Power output of the lower limb during variable inertial loading: a comparison between methods using single and repeated contractions

Daily variation in performance measures related to anaerobic power and capacity: A systematic review

Numerous functional measures related to anaerobic performance display daily variation. The diversity of tests and protocols used to assess anaerobic performance related to diurnal effects and the lack of a standardized approach have hindered agreement in the literature. Therefore, the aim of the present study was to investigate and systematically review the evidence relating to time-of-day differences in anaerobic performance measures. The entire content of PubMed (MEDLINE), Scopus, SPORTDiscus® (via EBSCOhost) and Web of Science and multiple electronic libraries were searched. Only experimental research studies conducted in male adult participants aged ≥ 18 yrs before May 2021 were included. Studies assessing tests related to anaerobic capacity or anaerobic power between a minimum of two time-points during the day (morning vs evening) were deemed eligible. The primary search revealed that a total of 55 out of 145 articles were considered eligible and subsequently included. Thirty-nine studies assessed anaerobic power and twenty-five anaerobic capacity using different modes of exercise and test protocols. Forty-eight studies found several of their performance variables to display time-of-day effects, with higher values in the evening than the morning, while seven studies did not find any time-of-day significance in any variables which were assessed. The magnitude of difference is dependent on the modality and the exercise protocol used. Performance measures for anaerobic power found jump tests displayed 2.7 to 12.3% differences, force velocity tests ~8% differences, sprint tests 2.7 to 11.3% differences and 5-m multiple shuttle run tests 3.7 to 13.1% differences in favour of the evening. Performance measures for anaerobic capacity found Wingate test to display 1.8 to 11.7% differences and repeated sprint tests to display 3.4 to 10.2% differences. The only test not to display time-of-day differences was the running based anaerobic sprint test (RAST). Time-of-day variations in anaerobic performance has previously been partially explained by higher core-body and/or muscle temperature and better muscle contractile properties in the afternoon, although recent findings suggest that differences in methodology, motivation/arousal, habitual training times and chronotypes could provide additional explanations. There is a clear demand for a rigorous, standardised approach to be adopted by future investigations which control factors that specifically relate to investigations of time-of-day.

On the Shape of the Force-Velocity Relationship in Skeletal Muscles: The Linear, the Hyperbolic, and the Double-Hyperbolic

The shape of the force-velocity (F-V) relationship has important implications for different aspects of muscle physiology, such as muscle efficiency and fatigue, the understanding of the pathophysiology of several myopathies or the mechanisms of muscle contraction per se, and may be of relevance for other fields, such as the development of robotics and prosthetic applications featuring natural muscle-like properties. However, different opinions regarding the shape of the F-V relationship and the underlying mechanisms exist in the literature. In this review, we summarize relevant evidence on the shape of the F-V relationship obtained over the last century. Studies performed at multiple scales ranging from the sarcomere to the organism level have described the concentric F-V relationship as linear, hyperbolic or double-hyperbolic. While the F-V relationship has most frequently been described as a rectangular hyperbola, a large number of studies have found deviations from the hyperbolic function at both ends of the F-V relation. Indeed, current evidence suggests that the F-V relation in skeletal muscles follows a double-hyperbolic pattern, with a breakpoint located at very high forces/low velocities, which may be a direct consequence of the kinetic properties of myofilament cross-bridge formation. Deviations at low forces/high velocities, by contrast, may be related to a recently discovered, calcium-independent regulatory mechanism of muscle contraction, which may also explain the low metabolic cost of very fast muscle shortening contractions. Controversial results have also been reported regarding the eccentric F-V relationship, with studies in prepared muscle specimens suggesting that maximum eccentric force is substantially greater than isometric force, whereas in vivo studies in humans show only a modest increase, no change, or even a decrease in force in lengthening contractions. This review discusses possible reasons reported in the literature for these discrepant findings, including the testing procedures (familiarization, pre-load condition, and temperature) and a potential neural inhibition at higher lengthening velocities. Finally, some unresolved questions and recommendations for F-V testing in humans are reported at the end of this document.

Preoperative quadriceps weakness preferentially predicts postoperative aberrant movement patterns during high-demand mobility following total knee arthroplasty

BACKGROUND

Nearly all patients with total knee arthroplasty show aberrant movement patterns during tasks requiring greater joint demand compared to matched peers. Greater movement compensation leads to increased loading onto other joints, decreased functional capacity and limited reserve for independence later in life. Understanding how preoperative predictors contribute to postoperative aberrant movement patterns is needed to make better decisions for patients considering total knee arthroplasty.

METHODS

Forty-seven patients were tested preoperatively and six months following primary total knee arthroplasty. Demographic (age, sex, body mass), self-reported (knee pain, perception of physical performance, physical activity level), physical performance (quadriceps strength, lower limb power and timed stair climbing) and surgical metrics were collected as predictor variables. Three-dimensional models based on joint mechanic asymmetry during a decline walking task were collected at six months postoperatively. Decline walking is a preferred means to assess the surgical knee's contribution to limb performance during high-demand tasks. Bootstrap inclusion fraction was employed to compare the stability of each predictor variable prior to the final regression model.

RESULTS

Preoperative quadriceps strength (β = 0.33; p = 0.04) showed a significant relationship with knee extensor angular impulse during loading phase. No other predictor variable had any meaningful relationship with aberrant movement patterns (p > 0.05).

CONCLUSION

Our findings highlight patients' preoperative quadriceps strength as a meaningful predictor of postoperative performance. Preoperative quadriceps strength should be addressed when considering the knee's ability to contribute to higher demanding mobility tasks following surgery.

Quadriceps weakness preferentially predicts detrimental gait compensations among common impairments after total knee arthroplasty

Patients with total knee arthroplasty (TKA) have large deficits in physical performance in comparison to their healthy age-matched peers. Limb asymmetry stemming from less relative load borne by the surgical limb during daily mobility is associated with diminished performance and worsens with greater mobility demands. How common targets of postoperative care, such as muscle weakness, lower limb extension power, residual knee pain, and poor balance confidence can influence asymmetrical limb loading remains unclear. Forty-six patients with unilateral TKA underwent testing of impairments and motion analysis during 10° decline walking at 3 and 6 months postoperatively. At 3 months, only quadriceps femoris strength asymmetry was found to be significantly related to both total support moment (M_T ) (β = 0.431; p < 0.001) and knee extensor moment (M_K ) (β = 0.493; p < 0.001) asymmetry. Again at 6 months, only quadriceps strength asymmetry was related to M_T (β = 0.432; p < 0.001) and M_K (β = 0.534; p < 0.001) asymmetry. Quadriceps strength significantly improved over time in both limbs, however, deficits between limbs remained. Persistent quadriceps weakness is a key factor associated with walking compensation patterns that are limiting the capacity for greater physical performance of patients with TKA. The pronounced asymmetry in limb and knee loading at 3 months remains unchanged until at least 6 months after surgery, and its association with quadriceps strength asymmetry does not substantially change over time. While other factors may also prompt gait compensations, emphasis on improved quadriceps strength should be a focus of efforts to resolve gait compensations and enhance physical performance outcomes. © 2018 Orthopaedic Research Society. Published by Wiley Periodicals, Inc. J Orthop Res 36:2355-2363, 2018.

Properties of the vastus lateralis muscle in relation to age and physiological function in master cyclists aged 55-79 years

In this study, results are reported from the analyses of vastus lateralis muscle biopsy samples obtained from a subset (n = 90) of 125 previously phenotyped, highly active male and female cyclists aged 55–79 years in regard to age. We then subsequently attempted to uncover associations between the findings in muscle and in vivo physiological functions. Muscle fibre type and composition (ATPase histochemistry), size (morphometry), capillary density (immunohistochemistry) and mitochondrial protein content (Western blot) in relation to age were determined in the biopsy specimens. Aside from an age‐related change in capillary density in males (r = −.299; p = .02), no other parameter measured in the muscle samples showed an association with age. However, in males type I fibres and capillarity (p < .05) were significantly associated with training volume, maximal oxygen uptake, oxygen uptake kinetics and ventilatory threshold. In females, the only association observed was between capillarity and training volume (p < .05). In males, both type II fibre proportion and area (p < .05) were associated with peak power during sprint cycling and with maximal rate of torque development during a maximal voluntary isometric contraction. Mitochondrial protein content was not associated with any cardiorespiratory parameter in either males or females (p > .05). We conclude in this highly active cohort, selected to mitigate most of the effects of inactivity, that there is little evidence of age‐related changes in the properties of VL muscle across the age range studied. By contrast, some of these muscle characteristics were correlated with in vivo physiological indices.

The Relationship between Pedal Force and Crank Angular Velocity in Sprint Cycling

PURPOSE

Relationships between tangential pedal force and crank angular velocity in sprint cycling tend to be linear. We set out to understand why they are not hyperbolic, like the intrinsic force-velocity relationship of muscles.

METHODS

We simulated isokinetic sprint cycling at crank angular velocities ranging from 30 to 150 rpm with a forward dynamic model of the human musculoskeletal system actuated by eight lower extremity muscle groups. The input of the model was muscle stimulation over time, which we optimized to maximize average power output over a cycle.

RESULTS

Peak tangential pedal force was found to drop more with crank angular velocity than expected based on intrinsic muscle properties. This linearizing effect was not due to segmental dynamics but rather due to active state dynamics. Maximizing average power in cycling requires muscles to bring their active state from as high as possible during shortening to as low as possible during lengthening. Reducing the active state is a relatively slow process, and hence must be initiated a certain amount of time before lengthening starts. As crank angular velocity goes up, this amount of time corresponds to a greater angular displacement, so the instant of switching off extensor muscle stimulation must occur earlier relative to the angle at which pedal force was extracted for the force-velocity relationship.

CONCLUSION

Relationships between pedal force and crank angular velocity in sprint cycling do not reflect solely the intrinsic force-velocity relationship of muscles but also the consequences of activation dynamics.

Deconstructing the power resistance relationship for squats: A joint-level analysis

Generating high leg power outputs is important for executing rapid movements. Squats are commonly used to increase leg strength and power. Therefore, it is useful to understand factors affecting power output in squatting. We aimed to deconstruct the mechanisms behind why power is maximized at certain resistances in squatting. Ten male rowers (age = 20 ± 2.2 years; height = 1.82 ± 0.03 m; mass = 86 ± 11 kg) performed maximal power squats with resistances ranging from body weight to 80% of their one repetition maximum (1RM). Three-dimensional kinematics was combined with ground reaction force (GRF) data in an inverse dynamics analysis to calculate leg joint moments and powers. System center of mass (COM) velocity and power were computed from GRF data. COM power was maximized across a range of resistances from 40% to 60% 1RM. This range was identified because a trade-off in hip and knee joint powers existed across this range, with maximal knee joint power occurring at 40% 1RM and maximal hip joint power at 60% 1RM. A non-linear system force-velocity relationship was observed that dictated large reductions in COM power below 20% 1RM and above 60% 1RM. These reductions were due to constraints on the control of the movement.

The growth plate's response to load is partially mediated by mechano-sensing via the chondrocytic primary cilium

Mechanical load plays a significant role in bone and growth-plate development. Chondrocytes sense and respond to mechanical stimulation; however, the mechanisms by which those signals exert their effects are not fully understood. The primary cilium has been identified as a mechano-sensor in several cell types, including renal epithelial cells and endothelium, and accumulating evidence connects it to mechano-transduction in chondrocytes. In the growth plate, the primary cilium is involved in several regulatory pathways, such as the non-canonical Wnt and Indian Hedgehog. Moreover, it mediates cell shape, orientation, growth, and differentiation in the growth plate. In this work, we show that mechanical load enhances ciliogenesis in the growth plate. This leads to alterations in the expression and localization of key members of the Ihh-PTHrP loop resulting in decreased proliferation and an abnormal switch from proliferation to differentiation, together with abnormal chondrocyte morphology and organization. Moreover, we use the chondrogenic cell line ATDC5, a model for growth-plate chondrocytes, to understand the mechanisms mediating the participation of the primary cilium, and in particular KIF3A, in the cell's response to mechanical stimulation. We show that this key component of the cilium mediates gene expression in response to mechanical stimulation.

An investigation into the relationship between age and physiological function in highly active older adults

KEY POINTS

The relationship between age and physiological function remains poorly defined and there are no physiological markers that can be used to reliably predict the age of an individual. This could be due to a variety of confounding genetic and lifestyle factors, and in particular to ill-defined and low levels of physical activity. This study assessed the relationship between age and a diverse range of physiological functions in a cohort of highly active older individuals (cyclists) aged 55-79 years in whom the effects of lifestyle factors would be ameliorated. Significant associations between age and function were observed for many functions. V̇O2max was most closely associated with age, but even here the variance in age for any given level was high, precluding the clear identification of the age of any individual. The data suggest that the relationship between human ageing and physiological function is highly individualistic and modified by inactivity.

ABSTRACT

Despite extensive research, the relationship between age and physiological function remains poorly characterised and there are currently no reliable markers of human ageing. This is probably due to a number of confounding factors, particularly in studies of a cross-sectional nature. These include inter-subject genetic variation, as well as inter-generational differences in nutrition, healthcare and insufficient levels of physical activity as well as other environmental factors. We have studied a cohort of highly and homogeneously active older male (n = 84) and female (n = 41) cyclists aged 55-79 years who it is proposed represent a model for the study of human ageing free from the majority of confounding factors, especially inactivity. The aim of the study was to identify physiological markers of ageing by assessing the relationship between function and age across a wide range of indices. Each participant underwent a detailed physiological profiling which included measures of cardiovascular, respiratory, neuromuscular, metabolic, endocrine and cognitive functions, bone strength, and health and well-being. Significant associations between age and function were observed for many functions. The maximal rate of oxygen consumption (V̇O2max) showed the closest association with age (r = -0.443 to -0.664; P < 0.001), but even here the variance in age for any given level was high, precluding the clear identification of the age of any individual. The results of this cross-sectional study suggest that even when many confounding variables are removed the relationship between function and healthy ageing is complex and likely to be highly individualistic and that physical activity levels must be taken into account in ageing studies.

Interdependence of torque, joint angle, angular velocity and muscle action during human multi-joint leg extension

PURPOSE

Force and torque production of human muscles depends upon their lengths and contraction velocity. However, these factors are widely assumed to be independent of each other and the few studies that dealt with interactions of torque, angle and angular velocity are based on isolated single-joint movements. Thus, the purpose of this study was to determine force/torque-angle and force/torque-angular velocity properties for multi-joint leg extensions.

METHODS

Human leg extension was investigated (n = 18) on a motor-driven leg press dynamometer while measuring external reaction forces at the feet. Extensor torque in the knee joint was calculated using inverse dynamics. Isometric contractions were performed at eight joint angle configurations of the lower limb corresponding to increments of 10° at the knee from 30 to 100° of knee flexion. Concentric and eccentric contractions were performed over the same range of motion at mean angular velocities of the knee from 30 to 240° s(-1).

RESULTS

For contractions of increasing velocity, optimum knee angle shifted from 52 ± 7 to 64 ± 4° knee flexion. Furthermore, the curvature of the concentric force/torque-angular velocity relations varied with joint angles and maximum angular velocities increased from 866 ± 79 to 1,238 ± 132° s(-1) for 90-50° knee flexion. Normalised eccentric forces/torques ranged from 0.85 ± 0.12 to 1.32 ± 0.16 of their isometric reference, only showing significant increases above isometric and an effect of angular velocity for joint angles greater than optimum knee angle.

CONCLUSIONS

The findings reveal that force/torque production during multi-joint leg extension depends on the combined effects of angle and angular velocity. This finding should be accounted for in modelling and optimisation of human movement.

Lower limb mechanical properties: determining factors and implications for performance

Limb stiffness or musculotendinous stiffness (MTS) has previously been examined in relation to performance and characterized using a number of different methods. However, the fact that MTS shows only low to moderate correlations to performances may indicate a lack of understanding of this parameter. In addition to this, variation is seen between studies examining the same factors. To date, our understanding of MTS and its components are not complete and thus it is unclear which characteristic value represents the ideal index of stiffness as it relates to performance. Moreover, it is uncertain how MTS stiffness as a functional measure relates to performance, and also if there is an optimal amount of MTS stiffness for specific functions or tasks. The knowledge of the interplay of MTU stiffness as it relates to performance and injury risk is also poorly understood in that there is likely a disparity between levels of stiffness required to optimize performance and those required to minimize injury risk. The aim of this article is to review the literature as it describes the components of MTS and to discuss these in terms of their relationship to functional performance; consider adaptations of the MTU with training along with associated performance changes; highlight and discuss how stiffness may affect loading of the soft and bony tissues in terms of the MTU components and gender, with respect to risk of injury; discuss the apparent differences in the literature regarding associations of the various forms of stiffness index to function; suggest recommendations for training in light of adaptation of the muscle and tendon and injury risk in context of gender; and, finally, to highlight potential limitations of current methodologies and suggest further work to gain insight into the mechanisms of stiffness. It is hoped that by suggesting future work, a more detailed and comprehensive understanding of MTS will be gained, thus enabling appropriate interventions to optimally modify this parameter for specific requirements.

Comparison of power and EMG during 6-s all-out cycling between young and older women

To investigate the effects of ageing on the neural control strategies governing sprint cycling on a friction-loaded cycle-ergometer, 10 older (aged 70-83 yr) and 8 young (aged 19-35 yr) healthy women completed seven 6-s all-out cycling trials against varying loads. Root mean square (RMS), median frequency and muscle fibre conduction velocity were determined from the vastus lateralis of the dominant limb during each pedal stroke. Peak power was 43% lower in the older group compared to the younger (p < 0.001) and was accompanied by a significantly lower RMS (p < 0.05). No differences were observed in the other electromyography (EMG) parameters between the groups (p > 0.05). ΔRMS from the first to the sixth second during each trial was found to increase significantly with the development of power output in both groups (p < 0.05). For the first time during an all-out 6-s cycle trial, it has been demonstrated that older women's lower mechanical power output was accompanied by a significantly lower RMS, which indicates a decline in either the number of active of motor units or a reduced discharge rate. Hence, changes in motor units can be regarded as a contributory factor to the decline of muscle power with advancing age. Overall, though, similar neural strategies are adopted in both younger and older populations.

Why is the force-velocity relationship in leg press tasks quasi-linear rather than hyperbolic?

Force-velocity relationships reported in the literature for functional tasks involving a combination of joint rotations tend to be quasi-linear. The purpose of this study was to explain why they are not hyperbolic, like Hill's relationship. For this purpose, a leg press task was simulated with a musculoskeletal model of the human leg, which had stimulation of knee extensor muscles as only independent input. In the task the ankles moved linearly, away from the hips, against an imposed external force that was reduced over contractions from 95 to 5% of the maximum isometric value. Contractions started at 70% of leg length, and force and velocity values were extracted when 80% of leg length was reached. It was shown that the relationship between leg extension velocity and external force was quasi-linear, while the relationship between leg extension velocity and muscle force was hyperbolic. The discrepancy was explained by the fact that segmental dynamics canceled more and more of the muscle force as the external force was further reduced and velocity became higher. External power output peaked when the imposed external force was ∼50% of maximum, while muscle power output peaked when the imposed force was only ∼15% of maximum; in the latter case ∼70% of muscle power was buffered by the leg segments. According to the results of this study, there is no need to appeal to neural mechanisms to explain why, in leg press tasks, the force-velocity relationship is quasi-linear rather than hyperbolic.

A Cyclic Robot for Lower Limb Exercise

This paper presents the design and simulation of a cyclic robot for lower-limb exercise robots. The robot is designed specifically for cyclic motions and the high power nature of lower-limb interaction—as such, it breaks from traditional robotics wisdom by intentionally traveling through singularities and incorporating large inertia. Such attributes lead to explicit design considerations. Results from a simulation show that the specific design requires only a reasonably sized damper and motor.

Steady-state force–velocity relation in human multi-joint movement determined with force clamp analysis

To study the force-velocity characteristics of human knee-hip extension movement, a dynamometer, in which force was controlled by a servo system, was developed. Seated subjects pressed either bilaterally or unilaterally a force plate, a horizontal position of which was servo-controlled so as to equalize the measured force and a force command generated by a computer at a time resolution of 2 ms (force clamp). The force command was based on the relation between maximum isometric force and foot position within the range between 70% and 90% of "leg length" (LL: longitudinal distance between the sole of the foot and the hip joint), so that the same force relative to the maximum isometric force was consistently applied regardless of the foot position. By regulating the force according to this function, the force-velocity relation was determined. The force-velocity relation obtained was described by a linear function (n=17, r=-0.986 for 80% LL, r=-0.968 for 85% LL) within a range of force between 0.1 and 0.8F(0) (maximum isometric force). The maximum force extrapolated from the linear regression (F(max)) coincided with F(0) (n=17, F(0)/F(max)=1.00+/-0.09 for 80% LL and 1.00+/-0.20 for 85% LL). Also, the velocity at zero force (V(max)) was obtained from the extrapolation. When compared to the bilateral movements, unilateral movements gave rise to a smaller F(max) but the same V(max), suggesting that V(max) is independent of force and therefore represents the proper unloaded velocity. It is suggested that some neural mechanisms may be involved in the force-velocity relation of the knee-hip extension movement, and make it exhibit a linear appearance rather than a hyperbola.

Power Output and Muscle Myosin Heavy Chain Composition in Young and Elderly Men

PURPOSE

Aging is associated with a decline in muscle volume, power output, and the velocity at which peak power (Vopt) occurs. The current study aimed to examine the relationship between lower-limb power output characteristics, muscle myosin heavy chain (MHC) composition, and lean limb volume.

METHODS

Lower-limb power output during repeated efforts on an inertial sprint cycle and single-leg thrusts on the modified Nottingham power rig was studied in seven young and seven old males in relation to muscle MHC isoform composition of the vastus lateralis.

RESULTS

Older subjects produced significantly lower power outputs and Vopt under all conditions (P < 0.01) and had lower proportions of fast MHC isoforms (P< 0.05). Peak power output during cycling was significantly related to lower-limb lean volume (r = 0.92, P < 0.05), whereas Vopt during sprint cycling was closely related to vastus lateralis MHC-II composition (r = 0.80, P < 0.05).

CONCLUSIONS

These results provide further evidence of the importance of fast myosin isoform composition in the maintenance of dynamic muscle function in later life and particularly for maximal cycling performance.