Higher, faster, stronger: what are the upper limits of human performance? In a published study from January 2008, Berthelot et al 1 estimated that world records had reached 99% of its asymptotic values and that human upper performance limits in sport would be reached within 1 generation. Only
Thomas Haugen, Gøran Paulsen, Stephen Seiler and Øyvind Sandbakk
Samantha L. Winter and John H. Challis
For a physiologically realistic joint range of motion and therefore range of muscle fiber lengths, only part of the whole muscle force-length curve can be used in vivo; that is, only a section of the force-length curve is expressed. Previous work has determined that the expressed section of the force-length curve for individual muscles can vary between subjects; however, the degree of intersubject variability is different for different muscles. This study determined the expressed section of both the rectus femoris and gastrocnemius—muscles with very different ratios of tendon slack length to muscle fiber optimum length—for 28 nonspecifically trained subjects to test the hypothesis that the value of this ratio affects the amount of variability in the expressed section. The force-length curves of the two muscles were reconstructed from moment-angle data using the method of Herzog & ter Keurs (1988). There was no relationship between the expressed sections of the force-length curve for the two muscles. Less variability was found in the expressed section of the gastrocnemius compared with the rectus femoris, supporting the hypothesis. The lack of relationship between the expressed sections of the two muscles has implications for motor control and for training muscle for rehabilitation.
Kristen E. Thomas and Leah R. Bent
The integration of vestibular and somatosensory information for the control of lower limb musculature remains elusive. To determine whether a subthreshold vestibular input influences the cutaneous evoked response, the isometric EMG activity in the posturally inactive soleus muscles of 13 healthy, seated subjects was collected. Vestibular afferents were activated using galvanic vestibular stimulation (GVS; 1.8–2.5mA, 500ms), while percutaneous electrical stimulation was delivered to the distal tibial nerve (11ms train of 3 × 1.0 ms pulses, 200Hz) to activate foot sole skin afferents. GVS elicited responses in soleus both independently and when combined with cutaneous stimulation. The responses to the combined sensory input showed an interaction between the two sensory modalities to influence muscle activation. Of note is the presence of significant muscle modulation in the combined condition, where subthreshold vestibular inputs altered the outcome of the cutaneous reflex response. This finding has implications for individuals with sensory deficiency. In the case of an absent or deficient sensory modality, balance protective reflexes to maintain postural equilibrium may be enhanced with targeted sensory augmentation.
Marc Monsour, Tanya D. Ivanova, Tim D. Wilson and S. Jayne Garland
The purpose of this study was to investigate whether application of bipolar galvanic vestibular stimulation (GVS) would influence the common modulation of motor unit discharge rate in bilateral soleus muscles during quiet standing. Soleus motor unit activity was recorded with fine wire electrodes in each leg. Subjects stood, with eyes closed, on two adjacent force platforms to record postural sway with the head facing straight ahead, turned to right, or turned left. Subjects also swayed voluntarily without GVS to the same position as evoked during the GVS. There was no difference in the common drive to bilateral soleus motoneurons during quiet standing and voluntary sway tasks. Common drive was significantly lower during right cathode GVS with the head straight or turned to the right. These results demonstrate that manipulation of vestibular afferent input influences the common modulation of bilateral soleus motor unit pairs during quiet standing.
Neil Chapman, John Whitting, Suzanne Broadbent, Zachary Crowley-McHattan and Rudi Meir
electrically stimulated in vivo human muscle. 23 However, it is important to note an apparent disparity in magnitude of RFE between electrically stimulated and voluntarily activated contractions. In fact, the magnitude of RFE has been found to vary greatly in vivo voluntarily contracted human muscle. 23
Scott W. Ducharme and Richard E.A. van Emmerik
Biological systems are inherently variable, and this variability has been the focus of much research in the movement sciences. In human locomotion research, a commonly accepted convention has been that variability is a one-size-fits-all parameter; more is bad. This “negative” perspective towards
Antoine Falisse, Sam Van Rossom, Johannes Gijsbers, Frans Steenbrink, Ben J.H. van Basten, Ilse Jonkers, Antonie J. van den Bogert and Friedl De Groote
Musculoskeletal models for biomechanical simulations have become increasingly popular to analyze human movement. In addition to joint kinematics and kinetics, musculoskeletal models enable researchers and clinicians to assess other biomechanical variables, such as muscle lengths and forces
Bart Roelands and Kevin De Pauw
Human performance optimization is probably the most studied topic in sport science, as it is in other closely related areas such as rehabilitation or settings like industry and the army. In the International Journal of Sports Physiology and Performance ( IJSPP ), an abundance of studies appears
Ilkka Heinonen, Jukka Kemppainen, Toshihiko Fujimoto, Juhani Knuuti and Kari K. Kalliokoski
-Czernik, 2012 ). Human bone marrow circulation responds to acute exercise ( Heinonen et al., 2013a , 2013b ) and local heat stress ( Heinonen et al., 2011a ), although to a lesser degree than in contracting skeletal muscles ( Heinonen et al., 2007 , 2010a , 2011b , 2015 ). In addition to its perfusion, the
Maarten F. Bobbert, Han Houdijk, Jos J. de Koning and Gert de Groot
To gain a better understanding of push-off mechanics in speed skating, forward simulations were performed with a model comprising four body segments and six muscles. We started with a simulated maximum height one-legged jump, obtained by optimization of muscle stimulation time histories. The simulated jump was very similar to one-legged jumps produced by a human, indicating that the model was realistic. We subsequently studied how performance was affected by introducing four conditions characteristic of speed skating: (a) We changed the initial position from that in jumping to that at the start of the push-off phase in skating. This change was accommodated by a delay in stimulation onset of the plantar flexors in the optimal solution. (b) The friction between foot and ground was reduced to zero. As a result, maximum jump height decreased by 1.2 cm and performance became more sensitive to errors in muscle stimulation. The reason is that without surface friction, the foot had to be prevented from slipping away, which constrained the solution space and reduced the tolerance to errors in stimulation. (c) We introduced the requirement to maintain the upper body in a more or less horizontal position. This change could be accommodated by a delay in stimulation onset of the hamstrings, which inevitably caused a reduction in maximum jump height by 11.6 cm. (d) We increased the effective foot length from 16.5 cm, representative of jumping, to 20.5 cm, representative of skating with klapskates. At the 20.5-cm foot length, rotation of the foot did not start during the buildup of plantar flexion moment as it did at smaller foot lengths, but was delayed until hip and knee extension moments decreased. This caused an unbalanced increase in segment angular velocities and muscle shortening velocities, leading to a decrease in muscle force and muscle work and a further decrease in maximum jump height by approximately 5 cm. Qualitatively, these findings help clarify why and how performance of speed skaters depends on the location of the hinge of their skate.