How Can Biomechanics Improve Physical Preparation and Performance in Paralympic Athletes? A Narrative Review

ent research in Paralympic biomechanics has offered opportunities for coaches, athletes, and sports practitioners to optimize training and performance, and recent systematic reviews have served to summarize the state of the evidence connecting biomechanics to Paralympic performance. This narrative review serves to provide a comprehensive and critical evaluation of the evidence related to biomechanics and Paralympic performance published since 2016. The main themes within this review focus on sport-speci c body posture: the standing, sitting, and horizontal positions of current summer Paralympic sports. For standing sports, sprint and jump mechanics were assessed in athletes with cerebral palsy and in lower-limb amputee athletes using running-speci c prosthe- ses. Our ndings suggest that running and jumping-speci c prostheses should be `tuned'to each athlete depending on specific event demands to optimize performance. Standing sports were also inclusive to athletes with visual impairments. Sitting sports comprise of athletes performing on a bike, in a wheelchair (WC), or in a boat. WC configuration is deemed an important consideration for injury prevention, mobility, and performance. Other sitting sports like

jumping-speci c prostheses should be `tuned'to each athlete depending on specific event demands to optimize performance. Standing sports were also inclusive to athletes with visual impairments. Sitting sports comprise of athletes performing on a bike, in a wheelchair (WC), or in a boat. WC configuration is deemed an important consideration for injury prevention, mobility, and performance. Other sitting sports like hand-cycling, rowing, and canoeing/kayaking should focus on specific sitting positions (e.g., arm-crank position, grip, or seat configuration) and ways to reduce aero/hydrodynamic drag. Para-swimming practitioners should consider athlete-specific impairments, including asymmetrical anthropometrics, on the swim-start and free-swim velocities, with special considerations for drag factors. Taken together, we provide practition- ers working in Paralympic sport with specific considerations on disability and event-specific training modalities and equipment configurations to optimize performance from a biomechanical perspective. Keywords:kinematics; prostheses; classi cation; amputee; cerebral palsy 1. Introduction In 2016, 176 countries and more than 4000 athletes competed at the Paralympic Games. These athletes remain signi cantly understudied compared to Olympic athletes, especially with regards to the role the eld of biomechanics can serve in improving physical prepa- ration and performance. Recently, Morriën et al. [1] conducted a systematic review of biomechanical studies in Paralympic research consisting of 41 articles published before July 2016, showing that the majority of the included studies contribute to our understanding of technical optimization, injury prevention, and evidence-based classi cation. Because of the nature of the systematic review itself, a critical analysis of the 41 biomechanical studies was not included. Here, we serve to update this review and to examine the im- pact that speci c biomechanical interventions may have on Paralympic performance. We construct this narrative review by considering Paralympic athletes and their speci c impair- ment(s) and their sport's speci c body posture: standing, sitting, and horizontal positions (Table). We summarize the newest biomechanical evidence related to these athletes and their physical preparation, performance, and potential use of technological innovation. We then offer practical suggestions for coaches, Paralympic athletes, and sports practitioners to optimize Paralympic athlete performance. Sports2021,9, 89.

speci c body posture: standing, sitting, and horizontal positions (Table). We summarize the newest biomechanical evidence related to these athletes and their physical preparation, performance, and potential use of technological innovation. We then offer practical suggestions for coaches, Paralympic athletes, and sports practitioners to optimize Paralympic athlete performance. Sports2021,9, 89.

Sports2021,9, 89 2 of 15 Table 1.2020(1) Paralympic sports, sport-speci c body posture, and impairment(s). SPORT POSTURE IMPAIRED IMPAIRED LIMB LEG LENGTH INTELLECTUAL INVOLUNTARY MUSCLE UNCOORDONATED SHORT VISION MUSCLE POWER PASSIVE ROM DEFICIENCY DIFFERENCE IMPAIRMENT MOVEMENTS TENSION MOVEMENTS STATURE IMPAIRMENT Archery STANDING SITTING x x x x x x Athletics STANDING x x x x x x x x x x SITTING Badminton STANDING SITTING x x x x x x x x Boccia SITTING x x x x x x Canoe Sprint SITTING x x x Cycling SITTING x x x x x x x x Equestrian SITTING x x x x x x x x x Football 5-a-side STANDING x Goalball STANDING x Judo STANDING x Powerlifting SUPINE x x x x x x x x Rowing SITTING x x x x x x x Shooting STANDING SITTING x x x x x x x Swimming SUPINE x x x x x x x x x x Table Tennis STANDING SITTING x x x x x x x x x Taekwondo STANDING x x x x x x x Triathlon STANDING SITTING x x x x x x x Wheelchair Basketball SITTING x x x x x x x Wheelchair Fencing SITTING x x x x x x Wheelchair Rugby SITTING x x x x x x Wheelchair Tennis SITTING x x x x x x x

Sports2021,9, 89 3 of 15 2. Standing Standing postures are part of many Paralympic events (Table). We focus our narrative on the main themes of biomechanics research published since 2016: sprinting and jumping with prostheses, sprinters with cerebral palsy, and considerations for athletes with a visual impairment. We recognize that other standing postures may be omitted here given the lack of biomechanics-related research published since 2016. 2.1. Sprinting Biomechanics: The Basics Running speed is the product of step length and step frequency. These changes in kinematics are produced as the result of changes in kinetic quantities. Step length increases by applying greater horizontal and vertical ground reaction forces during ground contact, which increases a runner's horizontal and vertical takeoff velocities and thus increases the aerial time and the horizontal distance covered by the runner's centre of mass [2,3]. Step frequency increases are achieved by reducing foot–ground contact time and/or the time taken to reposition the swinging limb for the next step. 2.2. Amputee Mechanics Unilateral and bilateral lower limb amputee runners use running-speci c prostheses made of carbon- bre sockets and blades attached in-series with the residual limb(s). The blade works as a leaf spring to store and release elastic energy during running [4], returning up to 95% of the mechanical energy stored in them [5]. Unlike running-speci c prostheses, lower limbs `return' greater than 100% mechanical energy since active muscle contraction contributes to positive joint work [6]; thus, compared to an intact lower limb, running- speci c prostheses are disadvantaged with regard to energy storage and return. The storage and return of the mechanical work in intact human limbs require metabolic energy through active muscle contraction [7], which is not the case with running-speci c prostheses. Running-speci c prostheses also do not allow any neural adjustments (from active muscle contraction) in stiffness during running. Thus, stark differences in lower limb kinematics between able-bodied and lower-limb amputee runners have been reported [8–13]. Running- speci c prostheses are also available in different models with different geometries [14] and mechanical properties [5], and they can be modi ed for running-speci c prosthetic-

also do not allow any neural adjustments (from active muscle contraction) in stiffness during running. Thus, stark differences in lower limb kinematics between able-bodied and lower-limb amputee runners have been reported [8–13]. Running- speci c prostheses are also available in different models with different geometries [14] and mechanical properties [5], and they can be modi ed for running-speci c prosthetic- socket alignment and prosthetic height [15], the latter of which is to respect the maximum allowable standing height as regulated by the International Paralympic Committee. The running kinematics and kinetics of elite Paralympians using running-speci c prostheses and whether these devices incur an advantage to the Paralympians has been contentious [16,17]. Unilateral transtibial lower-limb amputees achieve top speeds by eliciting different spatiotemporal and vertical GRF characteristics compared to bilateral lower-limb amputees or able-bodied sprinters [9]. Speci cally, sprinting with running- speci c prostheses results in longer step lengths compared to a biological limb as well as lower and longer vertical GRF compared to the sound leg [9,10]. Thus, similar vertical impulses can be produced with lower peak vertical GRF forces with running-speci c prostheses; the total resultant force application does not need to be as high nor the lower limb strength as great in sprinters using running-speci c prostheses. Running-speci c prosthetic height and stiffness, the latter of which is in uenced by running speed, can be modi ed. Taboga et al. [15] found, however, no effect of height or stiffness on maximum sprint speed (over a relatively narrow range of stiffnesses ( 1 stiff- ness category) or heights ( 2 cm)). This does not imply that changes in stiffness and/or height beyond those that were tested would not infer a performance advantage. Indeed, a recent ruling by the Court of Arbitration for Sport ruled a Paralympic Champion ineligible to compete at the Olympics largely as a result of his running-speci c prosthetic height [18]. Running-speci c prosthetic alignment in the sagittal plane relative to the intact limb has only recently been investigated. Migliore et al. [19] examined the impact of increasing the sagittal tilt from 5 to 12 relative

of Arbitration for Sport ruled a Paralympic Champion ineligible to compete at the Olympics largely as a result of his running-speci c prosthetic height [18]. Running-speci c prosthetic alignment in the sagittal plane relative to the intact limb has only recently been investigated. Migliore et al. [19] examined the impact of increasing the sagittal tilt from 5 to 12 relative to the line of gravity in a gold medal Paralympian. With this change in alignment, the athlete was able to increase step frequency and reduce

Sports2021,9, 89 4 of 15 step length with the sound side. With a higher sagittal tilt, the propulsive impulse was higher, and the braking impulse decreased. Importantly, the athlete perceived the increased tilt as the best running-speci c prosthetic alignment, and shortly after implementing the tilt alignment, she improved upon her personal best in the T42 100 m, narrowly missing the world record. Importantly, athletes looking to change their running-speci c prosthetic model, align- ment, and/or height must be aware that these changes likely alter the stiffness pro le of the running-speci c prosthetic [5]. Speci cally, the running-speci c prosthetic stiffness increases with the magnitude of the peak resultant GRF but decreases when the angle between the running-speci c prosthetic and resultant GRF is increased. This inverse re- lationship between GRFs and angle may explain previous reports that running-speci c prosthetic stiffness does not change during running [10,11]. The maximum resultant force an athlete can produce limits the maximal sprint speed around a curve [2,20]. Running around a curve requires athletes to apply an additional centripetal force to change direction in addition to the horizontal and vertical propulsive forces. In sprint events of 200 m and 400 m, an athlete must negotiate either one or two curves, respectively, with the inside leg relative to the curve always being the left leg (since all races are run in the counterclockwise direction). This may disadvantage left legged single lower-limb amputee sprinters since both the compliance and passive nature of running-speci c prostheses reduce the application of the maximal forces on the ground [10]. Indeed, lower-limb amputee sprinters were 4% slower with the affected leg on the inside of the curve compared to sprinting with the affected leg on the outside of the curve due to their inability to generate large forces with their affected leg [21]. Single-leg strength and speed training may be required in order to improve the affected limb's ability to generate large vertical and centripetal forces during sprint running, and/or athletes should consider a better prosthetic design to help improve curve running in single and double-leg amputee sprinters.

the curve due to their inability to generate large forces with their affected leg [21]. Single-leg strength and speed training may be required in order to improve the affected limb's ability to generate large vertical and centripetal forces during sprint running, and/or athletes should consider a better prosthetic design to help improve curve running in single and double-leg amputee sprinters. 2.3. Mechanics of Sprinters with Cerebral Palsy Cerebral palsy is associated with a series of permanent movement disorders resulting from an upper motor neuron lesion in the brain. This impairment is not progressive in nature, but it likely causes secondary adaptations to the muscle structure, function, and composition [22]. Athletes with cerebral palsy will experience muscle weakness [23], increased antagonist coactivation [24], spasticity [25], reduced muscle and joint power, and limited range of motion [26]. Sprinters with cerebral palsy show lower force production compared to their able- bodied counterparts, which is likely a result of the reduced lower-limb strength in cerebral palsy. For example, over a 10 m sprint, a Paralympic sprint medalist with cerebral palsy (T36) showed lower average horizontal power compared to able-bodied sprinters, at- tributable to both higher horizontal braking and lower propulsive forces resulting in a lower net propulsive impulse [27]. The larger horizontal braking forces in the athlete with cerebral palsy, along with a stiffer ankle joint, were attributed to the increased muscle co-contractions commonly observed in cerebral palsy [28–30]. Increased passive and active (from antagonist co-activation) joint stiffness contribute to the reduced joint angular ve- locities, external joint power, and reductions in step length during the initial acceleration phase of sprinting. In summary, training interventions should focus on strategies to increase lower limb joint angular velocities and positive joint powers. Increases in the strength and power of the muscles that cross the hip and knee extensors is recommended to improve sprint performance, primarily because the powerful hip joint performs negative work in cerebral palsy yet performs net positive work in able-bodied athletes [27].

and power of the muscles that cross the hip and knee extensors is recommended to improve sprint performance, primarily because the powerful hip joint performs negative work in cerebral palsy yet performs net positive work in able-bodied athletes [27].

Sports2021,9, 89 5 of 15 2.4. Biomechanics of Jumping The performance gap between the Olympic and unilateral lower-limb amputee Par- alympic winning jumps is reducing (Figure). In the past, Paralympic champions have been permitted to compete at the Olympics on the track [13], and it may soon be the case that Paralympic long jumpers will compete at the Olympics as well.Sports 2021, 9, x FOR PEER REVIEW 7 of 18 the muscles that cross the hip and knee extensors is recommended to improve sprint per- formance, primarily because the powerful hip joint performs negative work in cerebral palsy yet performs net positive work in able-bodied athletes [27]. 2.4. Biomechanics of Jumping The performance gap between the Olympic and unilateral lower-limb amputee Par- alympic winning jumps is reducing (Figure 1). In the past, Paralympic champions have been permitted to compete at the Olympics on the track [13], and it may soon be the case that Paralympic long jumpers will compete at the Olympics as well. Figure 1. Men′s gold medal winning long jumps for the Olympics and Paralympic Games from 1992 to 2016. Olympic-winning jumps are shown in filled circles. Paralympic-winning F44 jumps are shown in open circles. Filled bars indicate the percent difference between Olympic and Paralympic winning jumps. Biomechanically, the goal of the long jump is to maximize the horizontal distance of the jump. This is accomplished by generating as much vertical velocity at take-off while minimizing the loss in horizontal velocity gained on the approach run-up. To do so, Olym- pic and unilateral lower-limb amputee long jumpers lower their centre of mass on the approach to the board to generate as much vertical impulse (and thus vertical velocity) during take-off as possible. Compared to non-amputee jumpers, elite unilateral lower- limb amputees approach the board slower but achieve similar vertical velocities at take- off [31]. The slower horizontal velocity would imply a disadvantage to unilateral lower- limb amputees compared to Olympic jumpers as a result of their running-specific pros- theses. However, energy is stored (a portion of which is returned) in running-specific prostheses that may exceed that of a

lower- limb amputees approach the board slower but achieve similar vertical velocities at take- off [31]. The slower horizontal velocity would imply a disadvantage to unilateral lower- limb amputees compared to Olympic jumpers as a result of their running-specific pros- theses. However, energy is stored (a portion of which is returned) in running-specific prostheses that may exceed that of a biological limb. In support, Funken et al. [32] have shown that unilateral lower-limb amputees have a longer compression phase and a greater downward motion of the centre of mass, which would be effective in generating vertical impulse and storing and returning energy within the running-specific prostheses. An optimal running-specific prosthetic stiffness, one that maximizes the storage and re- turn of energy while also maximizing the pivot that translates a portion of the horizontal velocity to vertical velocity at take-off [31] should be investigated. This optimal stiffness may be different than the optimal running-specific prosthetic stiffness for straight sprint- ing. Furthermore, unilateral lower-limb amputee long jumpers could benefit from specific Figure 1. Men's gold medal winning long jumps for the Olympics and Paralympic Games from 1992 to 2016. Olympic-winning jumps are shown in lled circles. Paralympic-winning F44 jumps are shown in open circles. Filled bars indicate the percent difference between Olympic and Paralympic winning jumps. Biomechanically, the goal of the long jump is to maximize the horizontal distance of the jump. This is accomplished by generating as much vertical velocity at take-off while minimizing the loss in horizontal velocity gained on the approach run-up. To do so, Olympic and unilateral lower-limb amputee long jumpers lower their centre of mass on the approach to the board to generate as much vertical impulse (and thus vertical velocity) during take-off as possible. Compared to non-amputee jumpers, elite unilateral lower-limb amputees approach the board slower but achieve similar vertical velocities at take-off [31]. The slower horizontal velocity would imply a disadvantage to unilateral lower-limb amputees compared to Olympic jumpers as a result of their running-speci c prostheses. However, energy is stored (a portion of which is returned) in running-speci c prostheses that may exceed that

The review discusses how biomechanics can optimize training and performance for Paralympic athletes.