The Biomechanics of Spear Throwing: An Analysis of the Effects of Anatomical Variation on Throwing Performance, with Implications for the Fossil Record

Accurate, high velocity throwing is a skill unique to humans among living species. It likely provided an adaptive advantage for our hominin ancestors, either in the context of hunting, or protection from predators. Thus, understanding how variation in body form and anatomy influences throwing ability may provide insight into the evolution of human morphology. Research has been done on various forms of ball and javelin throwing, yet the biomechanics of spear throwing were completely unknown. Moreover, it has been suggested that early modern humans had lesser effective mechanical advantage: EMA, the ratio of moment arm to load arm) than Neandertals, as a result of selection for throwing, as this could provide increased joint angular velocity at the cost of strength. However the biomechanical principles underpinning this assertion had not been tested empirically. Thus, the purpose of this dissertation is to establish the patterns of kinetics and kinematics used during spear throwing, to examine the morphological correlates of throwing performance and strength in living humans, and to consider the implications of the patterns observed for Paleolithic fossil hominins. Throwing performance, measured as the energy imparted to a thrown object: kinetic energy, Ke), was assessed in 41 experienced throwers throwing balls and spear-like objects. Joint linear and angular velocities, torques, and kinetic energies were calculated using high-speed infrared cameras. Subject anthropometrics, including body mass, height, and segment lengths, were measured externally, and arm muscle cross-sectional areas and EMA of the elbow and wrist were measured from magnetic resonance images. The maximum isometric strength of the elbow and wrist were measured using a load cell. Additionally, EMA of the elbow and wrist were estimated skeletally for available early Homo, Neandertal, Middle Paleolithic, Upper Paleolithic, and recent human specimens. It was demonstrated that the shoulder experiences very low angular velocities and torques, and the elbow is relatively less mobile during spear as compared with ball throwing. Thus, the literature on the arm in ball throwing will be of less relevance to the evolution of throwing. Furthermore, there is no direct correlation between spear Ke and any morphological or anatomical variable of the arm: including EMA). Instead, spear Ke is primarily produced by the legs and torso, where musculature, leg length, and shoulder breadth are correlated with greater Ke. This implies that research on throwing should focus on the legs and torso, and variation in the morphology of the arm should not be used to infer differential selection for throwing in Paleolithic hominins. No correlation was found between the EMA of the elbow or wrist and joint strength independent of muscle cross-sectional area. This is likely due to variation in muscle recruitment and activation, and because r is correlated with muscle size independent of body size. Furthemore, it was demonstrated that EMA cannot be measured reliably in the fossil record, as load arms are highly variable within and between individuals, even performing a highly trained activity like throwing. Consequently, they cannot be predicted from skeletal dimensions available in the fossil record. Furthermore, although there is a trend towards greater EMA of the elbow in Neandertals as compared with other fossil hominin samples, this variation is not functionally or adaptively relevant. No significant differences exist for moment arms between groups; rather, the difference in EMA is due to variation in load arms: ulna length). However, load arms measured skeletally are not relevant to living behaviors, and variation in load arms between these groups is due to known differences in brachial indices, which reflect climatic adaptations. Thus, EMA of the arm is not a useful measure of performance in Paleolithic fossil hominins

The effect of alterations in effective seat tube angle on cycling performance, economy and muscle recruitment

Introduction: The bicycle seat tube angle (STA) has been used in scientific research to investigate cycling performance since the early 1980's and has led to inconclusive findings when manipulated between 69° and 82° STA configuration. Most of these studies did not clearly indicate the handlebar positioning in relation to the change in STAs. In addition, the studied duration and intensity were not a true reflection for cycling performance during races. Aim: The study aimed to compare the effect of independent alteration of effective seat tube angle (ESTA) on gross muscle activities, body kinematics and gross economy for well-trained cyclists. Methods: Ten well-trained male cyclists (mean ± SD; age 37.8 ± 3.6 years, height 178.2 ± 3.8 cm, body mass 76.9 ± 8.0 kg, VO₂ₘₐₓ 51.6 ± 5.3 ml/kg/min with 6.8 ± 2.6 years cycling experience and an average training load of 5.8 ± 2.3 hours per week for three months prior) were volunteered for this study. All cyclists were randomly assigned to either a forward or rearward saddle position after an initial preferred saddle cycling position. Each cycling position was performed at 60% of Wₚₑₐₖ for one hour with forty reflective markers placed on bony landmarks described by Vicon full body model Plug-in gait and EMG electrodes placed on the right lower limb on seven muscles. Results: The mean power output and cadences during one hour submaximal steady state cycling differed by a maximum of 0.7W and 3.5 repetitions per minute respectively between three trials. VO₂ values (P=0.95), respiratory exchange ratio (P=0.39) and heart rate (P=0.92) for the trials were not significantly different. Mean angles for each joint and gross muscle activation patterns across the three trials were not significantly different. Magnitude-based inferences statistics showed "possible beneficial effects" on knee and ankle joint kinematics when comparing the forward and rearward saddle displacement. A progressive increase in integrated EMG values was observed for gluteus maximus, biceps femoris and rectus femoris from forward to rearward position. Both vastus lateralis and vastus medialis decreased activation in forward and rearward positions as compared to preferred position. However, none of these changes were statistically significant. Conclusion: Preserving the joint kinematics of the elbow, shoulder, hip, knee and ankle joint of the cyclist when changing the saddle displacement effectively negate any change in heart rate, oxygen consumption and respiratory exchange ratio. Nonetheless, the knee and ankle joints were increased by 1° and decreased by 1.5° respectively when saddle was moved forward. Similar knee and ankle joints effects were also detected with when saddle was moved rearward, which were decreased by 3° and increased by 2° respectively. Therefore, dynamic joint angles should be controlled for future studies when manipulating saddle displacement during cycling. The seven lower limb muscles activations were not statistically significant different when using traditional statistical methods and magnitude type statistic also indicates most unlikely or very unlikely benefits for all surface EMG variables between saddle displacements. These could be due to the high degrees of variability in EMG signal during cycling. Therefore, greater numbers of participants are encouraged for future studies aimed at understanding the coordination of agonist and antagonist muscles at different ESTA. Key words: Effective seat tube angle, submaximal cycling, 3D joint kinematics, electromyography (EMG)

Sex Differences in Spatial Accuracy Relate to the Neural Activation of Antagonistic Muscles in Young Adults

Sex is an important physiological variable of behavior, but its effect on motor control remains poorly understood. Some evidence suggests that women exhibit greater variability during constant contractions and poorer accuracy during goal-directed tasks. However, it remains unclear whether motor output variability or altered muscle activation impairs accuracy in women. Here, we examine sex differences in endpoint accuracy during ankle goal-directed movements and the activity of the antagonistic muscles. Ten women (23.1 ± 5.1 years) and 10 men (23 ± 3.7 years) aimed to match a target (9° in 180 ms) with ankle dorsiflexion. Participants performed 50 trials and we recorded the endpoint accuracy and the electromyographic (EMG) activity of the primary agonist (Tibialis Anterior; TA) and antagonist (Soleus; SOL) muscles. Women exhibited greater spatial inaccuracy (Position error: t = −2.65, P = 0.016) but not temporal inaccuracy relative to men. The motor output variability was similar for the two sexes (P \u3e 0.2). The spatial inaccuracy in women was related to greater variability in the coordination of the antagonistic muscles (R 2 0.19, P = 0.03). These findings suggest that women are spatially less accurate than men during fast goal-directed movements likely due to an altered activation of the antagonistic muscles

Adaptive Neural Networks for Control of Movement Trajectories Invariant under Speed and Force Rescaling

This article describes two neural network modules that form part of an emerging theory of how adaptive control of goal-directed sensory-motor skills is achieved by humans and other animals. The Vector-Integration-To-Endpoint (VITE) model suggests how synchronous multi-joint trajectories are generated and performed at variable speeds. The Factorization-of-LEngth-and-TEnsion (FLETE) model suggests how outflow movement commands from a VITE model may be performed at variable force levels without a loss of positional accuracy. The invariance of positional control under speed and force rescaling sheds new light upon a familiar strategy of motor skill development: Skill learning begins with performance at low speed and low limb compliance and proceeds to higher speeds and compliances. The VITE model helps to explain many neural and behavioral data about trajectory formation, including data about neural coding within the posterior parietal cortex, motor cortex, and globus pallidus, and behavioral properties such as Woodworth's Law, Fitts Law, peak acceleration as a function of movement amplitude and duration, isotonic arm movement properties before and after arm-deafferentation, central error correction properties of isometric contractions, motor priming without overt action, velocity amplification during target switching, velocity profile invariance across different movement distances, changes in velocity profile asymmetry across different movement durations, staggered onset times for controlling linear trajectories with synchronous offset times, changes in the ratio of maximum to average velocity during discrete versus serial movements, and shared properties of arm and speech articulator movements. The FLETE model provides new insights into how spina-muscular circuits process variable forces without a loss of positional control. These results explicate the size principle of motor neuron recruitment, descending co-contractive compliance signals, Renshaw cells, Ia interneurons, fast automatic reactive control by ascending feedback from muscle spindles, slow adaptive predictive control via cerebellar learning using muscle spindle error signals to train adaptive movement gains, fractured somatotopy in the opponent organization of cerebellar learning, adaptive compensation for variable moment-arms, and force feedback from Golgi tendon organs. More generally, the models provide a computational rationale for the use of nonspecific control signals in volitional control, or "acts of will", and of efference copies and opponent processing in both reactive and adaptive motor control tasks.National Science Foundation (IRI-87-16960); Air Force Office of Scientific Research (90-0128, 90-0175

Coordination in adults with neurological impairment – a systematic review of uncontrolled manifold studies

Background: Analysis of sensorimotor synergies has been greatly advanced by the Uncontrolled Manifold (UCM) approach. The UCM method is based on partitioning inter-trial variance displayed by elemental variables into ‘good’ (VUCM) and ‘bad’ (VORT) variability that, respectively, indicate maintenance or loss of task stability. In clinical populations, these indices can be used to investigate the strength, flexibility, stereotypy and agility of synergistic control. Research question: How are synergies affected by neurological impairment in adults? Specifically, this study aimed to determine i) the impact of pathology on VUCM, VORT, and their ratio (synergy index); ii) the relationship between synergy indices and functional performance; iii) changes in anticipatory synergy adjustments (ASAs); and iv) the effects of interventions on synergies. Methods: Systematic review of UCM studies on adults with neurological impairment. Results: Most of the 17 studies had moderate to high quality scores in the adapted Critical Review Form and the UCM reporting quality checklist developed for this review. i) Most of the studies found reduced synergy indices for patients with Parkinson's disease (PD), olivo-ponto-cerebellar atrophy, multiple sclerosis and spinocerebellar degeneration, with variable levels of change in VUCM and VORT. Reduction in synergy indices was not as consistent for stroke, in three out of six studies it was unchanged. ii) Five of seven studies found no significant correlations between scores on motor function scales and UCM indices. iii) Seven studies consistently reported ASAs that are smaller in magnitude, delayed, or both, for patients compared to healthy controls. iv) Two studies reported increased synergy indices, either via increase in VUCM or decrease in VORT, after dopaminergic drugs for patients with PD. There were similar synergy indices but improved ASAs after deep brain stimulation for patients with PD. Significance: UCM can provide reliable and sensitive indicators of altered synergistic control in adults with neurological impairment