Introduction: Stiffness (k) describes a material's resistance to deformation and is useful for understanding neuromuscular function, performance, and injury risk. The aim of this study is to compare the lower limb stiffness method (kLLS), which uses only force plate data, with methods combining force plate and motion capture data to calculate stiffness during the eccentric phase of a countermovement. Material and Methods: Twelve resistance-trained males: age 24.9 (4.4) years, height 1.81 (0.05) m, weight 88.2 (14) kg) performed three maximal effort countermovement jumps (CMJ). Data were collected synchronously using three-dimensional (3D) kinematic and kinetic data (dual force plate setup). Lower limb stiffness (z), joint stiffness (x, y, and z), and leg stiffness (linear, sagittal plane, and 3D) were calculated for the eccentric phase of all CMs. Results: kLLS showed high concurrent validity with strong correlations to kinetic-kinematic methods (r = 0.90-0.97, p < 0.05). A linear mixed model revealed no significant differences in k-values between kLLS and leg stiffness, indicating high concurrent validity. Discussion: kLLS offers valid and valuable information affecting performance, injury risk, and return-to-sport decisions. Conclusion: The findings suggest that kLLS is a valid method for calculating stiffness in CMJs and equal to 3D leg stiffness.

In this study, a developed co-simulation method, which couples 1D-fluid and 3D-structural models, has been utilised to simulate wear in a hydraulic percussion unit. The effect of wear is generally detrimental on performance and lifetime for such units, but can also cause catastrophic failure and breakdown, requiring a total overhaul and replacement of core components. One experiment of standard straight impact was performed to investigate the tolerance against seizure. The percussion unit was operated at successively increasing operating pressures, and the level of wear was registered at each step, until seizure occurred. The co-simulation model was used to replicate the running conditions from the experiment to simulate the structural response to be used as input for the wear routine to calculate the wear depth. The wear pattern from the simulations corresponds well to the wear pattern from the experiment. Further, the effect of a misaligned impact on wear development was also studied, as this is a loading situation that typically occurs for hydraulic percussion units. The study demonstrates that the simulation method used has a potential for simulating wear and predicting seizure in hydraulic percussion units.

The aim of this study was to estimate and compare the muscular metabolic power produced in the human body using musculoskeletal inverse-dynamics during cross-country sit-skiing. Two sitting positions were adapted for athletes with reduced trunk and hip muscle control, knee low with frontal trunk support (KL-fix), and knee high (KH). Five female national class able-bodied cross-country skiers performed submaximal and maximal exercise in both sitting positions, while recording 3-D kinematics, pole forces, electromyography and respiratory variables. Simulations were performed from these experimental results and muscular metabolic power was computed. The main part of the muscle metabolic power was produced in the upper limbs for both sitting positions, but KH produced more muscle metabolic power in lower limbs and trunk during maximal intensity. KH was also more efficient, utilizing less muscular metabolic power during submaximal intensities, relatively less power in the upper limbs and more power in the trunk, hip and lower limb muscles. This implies that sitting position KH is preferable for high power output when using able-bodied simulation models. This study showed the potential of using musculoskeletal simulations to improve the understanding of how different equipment design and muscles contribute to performance.

Introduction: In Nordic skiing all sitting athletes compete in the same event competition. The sitting positions differ between athletes. Most of the athletes sit knee-seated, or with their thighs tilted downward (KL) and free to move their trunk. Some athletes do not have the possibility to sit in that position and therefore adjust their sitting position. For example, athletes with reduced muscle control in hips and lower trunk sit with their knees higher than their hips (KH) to increase stability.

Purpose: The purpose of this study was to examine how sitting position KL and KH affects the muscular power.

Methods: One female able-bodied athlete performed one test session in each sitting position (KL and KH) comprising five times 3 minutes sub-maximal exercise and a maximal time-trial in a double-poling ergometer (ThoraxTrainer A/S, Denmark). During the tests 3D kinematics (Qualisys AB, Sweden), pole forces and power output were measured. From the measured data, participant and test specific musculo-skeletal inverse-dynamics simulation models were created using the AnyBody Modelling system (AMS 6.0, Anybody Technology A/S, Denmark). From the simulations of submaximal exercise power output 37 W, 52 W and maximal time-trial the muscular metabolic power (mMP) was computed according to Holmberg (2013).

Results: The power output in maximal exercise was higher in KL (90.1 W) compared to KH (74.7 W). During both submaximal and maximal exercise, the total muscular metabolic power was larger in KL compared to KH (KL mean 861 W and KH mean 682 W). The muscular metabolic power also showed larger relative involvement of legs in KL (KL mean 18 % and KH mean 4 %) and larger relative involvement of arms and trunk in KH.

Conclusion: That sitting position KL compared to KH is related to higher performance for athletes without impairment in hips and trunk is known before (Gastaldi, 2012). However, the results from this study explains why performance is higher in KL, i.e. that larger muscular metabolic power are produced in the legs. This study also shows the size of the involvement of legs, which could be interesting for development of classification rules.

INTRODUCTION

For wheel-chair users shoulder injuries [1] and lower back injuries [2] are common. Lower back kyphosis of the spine, increases the anterior shear force in the lower back [3] and increases the risk of shoulder injuries [4].

Cross-country sit-skiing (CCSS) is an endurance sport where the athlete is seated in a sledge mounted on a pair of skis and propel themselves by poling with a pair of sticks. This sport creates more equal loading on the muscles around the shoulder than wheel-chair rolling [5] which is positive in an injury perspective for the gleno-humeral joint [1].

Athletes in CCSS with reduced trunk muscle control often sits in a sledge with their knees higher than their hips (KH) and a backrest. This position is hypothesized to be associated with spinal kyphosis and hence an increased risk of injuries. Therefore we have created a new sitting position with knees lower than hips (KL) with the trunk restrained on a frontal support.

The aim of this study was to compute the L4/L5 joint reactions and compare the results between the positions KH and KL.

METHODS

Five female abled-bodied cross-country skiing athletes (62.6 ± 8.1kg, 1.67 ± 0.05m)  performed one exercise test session in each sitting position; The sessions included a sub-maximal incremental test, including 4-6 exercise levels of 3 min (exercise intensity nr 4, 37W, reflected race-pace) and a maximal time-trial (MAX) of 3 min on a commercial skiing ergometer (ThoraxTrainer A/S, Denmark).

Full-body kinematics (Qualisys AB, Sweden) and pole forces (Biovision, Germany) were measured in 200 Hz. These data served as input to inverse dynamic simulations in The AnyBody Modelling system (AMS 6.0, Anybody Technology A/S, Denmark). For each participant and sitting position, simulations were made for exercise intensity 37W and MAX over four poling cycles using a 5^th order polynomial muscle recruitment criteria. Compression forces and anterior shear forces between L4 and L5 were computed and normalized to each participant’s standing joint reactions. Data were compared pair-wise between the two sitting positions.

Statistical significance (p ≤ 0.05) were marked with asterisk (*). Tendency of difference (0.05 ≤ p < 0.10) were marked (ǂ).

RESULTS AND DISCUSSION

Performance was higher in position KH (KL: 0.77±0.08 W/kg, KH: 1.00±0.14 W/kg, p < 0.01). No difference were observed in cycle length or cycle time. Kinematics results showed that KL had less spine flexion and range of motion in flexion. KH showed higher mean pole force in 37W and tendency of higher peak pole force in MAX.

In standing, L4/L5 compression and anterior shear forces were 354 ± 45N and 32 ± 11N respectively. The normalized L4/L5 reaction forces (fig. 1) were larger in KH, especially during MAX intensity due to higher power. For equal power output, 37W, the mean anterior shear force was larger in KH and the mean compression force showed tendency of larger in KH (p=0.077).

Figure 1: Normalized joint reaction forces, compression and anterior shear forces, between vertebrae L4/L5 for the two sitting positions KH and KL with trunk restraint. Min – minimal force, Maximal force and Mean – mean force over the four poling cycles.

CONCLUSIONS

Based on inverse-dynamics musculo-skeletal simulations of 5 abled-bodied athletes, the sitting position KL with frontal restraint reduced the compression and shear force between the L4/L5 vertebrae but impeded performance. This study shows the difficulty of comparing performance and safety in the same piece of equipment.

ACKNOWLEDGEMENTS

The authors acknowledge the Rolf & Gunilla Enström foundation and the Promobilia foundation, Sweden, for financial support, and the Ableway AB (Sweden) for construction of the sledges.

REFERENCES

1. Burnham RS, et al., Am J Sports Med, 21: 238-242, 1993.
2. Thyberg M, et al., Disabil rehabil. 23:677-682, 2001.
3. McGill SM, et al., Clin Biomech, 15: 777-780, 2000.
4. Samuelsson KA, et al., J Rehabil Res Dev, 41: 65-74, 2004.
5. Bjerkefors A, et al., Int J Sports Med, 34: 176-182, 2013.