Iain R. Spears

Optimizing Performance by Improving Core Stability and Core Strength

Core stability and core strength have been subject to research since the early 1980s. Research has highlighted benefits of training these processes for people with back pain and for carrying out everyday activities. However, less research has been performed on the benefits of core training for elite athletes and how this training should be carried out to optimize sporting performance. Many elite athletes undertake core stability and core strength training as part of their training programme, despite contradictory findings and conclusions as to their efficacy. This is mainly due to the lack of a gold standard method for measuring core stability and strength when performing everyday tasks and sporting movements. A further confounding factor is that because of the differing demands on the core musculature during everyday activities (low load, slow movements) and sporting activities (high load, resisted, dynamic movements), research performed in the rehabilitation sector cannot be applied to the sporting environment and, subsequently, data regarding core training programmes and their effectiveness on sporting performance are lacking.There are many articles in the literature that promote core training programmes and exercises for performance enhancement without providing a strong scientific rationale of their effectiveness, especially in the sporting sector. In the rehabilitation sector, improvements in lower back injuries have been reported by improving core stability. Few studies have observed any performance enhancement in sporting activities despite observing improvements in core stability and core strength following a core training programme. A clearer understanding of the roles that specific muscles have during core stability and core strength exercises would enable more functional training programmes to be implemented, which may result in a more effective transfer of these skills to actual sporting activities.

Enamel microstructure-a truly three-dimensional structure

Paleoanthropological studies often center on teeth, not only because these elements are commonly preserved in the fossil record, but because they apparently contain a wealth of information with regard to development, phylogeny, and function. However, despite a plethora of studies, some fundamental problems are still unresolved. For example, while it is recognized that the 3dimensional arrangement of enamel prisms may hold important information with regard to phylogeny (von Koenigswald and Sander, 1997) and function (Rensberger, 2000), many paleoanthropological studies have thus far relied on investigating enamel microanatomy as a 2dimensional structure (e.g., Dean et al., 2001). This is mainly due to difficulties in visualizing and quantifying the 3-D structure of prisms. In order to overcome these limitations a computer model was developed (Jiang et al., 2003) which attempted to simulate the effects of biophysical processes governing enamel formation in modern humans (adapted from Osborn, 1970). Here we extend our model and present preliminary data on interspecific variation in prism arrangement among primates. Furthermore, during our work to recreate the 3D microstructure of prismatic enamel it became increasingly clear that there are not only limitations with previous dental growth studies, but that these studies are based on fundamentally different concepts regarding evolutionary processes from those assumed in our approach. These limitations and differences will be highlighted also.

The effect of interfacial parameters on cup-bone relative micromotions. A finite element investigation.

Achieving stability is a prerequisite for allowing bone to grow into the porous surface of non-cemented acetabular cups. The purpose of this study is to estimate the effects of interfacial characteristics on relative cyclical micromotion between cup and bone during gait in the immediate postoperative phase. The technique used is finite element analysis. Six models with different interfacial characteristics are created in order to study the effects of fixation technique. These include representation of a 1 mm press-fit, 2 mm press-fits (with and without an initial polar gap) and exact-fit conditions (with and without additional screw fixation). Although direct validation of the model has not been performed, the calculated micromotions under a static load of 1112 N are compared with appropriate experimental data. Generally, the model tends to underestimate micromotion and this underestimate is significant in the case of relative surface-normal micromotion in polar regions for models with low- and no-interference. The most likely cause of this significant underestimate is a failure of the model to accurately represent penetration of rough contacting surfaces under compression. Other types of micromotion, although low, are within standard deviations reported by Kwong et al. (1994 Journal of Arthroplasty 9, 163-170). Quasi-static joint contact and muscle forces, representative of the stance phase of gait are then applied and maximum micromotions are found to occur consistently prior to toe off: this being the point of maximum force. With regard to the press-fit simulations, good cup-bone contact in the superior region of the interface is required for stability and the greatest micromotions occur in the models with the larger interference and larger polar gaps. In contrast to the press-fit models, muscle activity in exact-fit models influences the calculations. Specifically, the early activity of m.semimembranosus modelled causes opening of the peripheral seal. Taken together it is found that polar gaps reduce the stability of the model and lack of pre-compresssion in the periphery allows this region of the interface to be opened up.

Peak and average rectified EMG measures: Which method of data reduction should be used for assessing core training exercises?

Core strengthening and stability exercises are fundamental for any conditioning training program. Although surface electromyography (sEMG) is used to quantify muscle activity there is a lack of research using this method to investigate the core musculature and core stability. Two types of data reduction are commonly used for sEMG; peak and average rectified EMG methods. Peak EMG has been infrequently reported in the literature with regard to the assessment of core training while even fewer studies have incorporated average rectified EMG data (ARV). The aim of the study was to establish the repeatability of peak and average rectified EMG data during core training exercises and their interrelationship. Ten male highly trained athletes (inter-subject repeatability group; age, 18 ± 1.2 years; height, 176.5 ± 3.2 cm; body mass, 71 ± 4.5 kg) and one female highly trained athlete (intra-subject repeatability group; age; 27 years old; height; 180 cm; weight; 53 kg) performed five maximal voluntary isometric contractions (MVIC) and five core exercises, chosen to represent a range of movement and muscle recruitment patterns. Peak EMG and ARV EMG were calculated for eight core muscles (rectus abdominis, RA; external oblique, EO; internal oblique, IO; multifidis, MF; latissimus dorsi, LD; longissimus, LG; gluteus maximus, GM; rectus femoris, RF) using sEMG. Average coefficient of variation (CV%) for peak EMG across all the exercises and muscles was 45%. This is in comparison to 35% for the ARV method, which was found to be a significant difference (P<0.05), therefore implying that the ARV method is the more reliable measure for these types of exercise. Analysis of the inter-subject and intra-subject CV% values suggest that these exercises and muscles are sufficiently repeatable using sEMG. Five muscles were highly correlated (R>0.70; RA, EO, MF, GM, LG) between peak and ARV EMG suggesting, that for these core muscles, the two methods provide a similar evaluation of muscle activity. However, for other muscles (IO, RF, LD) the relationship was found to range from poor to moderate (R=0.10-0.70). The relationship between peak and ARV EMG was also affected by exercise type. Dynamic low and high-threshold exercises and asymmetrical low-threshold exercises had a moderate correlation between the variables (R=0.74-0.81), while the static exercise showed a poor correlation (R=0.46). It can be concluded that there are similarities between the two EMG variables, however due to the effect of type of exercise and muscle on the EMG data, both methods should be included in any future EMG study on the core musculature and core stability exercises.

The influence of friction and interference on the seating of a hemispherical press-fit cup: a finite element investigation

The formation of gaps in the polar region of acetabular cups is seen as a drawback of press-fit fixation of non-cemented acetabular cups. Recent findings indicate a link between long-term polar gaps and the gaps present directly after implantation. In this study the process of press-fitting is simulated with a linear-elastic two-dimensional axisymmetric finite-element model. The aim of this paper is to investigate the possible importance of friction and interference on the formation of these gaps. A range of cup-bone friction coefficients (mu = 0.1-0.5) is assigned to the cup-bone interface in order to represent the unknown amount of friction occurring during press-fitting. The cup is modeled with a radius of 27 mm, whereas the radius of the cavity is varied between 26.50 and 26.75 mm, thus, creating 0.50 and 0.25 mm radial interference fits. The difference in cavity radius represents the discrepancy between the radius of the last-reamer-used and radius of the cavity it creates. The subchondral plate is considered as being completely removed during reaming. The effects of impact blows via the surgeons mallet during surgery are modeled as a series of four load pulses, in which peak force is gradually increased from 0.5 to 4.0 kN. The effects of load removal as well as those of load application are investigated. On load application, the cup penetrates into the cavity, and on load removal, the cup rebounds. Depending on the friction, interference and load applied, the position of the cup after the load pulse is somewhere between its position at peak force and its position at the beginning of the pulse. Although the simplifications and conditions involved in the creation of the model necessitate caution when interpreting the results for all clinical cases, it is found that the seating of hemispherical cups in trabecular bone could be more satisfactory for intermediate values of friction (mu = 0.2-0.3) and smaller interference fits (0.25 mm).

The effect of saddle design on stresses in the perineum during cycling

PURPOSE Repetitive internal stress in the perineum has been associated with soft-tissue trauma in bicyclists. Using an engineering approach, the purpose of this study was to quantify the amount of compression exerted in the perineum for a range of saddle widths and orientations. METHODS Computer tomography was used to create a three-dimensional voxel-based finite element model of the right side of the male perineum-pelvis. For the creation of the saddle model, a commercially available saddle was digitized and the surface manipulated to represent a variety of saddle widths and orientations. The two models were merged, and a static downward load of 189 N was applied to the model at the region representing the sacroiliac joint. For validation purposes, external stresses along the perineum-saddle interface were compared with the results of pressure sensitive film. Good agreement was found for these external stresses. The saddles were then stretched and rotated, and the magnitude and location of maximum stresses within the perineum were both recorded. In all cases, the model of the pelvis-perineum was held in an upright position. RESULTS Stresses within the perineum were reduced when the saddle was sufficiently wide to support both ischial tuberosities. This supporting mechanism was best achieved when the saddle was at least two times wider than the bi-ischial width of the cyclist. Stresses in the anterior of the perineum were reduced when the saddle was tilted downward, whereas stresses in the posterior were reduced when the saddle was tilted upward. CONCLUSIONS Recommendations that saddles should be sufficiently wide to support the ischial tuberosities appear to be well founded. Recommendations that saddles be tilted downward (i.e., nose down) are supported by the model, but with caution, given the limitations of the model.

An investigation into fractured surfaces of enamel of modern human teeth: a combined SEM and computer visualisation study

It has long been recognised that the enamel microstructure may hold important information with regards to phylogeny and masticatory biomechanics. Further, the biophysical and adaptive processes involved in enamel formation and in the creation of different microstructures are poorly understood. This lack of understanding is in part due to technical difficulties when visualising the 3D structure of enamel. Using modern visualisation techniques, models of various regions of different modern human teeth were created. Underlying these models are consistent mathematical representations of the interplay between cell-to-cell adhesion, integrity of the advancing enamel front and (potentially decreasing) constraints on the prism course from the dentino-enamel junction (DEJ) to the outer surface. Seven modern human teeth (I1, 1 lower C, 1 P4, 1 M2, 2 M2 and 1 M3) were fractured longitudinally and formed the basis for the creation of the models. For validation purposes the teeth were then fractured transversely, thus allowing quantitative comparisons between the prism pathways on the newly fractured transverse plane and the transverse pathways as predicted by the model. It was found that these predictions were fairly accurate provided that (a) the light position with respect to the model corresponds with the beam position with respect to the scanned surface and (b) the path of prisms was carefully reconstructed/extrapolated from SEM in cases where prisms were broken. Given that these predictions were based on the mechanisms governing enamel formation as applied to the model, it is suggested that such theories must be reasonable. In other words, biophysical processes, rather than complicated (genetic) positional information, suffice to create different enamel microstructures. In addition, systematic differences were found in prism deviation from their c-axis in different enamel pieces. Given the nature of these differences it is suggested that enamel formation is not only the result of biophysical processes (proximal causes), but could be due to the structures having been selected for in order to counteract masticatory stress exerted during the lifetime of the species (ultimate causes). As to whether and to what extent this may be the case is not yet clear but it is apparent that computer visualisation does have potential to quantify enamel microstructure and to address such questions. Given its non-destructive nature, computer modelling could have particular relevance for studying fragmented fossilised remains.

The effect of mouthguard design on stresses in the tooth-bone complex

PURPOSE Mouthguards protect the tooth-bone complex from impact loads that occur during sporting activity. The aim of this study is to investigate the effects of varying mouthguard thickness and stiffness on the magnitude of tensile stresses in the tooth-bone-complex. METHODS A two-dimensional, plane stress, finite element representation of a central maxillary incisor (CMI) is created. For validation purposes, displacements of the incisal edge of the unprotected tooth model are compared with in vivo displacements under similar loads. A protective mouthguard is then superimposed over the model with varied labial thickness (1-6 mm) and stiffness (9-900MPa) representing a range of designs available. A large horizontal static load of 500N is then applied to the anterior surface of the mouthguard and the resulting stresses in the tooth-bone complex are presented. It is suggested that this loading condition most accurately represent the situation occurring when a guarded tooth collides with a soft object (e.g. boxing glove). RESULTS It is generally found that mouthguard thickness and stiffness are both desirable in terms of reducing stresses. However, the protection offered by the low-stiffness guards, regardless of thickness, is minimal. Since this low-stiffness (9MPa) is representative of the most common choice of material in mouthguard fabrication, such findings may cast doubt on the ability of popular mouthguards to redistribute stress. CONCLUSION While few would disagree that these low-stiffness guards absorb shock during hard-object collisions (e.g. baseballs), they may not protect the tooth-bone during soft-object collisions (e.g. boxing gloves). In order to optimize their protective capabilities for a range of loads, the range of materials used in mouthguard construction may have to be reconsidered.

Isolated Core Training Improves Sprint Performance in National-Level Junior Swimmers

PURPOSE To quantify the effects of a 12-wk isolated core-training program on 50-m front-crawl swim time and measures of core musculature functionally relevant to swimming. METHODS Twenty national-level junior swimmers (10 male and 10 female, 16±1 y, 171±5 cm, 63±4 kg) participated in the study. Group allocation (intervention [n=10], control [n=10]) was based on 2 preexisting swim-training groups who were part of the same swimming club but trained in different groups. The intervention group completed the core training, incorporating exercises targeting the lumbopelvic complex and upper region extending to the scapula, 3 times/wk for 12 wk. While the training was performed in addition to the normal pool-based swimming program, the control group maintained their usual pool-based swimming program. The authors made probabilistic magnitude-based inferences about the effect of the core training on 50-m swim time and functionally relevant measures of core function. RESULTS Compared with the control group, the core-training intervention group had a possibly large beneficial effect on 50-m swim time (-2.0%; 90% confidence interval -3.8 to -0.2%). Moreover, it showed small to moderate improvements on a timed prone-bridge test (9.0%; 2.1-16.4%) and asymmetric straight-arm pull-down test (23.1%; 13.7-33.4%), and there were moderate to large increases in peak EMG activity of core musculature during isolated tests of maximal voluntary contraction. CONCLUSION This is the first study to demonstrate a clear beneficial effect of isolated core training on 50-m front-crawl swim performance.

Gait retraining and incidence of medial tibial stress syndrome in army recruits.

PURPOSE Gait retraining, comprising biofeedback and/or an exercise intervention, might reduce the risk of musculoskeletal conditions. The purpose was to examine the effect of a gait-retraining program on medial tibial stress syndrome incidence during a 26-wk basic military training regimen. METHODS A total of 450 British Army recruits volunteered. On the basis of a baseline plantar pressure variable (mean foot balance during the first 10% of stance), participants classified as at risk of developing medial tibial stress syndrome (n = 166) were randomly allocated to an intervention (n = 83) or control (n = 83) group. The intervention involved supervised gait retraining, including exercises to increase neuromuscular control and flexibility (three sessions per week) and biofeedback enabling internalization of the foot balance variable (one session per week). Both groups continued with the usual military training regimen. Diagnoses of medial tibial stress syndrome over the 26-wk regimen were made by physicians blinded to the group assignment. Data were modeled in a survival analysis using Cox regression, adjusting for baseline foot balance and time to peak heel rotation. RESULTS The intervention was associated with a substantially reduced instantaneous relative risk of medial tibial stress syndrome versus control, with an adjusted HR of 0.25 (95% confidence interval, 0.05-0.53). The number needed to treat to observe one additional injury-free recruit in intervention versus control at 20 wk was 14 (11 to 23) participants. Baseline foot balance was a nonspecific predictor of injury, with an HR per 2 SD increment of 5.2 (1.6 to 53.6). CONCLUSIONS The intervention was effective in reducing incidence of medial tibial stress syndrome in an at-risk military sample.