Force application during handcycling and handrim wheelchair propulsion: an initial comparison.

The aim of the study was to evaluate the external applied forces, the effectiveness of force application and the net shoulder moments of handcycling in comparison with handrim wheelchair propulsion at different inclines. Ten able-bodied men performed standardized exercises on a treadmill at inclines of 1%, 2.5% and 4% with an instrumented handbike and wheelchair that measured three-dimensional propulsion forces. The results showed that during handcycling significantly lower mean forces were applied at inclines of 2.5% (P < .001) and 4% (P < .001) and significantly lower peak forces were applied at all inclines (1%: P = .014, 2.5% and 4%: P < .001). At the 2.5% incline, where power output was the same for both devices, total forces (mean over trial) of 22.8 N and 27.5 N and peak forces of 40.1 N and 106.9 N were measured for handbike and wheelchair propulsion. The force effectiveness did not differ between the devices (P = .757); however, the effectiveness did increase with higher inclines during handcycling whereas it stayed constant over all inclines for wheelchair propulsion. The resulting peak net shoulder moments were lower for handcycling compared with wheelchair propulsion at all inclines (P < .001). These results confirm the assumption that handcycling is physically less straining.

Determination of functional rotation axes during elevation of the shoulder complex.

STUDY DESIGN: A cross-sectional, descriptive study of shoulder movements conducted on nonimpaired subjects. OBJECTIVE: To investigate whether a single functional rotation axis about the shoulder complex can be determined during elevation in the coronal or sagittal planes, and to identify their location. BACKGROUND: Accurate measurement of isokinetic torques about a joint requires alignment of the dynamometer axis with an assumed rotation axis of the joint. To assess shoulder function on a dynamometer, the location of a single rotation axis is not evident because the shoulder joint motion is based on several anatomical joints. Therefore, the rotation axis where humerothoracic movements occur should be judged as a functional rotation axis. METHODS AND MEASURES: During slow elevation movements in the sagittal and coronal plane, the position of the epicondyle and acromion were recorded with a motion analysis system. The motion trajectory of the elbow coordinates was fitted to a circle and considered an estimate of the functional shoulder joint rotation axis in the specified plane. RESULTS: The fitted trajectory appeared to be very accurate (root-mean-square error < 2%; N = 7). In the sagittal plane, the estimated functional rotation axis was found at the humeral head; in the coronal plane, it was located about 13 centimeters medial relative to the acromion. CONCLUSION: The shoulder complex of nonimpaired subjects can act as a hinge joint of the upper arm relative to the thorax during elevation in each measured plane.

Relevance of the force-velocity relationship in the activation of mono- and Bi-articular muscles in slow arm movements in humans.

We have investigated whether differences in EMG activity in mono- and bi-articular muscles for concentric and eccentric contractions (van Bolhuis, Gielen, & van Ingen Schenau, 1998) have to be attributed to a specific muscle coordination strategy or whether they are merely a demonstration of adaptations necessary to adjust for muscle contractile properties. Slow, multi-joint arm movements were studied in a horizontal plane with an external force applied at the wrist. Kinematics and electromyography data from 10 subjects were combined with data from a 3-D model of the arm and a Hill-type muscle model. Data for both mono- and bi-articular muscles revealed a higher activation in concentric than in eccentric contractions. The model calculations indicated that the measured difference in activation (20%) was much larger than expected based on the force-velocity relationship (predicting changes of approximately 5%). Although these findings eliminate the force-velocity relationship as the main explanation for changes in EMG, it cannot be ruled out that other muscle contractile properties, such as history dependence of muscle force, determine muscle activation levels in the task that was studied.

On the effectiveness of force application in guided leg movements.

In guided leg movements (e.g., in cycling or wheelchair propulsion), the kinematics of a limb are determined by the object on which a force is applied. As a consequence, the force direction can vary and may deviate from the movement direction, that is, the effective direction. In the present study, the relation of effective force application and maximal power output was examined. Subjects (n = 5) performed guided leg tasks on a special dynamometer. They were instructed to exert a maximal force against a moving forceplate in the direction of the movement, as if they were pushing the plate away. Three different movement directions were tested: perpendicular to the horizontal, rotated 30 degrees backward, and rotated 30 degrees forward. For each trial, force and position data were recorded. The results of the experiments showed that in the extreme movement directions (both 30 degrees conditions), the force vector deviated significantly from the direction of the movement. Apparently, maximal power output was achieved with a low force effectiveness in these tasks. The background of this phenomenon was revealed by using the kinematics of one of these tasks in a simulation model. The stimulation level of 6 leg muscles was optimized toward a maximal effective force component (a) without a constraint on the direction of the total force or (b) with a constraint on the force component perpendicular to the effective force. The muscle stimulation pattern that resulted in the highest effective force coincided with a low force effectiveness. Apparently, this is a prerequisite for maximal power transfer from the muscles to the plate in these guided movements.

Anaerobic power output and propulsion technique in spinal cord injured subjects during wheelchair ergometry.

In order to investigate the influence of the level of the spinal cord injury (SCI) on anaerobic or short-term power production and propulsion technique, 23 male SCI subjects performed a 30-second sprint test on a stationary wheelchair ergometer. Kinematic parameters were studied both inter- and intra-individually. Subjects with a cervical lesion showed a lower mean power output (21.5 Watt, one-sided) than the other subjects; whereas, no differences were found between subjects with a thoracic or lumbar injury (46.9, 63.7, and 49.1 Watt, one-sided). Unexpectedly, no differences were found for the effectiveness of the force applied on the rim between subjects with a cervical injury and the other subjects. It is suggested that the high hand rim velocity reached by subjects with a lower injury cause coordination problems. Reduced arm functionality of subjects with a cervical lesion appeared to cause a higher inward directed force. Arm functionality and rim velocity may have a compensating effect with respect to the effectiveness of force. The kinematics of subjects with a cervical lesion differed strongly from subjects with a lower lesion. Propulsion technique appeared to be intra-individually consistent, which is reflected in the consistency of the force curves, the power output curves, and the movement patterns. Large inter-individual differences in propulsion technique were found. It is concluded that the large diversity in capacity of the SCI population should be taken into account with respect to guidelines and requirements for the environmental space of the SCI population.