GCM lies slightly more ventral and caudal because the arms are moving in a posterior-anterior direction, shifting a portion of the weight of the body forward.

Phase 2

The GCM lies more ventrally than the previous phase due to the forward shift of weight because of the movement of the arms.

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Phase 3

The GCM lies in an almost unchanged position from the previous phase due to the fact that the weight of the body is the same, only the hands are no longer making contact with the handrim of the wheels.

Phase 4

The GCM lies slightly more dorsally than the last phase due to the changed position on the arms, and the backward movement of them.

Phase 5

During this phase the GCM has shifted more dorsally again, due to the backward position of the arms.

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Phase 6

At this point the GCM has reached its initial position, which is slightly more cranial than the last frame.

** The length of the line of gravity is estimated and tries to depict -800N that are acting on the GCM. This line is of equal length in each picture.

GCM Displacement

To find the translation of the GCM throughout the movement, I overlapped the images where the GCM was shown, and tracked the movement.

The photo above shows the GCM displacement in space throughout the propulsion cycle of a wheelchair user. Some images are very blurry because of the blending effect; however, the GCM of each phase is still visible. The displacement shows a forward translation, and minimal displacements cranially and caudally, this is because of the (very slight) forward bending of the torso as well as the movement in the arms.

The total forward displacement of the GCM in meters was: 1.37m / 1 sec (real time). Therefore the velocity of the displacement is 1.37m/1sec.

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Because of the constant speed at which the movement is happening, I can assume that the acceleration is 0. Joint Angles – Absolute angles *Angles of hip, knee and ankle will be neglected.

Phase Shoulder Elbow Wrist

295° - 294° Extension *Change in angle could be negligible

239° - 256° Flexion

193° - 192° Ulnar dev

294° - 350° Flexion

256° -215° Extension

192° - 157° Ulnar dev

350° - 352° Flexion

215°- 201° Extension

157°- 163° Radial dev

352° - 341° Extension

201°-207° Flexion

163° - 168° Radial dev

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341° - 314° Extension

207°- 218° Flexion

168° - 181° Radial dev

314° - 294° Extension

218° - 240° Flexion

181° - 186° Radial dev

I used the program Kinovea to calculate and trace the joint angles in each frame. All angles were calculated in a counterclockwise direction. Shoulder angle of frame 2 was mistakenly calculated in the opposite direction, so that angle was just subtracted from 360 in order to get the correct calculation. The change of angles in the lower limb, especially knee and ankle can be reasoned to be due because of the fact that the person in the wheelchair is not paralyzed and therefore unconsciously contracted muscles in her legs due to the effort that was needed to push the chair. These changes can be neglected. External Forces

(Hoffman. 2009) For this analysis I have decided to consider the individual and the wheelchair as one “object”, therefore I would like to use the example shown in the image on the right to depict the external forces generated on a wheelchair (and the user). There is a vertical ground reaction force acting on the wheels, a forward friction force (because the wheel is in turning backward direction -so against the friction force-), there is also an air (fluid) resistance, which, in my movement analysis I will ignore due to the relative slow speed at which the individual is propelling forward. In order to calculate how these external forces act on the individual, I need to know some details. The weight of the person (60KG), the weight of the wheelchair (20KG). By having these facts, I know that the force with which gravity is pulling on the GCM of the P+W (person and wheelchair) is -800N.

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The ground reaction force on the wheel is always the same, and always vertical. By combining the force of friction and the vertical ground reaction force we can assume the GRF would be slightly tilted forward in order for the wheelchair to be able to propel forward (or the wheel to turn backward without slipping on the surface). In this movement analysis I must focus instead on the reaction forces applied by the hand on the pushrim of the wheel. In order to understand the relationship between the forces applied on the pushrim by the hand, and the forces that the pushrim exerts on the hand I had to rely on a few articles that wrote about this specific subject (Bednarczyk et al. 1995, Robertson et al. 1996, Morrow et al. 2010). Despite the publication date of these articles, they are of high quality and very commonly referenced to in other scientific papers.

External (handrim) forces acting on the hand. Summation

of tangential and radial forces (Robertson et al. 1996) The pictures above and on the left display the forces the hand applies on the pushrim throughout the propulsion cycle, while the picture to the right display the pushrim reaction force throughout the same cycle (the equivalent of the GRF on a foot). The lengths of the lines that represent the magnitude of the forces that act on the hand are estimated based on the articles used. The picture above shows what an estimation of the external forces taken from Robertson et al (1996) would look like on my analysis.These forces would only be applied to the patient during push phase (black pushrims and red arrows). During recovery phase the only force acting on the arm (regardless of the specific joint analyzed) would be gravity (which in the picture is shown as the orange arrow)

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The picture below displays the forces and moment that Robertson et al used to analyze the propulsive stroke. Fx represents horizontal force, Fy vertical force; and Mz moment around the hub (or axis of the wheel). Fr represents the radial force, and Ft the tangential force. These forces and their paths are very important when analyzing the joints and considering the clinical impact of them. These will be reviewed in the next section.

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Joint Analysis

Shoulder Joint Analysis Free Body Elbow Joint Analysis Free Body Wrist Joint Analysis Free Body

For the joint analysis I have chosen to determine the free body parts as the body part distal from the joint that I am analyzing. This is because of the forces applied to the arm through the handrim.

In order to calculate the PCM of the arm in each frame, I made use of the displacement method (Loozen 2014) for PCM calculation. In this stance, and for calculation reasons, I am only including the weight of the person’s body, and neglecting the weight of the wheelchair, therefore I will use the GCM of the person alone as a reference point, and not the GCM of wheelchair and person combined.

Picture showing the mean external forces acting on the individual during push phase

The position of the PCM of the arm has slightly shifted in each frame due to the reduction of the weight of the rest of the body (purple star). The PCM of the arm shows a forward and vertical displacement across the cycle. The red arrows display the reaction force of the rim on the arm at the specific frame,

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while the orange arrows display the gravitational force acting on the arm during recovery phase; the black circles represent the wheel at which the hand is in contact with the pushrim. This force was approximated based on the study by Robertson et al. There are no external forces applied onto the arm when there is no contact with the pushrim. At this point the only force acting on the arm is gravitational force pulling the arm down.

The table below shows the calculated lever arms and moments in each frame of each phase.The total weight of the arm was calculated by finding 7% of the total body weight (60kg). The forearm and hand is equal to 3% of the total body weight, and the hand only 0.5%. All results are shown in the table.

The scale used to measure the lever arm 1:8 (size of the upper arm, forearm or hand) depending on the joint analyzed.

The formula used to calculate the force of gravity was: N% body weight below jointbody weight acceleration of gravity* = x

The formula used to calculate moment of gravity was: orce of gravity xternal lever arm x Nmf × e =

Frame Shoulder Joint

Total weight of arm = 7% total body weight = 4.2kg Force of gravity: 4.2 * -10m/s2 = -42N Moment of Gravity: FG * external lever arm= - x Nm

Elbow Joint

weight of forearm + hand= 3% total body weight = 1.8kg Force of gravity: 1.8 * -10m/s2 = -18N Moment of Gravity: FG * external lever arm= - x Nm

Wrist Joint

Total weight of hand = 0.5% total body weight = .3 kg Force of gravity: .3 * -10m/s2 = -3N Moment of Gravity: FG x external lever arm= - x Nm

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When the individual applies a moment to the pushrim, the moment at the hub is the sum of the hand moment and the tangential component transmitted from the hand to the pushrim (Mhub = Mzhand + Ft xR-1) and it can also be used to calculate net joint moments. (Bednarczyk et al. 1995, Robertson et al. 1996) Net joint forces and moments represent the overall forces seen across joint structures. They represent a combination of inertial factors, muscle contractions, and segmental weights.

The horizontal and vertical forces seen at the pushrim are the major contributors to the net joint reaction forces, and analysis of these forces at the pushrim reflects what is happening at the joints. The tangential and radial pushrim forces represent how effectively an individual applies force to the pushrim, therefore how efficiently they propel.

The stick figure represents the position of the arm, forearm, and hand when the peak vertical reaction force (mean value) was reached. The mean start position (x) and end position (o) are indicated. The peak value was reached at 62%. As it is visible in my screenshots, the peak force is different than the one shown on the analysis by Robertson et al. This can be assumed that is due to several reasons, including: 1. The wheelchair user in my video is not an experienced wheelchair user, 2. The wheelchair used is not designed to be pushed by the person sitting in it 3. The wheels were not pumped correctly, forcing the person to adapt the pushing technique for the wheelchair to follow a somewhat straight line, and 4. The ground was uneven and slightly tilted.

Robertson et al showed peak pushrim forces to average between 66 and 95N tangentially and 43 to 39N radially for a propulsion speed of approximately .75m/s.

According to Robertson et al, average peak shoulder, elbow, and wrist moments are -19.6, -12.3, and 5.8 Nm. Vertical forces at the pushrim averaged 57N. Veeger et al reported peak vertical forces between 88 and 152N for propulsion speeds between 1.11 and 1.67m/sec. Joint moments were between 22 to 36Nm for the shoulder, 5 to 10Nm for the elbow and 4 to 9Nm for the wrist. Robertson et al. analysis focused on tangential forces (Ft), the portion of the force driving the wheelchair forward, and radial forces (Fr), the portion of the force directed radially. Fr, although not involved in moving the wheelchair, is required to create friction between the hand and the pushrim. (Veeger et al. 1992 Shimada et al. 1998, Kootz et al. 2005) As it will be shown in the next sections, the values calculated in the analysis of the video used for this study vary dramatically from the ones found by Robertson et al and Veeger et al. This

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can be due to the limitations in image capturing and analyzing tools, as well as the reasons previously stated on the difference of peak forces.

From an efficiency standpoint, the greater proportion of force directed tangential to the pushrim, the greater the moment developed at the hub. Some radial force is required to provide friction such that a tangential force can be produced. However, people who apply large non-tangential forces will need larger total forces to maintain the same velocity. This has implications for injury meaning that if larger forces are required at the pushrim, then greater joint forces and moments are developed. Lower peak forces paired with longer stroke time decreases exposure of joints to harmful forces without decreasing speed. A rapid force at pushrim contact may expose joint structures to rapid rates of loading which will ultimately produce trauma. This implies that forces at the pushrim are the main contributor to the reaction forces seen at the joints. It is expected that at higher speeds the larger inertial component would increase the forces across the shoulder. The vertical forces the pushrim force is directed vertically with reference to the global coordinate system. These forces averaged 57N. Vertical forces, when transmitted to the shoulder, have a tendency to drive the head of the humerus into the acromion This upward driving force may place the shoulder at risk for the development of rotator cuff tears or impingement syndromes. (Robertson et al. 1996, Curtis et al. 1999, Morrow et al 2011)

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Muscle activity

Phase 1

Joint analyzed Shoulder Elbow Wrist

Angle Start 295 239 193

Angle End 294 256 192

Movement Extension Flexion Ulnar dev

Moment of Gravity Initial position: -9.24Nm Final position: -7.56Nm Counterclockwise: Flexion

Initial position: -.54Nm Final position: -1.26Nm Clockwise: Extension

Initial position: 0Nm Final position: 0Nm No moment of gravity

Moment PRF Extension Extension Radial dev

Moment Muscles Extension Flexion None

Monoarticular Muscles Flexors: Isometric Extensors: Isometric

Flexors: Concentric Extensors: Eccentric

Ulnar deviators: Isometric (change in angle can be neglected) Radial deviators: Isometric

Biarticular muscles Biceps: Concentric (shoulder is not moving [measurement can be neglected], but elbow is flexing) Triceps: Eccentric (the change of angle is negligible. The triceps is eccentrically contracting by extending the shoulder while the elbow is flexing)

Biceps: Concentric (elbow is flexing to a bigger extent than shoulder is extending) Triceps: Eccentric

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Discussion: The reason why biarticular muscles are considered to be acting concentrically and eccentrically as opposed to isometrically is due to the very small (negligible) change of angle in shoulder extension, but more drastic elbow flexion, leading us to believe that monoarticular shoulder extensors and flexors are acting isometrically to maintain the shoulder in its extended position throughout this phase. Biarticular muscles (biceps and triceps), unlike in other phases, are acting concentrically and eccentrically in this phase because of the elbow flexion. Wrist calculations were difficult to carry out due to the picture quality, but because of the very small change in angle it can be assumed that muscle contractions affecting the wrist are isometric. Due to the very small lever arm on the wrist joint, the moment of gravity is equal to 0.

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Phase 2

Joint Analyzed Shoulder Elbow Wrist

Angle Start 294 256 192

Angle End 350 215 157

Movement Flexion Extension Ulnar dev

Moment of Gravity Initial position: -7.56Nm Final position: -0.42Nm Counterclockwise: Flexion

Initial position: -1.26Nm Final position: -1.8Nm Clockwise: Extension

Initial position: 0Nm Final position: -0.03Nm Clockwise: Ulnar dev *Can be neglected because of the small pull

Moment PRF Extension Flexion Radial dev

Moment Muscles Extension Flexion Radial dev

Monoarticular Muscles Flexors: Concentric Extensors: Eccentric

Flexors: Eccentric Extensors: Concentric

Ulnar deviators: Concentric Radial deviators: Eccentric

Biarticular muscles Biceps: Isometric (Shoulder is flexing and elbow is extending) Triceps: Isometric (Shoulder is flexing and elbow extending)

Biceps: Isometric Triceps: Isometric

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Discussion: Based on the theory of biarticular muscles proposed by van Ingen Schenau (1990), the biarticular muscles are acting isometrically during this movement (it can be compared to a jumping leg) by transforming the large muscle actions into joint actions. The shoulder is flexing, and the elbow is extending, consequentially pushing the wheel forward. These biarticular muscles also are contracting isometrically when analyzing the elbow joint.

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Phase 3

Joint Analyzed Shoulder Elbow Wrist

Angle Start 350 215 157

Angle End 352 201 163

Movement Flexion Extension Radial dev

Moment of Gravity Initial position: -0.42Nm Final position: -2.94Nm Clockwise: Extension

Initial position: -1.8Nm Final position: -1.62Nm Clockwise: Extension

Initial position: -0.03Nm Final position: 0Nm Clockwise: Ulnar dev *Can be neglected because of the small pull

Moment PRF Flexion Flexion Radial dev

Moment Muscles Flexion Flexion Radial dev

Monoarticular Muscles Flexors: Isometric* Extensors: Isometric* *Because of the very small change in angle

Flexors: Eccentric Extensors: Concentric

Ulnar deviators: Eccentric Radial deviators: Concentric

Biarticular muscles Biceps: Eccentric Triceps: Concentric (the higher extension in elbow compared small flexion in shoulder leads to this conclusion)

Biceps: Eccentric Triceps: Concentric

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Discussion: Same reasoning applies to this phase as to the first phase. There is a very small change in angle on the shoulder joint, leading us to believe that it either is a miscalculation due to restricted tools and image quality, or simply the bigger change in angle at the elbow joint means that biarticular muscles crossing the shoulder and the elbow are acting concentrically and eccentrically as opposed to isometrically. In this case, the bigger extension movement at the elbow compared to the flexion movement at the shoulder leads us to believe that the biceps is contracting eccentrically and the triceps is contracting concentrically.

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Phase 4

Joint Analyzed Shoulder Elbow Wrist

Angle Start 352 201 163

Angle End 341 207 168

Movement Extension Flexion Radial dev

Moment of Gravity Initial position: -2.94Nm Final position: -2.1Nm Clockwise: Extension, then flexion

Initial position: -1.62Nm Final position: -0.9Nm Clockwise: Extension

Initial position: 0Nm Final position: 0Nm No moment of gravity

Moment PRF None None None

Moment Muscles Flexion, then extension Flexion No muscle moment

Monoarticular Muscles Flexors: Eccentric Extensors: Concentric

Flexors: Concentric Extensors: Eccentric

Ulnar deviators: Eccentric Radial deviators: Concentric

Biarticular muscles Biceps: Isometric (shoulder is extending but elbow is flexing) Triceps: Isometric

Biceps: Isometric (shoulder is extending but elbow is flexing) Triceps: Isometric

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Discussion: Biarticular muscles are isometrically contracting in this phase (following Ingen Schenau theory), allowing the shoulder to perform an extension movement and the elbow to flex. During this phase, the hand is no longer in contact with the pushrim, and therefore the only force acting on the shoulder, elbow and wrist joints is the pull of gravity and the muscle pull.

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Phase 5

Joint Analyzed Shoulder Elbow Wrist

Angle Start 341 207 168

Angle End 314 218 181

Movement Extension Flexion Radial dev

Moment of Gravity Initial position: -2.1Nm Final position: -8.4Nm Counterclockwise: Flexion

Initial position: -0.9Nm Final position: -0.18Nm Clockwise: Extension then flexion

Initial position: 0Nm Final position: -0.03Nm Clockwise: Ulnar dev

Moment PRF None None None

Moment Muscles Extension Flexion then extension Radial dev

Monoarticular Muscles Flexors: Eccentric Extensors: Concentric

Flexors: Concentric Extensors: Eccentric

Ulnar deviators: Eccentric Radial deviators: Concentric

Biarticular muscles Biceps: Isometric (shoulder is extended but elbow is flexing) Triceps: Isometric (shoulder is extended but elbow is flexing)

Biceps: Isometric (shoulder is extended but elbow is flexing) Triceps: Isometric (shoulder is extended but elbow is flexing)

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Discussion: During this phase biarticular muscles crossing the shoulder and the elbow are isometrically contracting due to the opposing movements of these two joints. The radial deviators of the wrist are concentrically contracted in preparation for the hand to come in contact with the pushrim.

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Phase 6

Joint Analyzed Shoulder Elbow Wrist

Angle Start 341 218 181

Angle End 294 240 186

Movement Extension Flexion Radial dev

Moment of Gravity Initial position: -8.4Nm Final position: -9.24Nm Counterclockwise: Flexion

Initial position: -0.18Nm Final position: -0.54Nm Clockwise: Extension

Initial position: -0.03Nm Final position: 0Nm Clockwise: Ulnar dev

Moment PRF None *Until final contact when there is an extension moment

None *Until final contact when there is a flexion moment

None *Until final contact when there is a radial dev moment

Moment Muscles Extension Flexion Radial dev

Monoarticular Muscles Flexors: Eccentric Extensors: Concentric

Flexors: Concentric Extensors: Eccentric

Ulnar deviators: Eccentric Radial deviators: Concentric

Biarticular muscles Biceps: Isometric (shoulder is extending but elbow is flexing) Triceps: Isometric (shoulder is extending but elbow is flexing)

Biceps: Isometric (shoulder is extending but elbow is flexing) Triceps: Isometric (shoulder is extending but elbow is flexing)

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Discussion: During this phase the arm is preparing to come back in contact with the pushrim, to start a new push cycle. At this time the monoarticular shoulder extensors, elbow flexors, and radial deviators are contracting concentrically, while the biarticular flexors and extensors crossing the shoulder and elbow are contracting isometrically.

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The anterior fibers of the deltoid, pectoralis major and biceps brachii act primarily during the push phase and contraction start towards the end of the recovery phase with load peaks at around 10% of the pushing phase; conversely, the activity of the triceps brachii is initially quite modest during push phase (acting as a synergist to the shoulder -providing stability-, and having a isometric contraction as an elbow extensor), and then it gradually increases during the recovery phase. The deactivation of the shoulder flexors occurs in the final pushing phase, where the muscles of the recovery phase begin to act: middle and posterior deltoid, subscapularis, supraspinatus and medium trapezius (the latter 3 muscles acting on the joint on different planes) (Dellabiancia et al. 2013).

Figure 5 –taken from ‘Early motor learning changes in upper-limb dynamics and shoulder complex loading during handrim wheelchair propulsion’ by Vegter et al– shows a typical example of the different muscle contributions that counteract the external moment around the glenohumeral joint for each of the three global axes. Around the global x-axis (sagittal plane), mainly the infraspinatus, subscapularis and biceps muscles are responsible for the ‘flexion’ moment, with smaller contributions of the coracobrachialis and pectoralis major. Around the global y-axis (frontal plane) the supraspinatus, subscapularis and biceps mostly account for the ‘adduction’ moment. The moment around the global z-axis (transverse plane) is mainly expressed by pectoralis major, biceps and coracobrachialis activity, but besides the external moment these muscles also have to counteract the vector components of the infra- and supraspinatus in this plane. DSEM stands for the Delft Shoulder and Elbow model, and is a finite element, inverse dynamic model describing musculoskeletal behavior of the upper extremity. (Vegter et al. 2015) The elbow joint is the least affected joint in the upper limb during wheelchair propulsion, due to the direction of the forces applied, the wrist and GH joint are more largely affected. (Vegter et al. 2015). It is known that wheelchair bound people often complain of wrist pain and injuries such as carpal tunnel syndrome (Sawatzky et al. 2015). The chart below (figure 6) is taken from ‘Early motor learning changes in upper-limb dynamics and shoulder complex loading during handrim wheelchair propulsion’ by Vegter et al., and it shows the mean force and power exerted per push and recovery phase in three different attempts, showing how with practicing the correct propulsion technique, power is exerted in a more efficient way, and forces are directed more proportionally.

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This chart also shows the activity of muscles throughout the main two phases, which gives relevant information to physiotherapists when it comes to training patients with SCI and teaching them how to push in their wheelchair. As it can be seen, the triceps and rotator cuff are the biggest actors during the cycle, together with other important muscles such as the pectoralis major and deltoideus. The fact that the arms are limited to a certain position during the push cycle also help understand the degree of the wear and tear injuries present in wheelchair users. The forces acting on the joints are always directed in the same direction, leaving little room for finding a way to prevent these injuries.

Arnet U. et al. (2012) showed in their study comparing hand cycling to wheelchair propulsion that the mean power of the forces exerted on the joints during motion were significantly smaller with the hand cycle than the conventional wheelchair.

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Conclusion Wheelchair propulsion is a very repetitive activity that many people need to perform in order to get around and complete their activities of daily living. Often times, people who rely on wheelchairs have disabilities that impair their muscle activity on important levels. The analysis performed for this course shows the importance of correct technique during wheelchair propulsion, as well as the need to always create training programs that are unique to the patient’s needs and capabilities. As it was stated before, the triceps and rotator cuff are the biggest actors during the wheelchair propulsion cycle, together with the pec major and deltoid. As physical therapists, it is very important to assess the strength of this group of muscles on patients after injury to create an appropriate strength rehabilitation plan together with a wheelchair mobility technique class plan to avoid early injuries, as well as bad propulsion habits that may lead to chronic injuries. The upper limb is not anatomically designed to undergo such repetitive activities on a daily basis, as well as to endure such forces that are exerted on it during propulsion. Although these findings have been known for a long time, more efforts should be made in order to improve the quality of life of wheelchair users, decrease their pain and physical complaints, and find a way to enhance their ambulation. The constant strain together with the high forces these muscles have to overcome to keep the wheelchair moving shines a light on the common injuries and complaints wheelchair users have. It should be interesting to have a study evaluate the forces needed to propel a wheelchair in day-to-day activities, (environments that are not ideal for wheelchair propulsion) to get a more realistic picture of what the anatomical structures of patients endure on a daily basis.

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References

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Appendix 1 - Moments of Gravity on each joint

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Appendix 2 - Lever arm and Moments of Gravity Calculations

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