The physical environment influences how and where a manual wheelchair is used. Richter et al. (2007b) define cross slope as the slope of a surface perpendicular to one’s path of travel. Sidewalks, pathways and roads have some degree of cross slope to drain water.
This subsection reviews research articles that examined specifically the kinetics and/or kinematic properties of propulsion on non-level surfaces. Several of the articles used the test items from some of the formal wheelchair skills programs to frame the study but did not report on outcomes of the programs therefore were included in this section as opposed to the wheelchair skills subsection. The non-level surfaces explored in these studies included wheelies, curb ascent, ramps, soft surfaces such as carpet and grass, and cross slopes.
Richter et al. (2007b) investigated the effect of cross slope on wheelchair handrim biomechanics. The data from this study indicates that more pushes are required to cover the same distance when on a cross slope and that the power required increased by a factor of 2.3 on a six-degree cross slope. Users must push harder on the downhill handrim and this increased loading may result in overuse injuries.
LaLumiere et al. (2013b) compared the effects of different rolling resistances on hand rim kinetics during manual wheelchair wheelies performed by people with a spinal cord injury (T12-C7) who had no history of shoulder pain. The rolling resistance (RR) was created by the surface on which the wheelie was performed; painted, high grade smooth composition board (NAT); five cm thick urethane soft yellow foam (LOW), 5-cm medium viscoelastic pink memory foam (MOD), and two five-cm high wooden blocks with rear wheels completely blocked (HIGH). The wheelie was analyzed in four phases; preparation, take-off, balance and, landing. Findings indicate that the HIGH RR was the least desirable surface for performing wheelies. The HIGH RR produced the greatest mean and total hand rim forces at all phases, showed a forward force application to lift the casters off the ground whereas all others used a quick backward force. The authors also found that the take-off phase mean and maximum resultant forces and mean and maximum of the tangential components of the resultant forces were greater than all other phases regardless of the RR. The authors conclude that completing wheelies with the rear wheels blocked requires different motor learning strategies than on the other surfaces. Symmetry between dominant and non-dominant upper extremities was also evaluated in this study, with the findings suggesting that exertion forces are symmetrical in each phase. However, during the balance phase, the direction of the exerted forces differed on the NAT and LOW surfaces with the different direction oscillating between the dominant and non-dominant upper extremities to maintain balance. The authors reported looking to another study (Boninger et al 1999) in which the same propulsion forces were used to compare propulsion forces to the forces required to complete a wheelie on the NAT surface. From this comparison, they conclude that the forces are similar between these two skills, and that given the frequency of propulsion compared to performing wheelies, wheelies may represent a decreased risk to UL’s versus propulsion. However, the authors did not expand on this comparison so it is questioned that if the intensity of these two skills are the same, would an increase in the frequency of wheelies result in a similar risk exposure as propulsion.
LaLumiere et al. (2012a) compared movement strategies (kinematics), mechanical loads (kinetics) and relative muscle demands on the non-dominant side while 15 people with paraplegia ascended curbs of four, eight and 12 cm heights; participants propelled a three-metre approach at a self-selected speed. The authors hypothesized that the mechanical loads and muscular demands, especially at the shoulder would increase as curb height increased. The curb ascent was divided into the phases of caster pop-up, rear wheel ascent and post ascent phases. The authors report that the greatest net joint moment for all curb heights was shoulder flexion, closely followed by shoulder internal rotation and elbow flexion, which were corroborated by their EMG results. This study found limited elbow extension effort with this skill of curb ascension; in fact, the elbow flexors (long head of biceps) were used to succeed with ascending curbs. The muscle utilization ratio (MUR) at the pectoralis major, anterior deltoid and biceps brachii indicate these muscles contribute highly to these primary moments involved in ascending curbs. The moment demands placed on the shoulder and elbow joints progressively increased from a four to 12 cm curb, specifically 2.2 times for shoulder flexion and internal rotation, 2.8 times for shoulder adduction and 1.8 times for elbow flexion. Similarly, the muscle demands as measured by EMG, increased as the curb height progressively increased. Considering the substantial shoulder and elbow demands with this task found in this study, the authors suggest that it is plausible that a decreased strength-generating capability at the shoulder flexors/adductors or at the elbow flexors could increase the mechanical demand and increase risk of musculoskeletal injury. The authors also found that forward trunk flexion increased as the curb height increased, suggesting that the forward momentum created by flexing the trunk and head in the direction of movement assisted in the second phase of rear wheels ascending the curb. The authors do report that the possible contributions of using forward trunk flexion were not fully examined in this study but they do propose there is benefit to include trunk flexion strategies in curb ascent training to augment the increasing demands on the shoulders and elbows as the curb height increases. Based on this study’s findings, the authors highlight clinical implications for injury prevention focused on 1) the individual and optimizing strength at shoulder flexors, shoulder adductors, and elbow flexor muscles, and determining the ability to use forward trunk flexion and 2) the environment by continuing to advocate for barrier free environments to decrease upper extremity risk exposure.
Marchiori et al. (2014) examined the joint angle and velocity during obstacle ascent in a manual wheelchair by 11 people up an 8 cm curb. Findings suggest increases in peak moments in the wrist, elbow and shoulders compared to propulsion, although their study did not measure level propulsion. Forward trunk flexion during the caster pop phase was stated to be supported by other study results, suggesting forward trunk flexion during this phase may reduce upper extremity strain, but this study did not provide supporting data.
Nagy et al. 2012 examined the pushrim forces during various advanced manual wheelchair skills compared to forces exerted during propulsion over a 10-metre tile surface. Advanced skills tested were from the Wheelchair Skills Test developed at Dalhousie University, which included; 10 meters of carpet, a soft surface, 5° and 10° ramps and 2 cm, 5 cm, and 15 cm curbs. The primary finding that the more advanced the skill the more force required. The authors note an increase in forces ranging from 18 to 130% but do not provide details of calculations. Discussion in this article focuses on the need to consider the forces being exerted during advanced wheelchair skills and the need to preserve upper extremity integrity through minimizing repetitive forces. However, the authors did not note if the participants were experienced with basic or advanced wheelchair skills nor the potential influence of skill experience on the forces exerted during the skills measured. The authors also did not discuss the implications or the need to balance minimizing the impact of pushrim forces with maintaining an active lifestyle or to the impact of wheelchair set-up/technique on the force.
Hurd et al. (2008) examined the symmetry of propulsion across a variety of terrains, for people with paraplegia (11 SCI, T4-L10, and one spina bifida). Findings indicated that propulsion asymmetries exist for all conditions with the magnitude of the difference being affected by the environment/terrain. Outdoor condition had the greatest magnitude of propulsion asymmetry. No differences were found in the magnitude between laboratory (tile floor and dynamometer) and indoor community conditions. The authors note that their results could not explain these differences but they question the effect of fatigue on the results as the outdoor conditions were completed in one continuous pathway to simulate actual outdoor conditions, whereas the others were single testing conditions with rests in between. Dominance did not appear to have a role as no patterns of dominant versus non-dominant upper limb use during propulsion was detected for any condition. For these reasons the authors caution the use of single or averaged bilateral data for propulsion based studies. The authors also highlight that these results, despite the limitations, underscore the need to complete propulsion evaluations and training in the person’s own natural environments to fully understand propulsion kinetics and kinematics.
Morrow et al (2010) examined intersegmental shoulder forces and moments during everyday propulsion activities for daily life and mobility. Findings indicated that forces and moments vary significantly across the conditions used to simulate daily life and mobility activities. Not surprisingly, the weight relief condition (push up and hold for three seconds) produced significantly higher shoulder forces than the level, start and stop propulsion conditions. The magnitude of forces at the shoulder was highest for the weight relief followed by the ramp condition in most directions of force. The weight relief maneuver resulted in a peak superior direct force two times greater than the magnitude of ramp propulsion and three times the magnitude of level propulsion. The authors suggest that the findings indicate that the weight relief maneuver and propelling up a ramp are very high loading activities compared to level propulsion and as such the frequency of these high loading activities needs to be considered as part of maintaining shoulder health. Regarding shoulder moments, most shoulder moments during ramp propulsion and start conditions were equivalent but higher than level propulsion. Extension and abduction moments were higher in ramp propulsion, weight relief and start conditions compared to level propulsion. The authors suggest these findings are indicative of weight relief, ramp propulsion and start conditions placing the largest estimated loads on the shoulder during propulsion.
Gagnon et al. (2014) examined the spatiotemporal propulsion cycle and push rim kinetics of the non-dominant hand during manual wheelchair propulsion in 18 people with spinal cord injury on a level surface and up four different slopes on a wheelchair treadmill. The slopes chosen corresponds to a 1:20, 1:16, 1:12 and 1:8 ratio of vertical height to horizontal length of the slope, similar to standards for ramps. Overall, they found that the push phase remained relatively the same on all slopes however, the recovery phase became shorter as the slope increased, with the recovery phase at the level surface being significantly longer than the slopes (54% – 70%). Therefore, as the authors suggest, the pushing frequency increases to offset the gravitational effect of the slope on the wheelchair. The initial contact on the rim moved forward with increasing slope and contact angle remained similar on the slopes equal to or greater than 3.6⁰. The authors question if the contact angle results are related to the forward flexion of the trunk during propulsion on slopes which was explored in their 2015 study (see below). Forces applied to the push rim increased as the slope increased, 200% at the greatest slope, however no similarities between the slopes were found which, the authors suggest, indicates the relationship between slope and push force is not linear. The authors suggest that these findings support the need for ramps with smaller slopes (2.7⁰ or 3.6⁰ which correspond to 1:20 and 1:16 respectively) as these slopes require similar effort and the greater slopes of 1:12 (4.8⁰) require greater effort, use greater forces, require more frequent push phases therefore have greater implications for shoulder integrity maintenance.
Gagnon et al. (2015) also examined the kinematic changes of the trunk and non-dominant shoulder in 18 people with spinal cord injury during manual propulsion up five different slopes (0°, 2.7°, 3.6°, 4.8° & 7.1) at a self-selected speed. All participants could maintain their self-selected propulsion speed of 1.17±0.18 m/s on the level surface and the 2.7° slope but only 88.9%, 77.8% and 55.6% were able to maintain it on the 3.6°, 4.8° and 7.1° slopes respectively. Forward trunk flexion, peak shoulder flexion, and shoulder mechanical and muscular efforts all increased as the slope increased. The authors suggest that the forward trunk flexion in conjunction with the forward trunk excursion may assist in moving the centre of mass anteriorly to prevent backward tipping as the slope increased. The authors also suggest that the increase in shoulder flexion but comparable flexion excursion across all slopes may be related to the need to accommodate at the shoulder for the forward trunk flexion. Additionally, the muscular and mechanical demands of the shoulder, particularly of the posterior deltoid muscle at the end of the push phase, also increased as the slope increased. The authors suggest that these finding support the clinical practice of high-intensity, short duration strength training for the upper extremity, especially the shoulders, to reduce the risk of shoulder integrity issues.
Pierret et al. 2014 examined the cardiorespiratory effect and perceived strain experienced by 25 men who sustained a thoracic or lumbar spinal cord injury during manual wheelchair propulsion on cross slopes of zero, two, eight and 12%. They found that cross slopes of zero percent and two percent did not differ in cardiorespiratory and subjective strains but that the 8% cross slope was found to have significant effects on cardiorespiratory strain and perceived strain but all participants were able to manage; not all participants were able to manage the 12% cross slope.
There is level 4 evidence (from one case series study; Richter et al. 2007b) that wheeling cross slope results in increased loading on users’ arms and may lead to overuse injuries.
There is level 4 (from one case series study by Nagy et al. 2012) evidence that advanced wheelchair skills require greater peak forces at the hand rim, however there is level 4 (from one cross sectional repeated measures study by LaLumiere et al. 2013b) evidence that wheelies require a mean peak hand rim force similar to that of wheelchair propulsion.
There is level 4 (from one cross sectional repeated measures study by LaLumiere et al 2013a) evidence that ascending curbs of increasing height increases the mechanical and muscular demands at the shoulder and elbow joints placing these joints at risk of injury especially if adequate strength in the associated muscles is not present.
There is level 4 (from one case series study by Hurd et al. (2008)) evidence upper limb asymmetries exist in manual wheelchair propulsion with greater asymmetry in outdoor versus laboratory (tile floor and dynamometer) conditions.
There is level 4 (one case series study by Morrow et al. 2010) evidence that the daily life and mobility activities of weight relief, ramp propulsion and the start phase of propulsion place the larger estimated loads on the shoulder and use greater shoulder abduction and extension moments compared to level propulsion.
There is level 4 evidence (from one pre-post study; Pierret et al. 2014) that suggests the physiological demands of propulsion increase with increasing cross slopes beyond 2%, and that slopes greater than 8% significantly pose significant challenges both physiologically and physically.
Wheeling cross slope can negatively affect the cadence and power that is required for wheelchair propulsion.
The strength of specific shoulder and elbow muscles, and the ability to flex the trunk forward all affect the efficiency in performing advanced wheelchair skills particularly those associated with wheelies and caster pop-ups. Given the increased mechanical and muscular demands in these types of advanced skills, the quality of shoulder, elbow and trunk movements should be considered to balance protection of the upper extremity shoulder with being functional in the community.