Testing protocols and measurement techniques when using pressure sensors for sport and health applications: A comparative review

Plantar pressure measurement systems are routinely used in sports and health applications to assess locomotion. The purpose of this review is to describe and critically discuss: (a) applications of the pressure measurement systems in sport and healthcare, (b) testing protocols and considerations for clinical gait analysis, (c) clinical recommendations for interpreting plantar pressure data, (d) calibration procedures and their accuracy, and (e) the future of pressure sensor data analysis. Rigid pressure platforms are typically used to measure plantar pressures for the assessment of foot function during standing and walking, particularly when barefoot, and are the most accurate for measuring plantar pressures. For reliable data, two step protocol prior to contacting the pressure plate is recommended. In-shoe systems are most suitable for measuring plantar pressures in the field during daily living or dynamic sporting movements as they are often wireless and can measure multiple steps. They are the most suitable equipment to assess the effects of footwear and orthotics on plantar pressures. However, they typically have lower spatial resolution and sampling frequency than platform systems. Users of pressure measurement systems need to consider the suitability of the calibration procedures for their chosen application when selecting and using a pressure measurement system. For some applications, a bespoke calibration procedure is required to improve validity and reliability of the pressure measurement system. The testing machines that are commonly used for dynamic calibration of pressure measurement systems frequently have loading rates of less than even those found in walking, so the development of testing protocols that truly measure the loading rates found in many sporting movements are required. There is clear potential for AI techniques to assist in the analysis and interpretation of plantar pressure data to enable the more complete use of pressure system data in clinical diagnoses and monitoring.

Commercially available pressure sensors for sport and health applications: A comparative review

Pressure measurement systems have numerous applications in healthcare and sport. The purpose of this review is to: (a) describe the brief history of the development of pressure sensors for clinical and sport applications, (b) discuss the design requirements for pressure measurement systems for different applications, (c) critique the suitability, reliability, and validity of commercial pressure measurement systems, and (d) suggest future directions for the development of pressure measurements systems in this area. Commercial pressure measurement systems generally use capacitive or resistive sensors, and typically capacitive sensors have been reported to be more valid and reliable than resistive sensors for prolonged use. It is important to acknowledge, however, that the selection of sensors is contingent upon the specific application requirements. Recent improvements in sensor and wireless technology and computational power have resulted in systems that have higher sensor density and sampling frequency with improved usability - thinner, lighter platforms, some of which are wireless, and reduced the obtrusiveness of in-shoe systems due to wireless data transmission and smaller data-logger and control units. Future developments of pressure sensors should focus on the design of systems that can measure or accurately predict shear stresses in conjunction with pressure, as it is thought the combination of both contributes to the development of pressure ulcers and diabetic plantar ulcers. The focus for the development of in-shoe pressure measurement systems is to minimise any potential interference to the patient or athlete, and to reduce power consumption of the wireless systems to improve the battery life, so these systems can be used to monitor daily activity. A potential solution to reduce the obtrusiveness of in-shoe systems include thin flexible pressure sensors which can be incorporated into socks. Although some experimental systems are available further work is needed to improve their validity and reliability.

The MMAAS Project: An Observational Human Study Investigating the Effect of Anabolic Androgenic Steroid Use on Gene Expression and the Molecular Mechanism of Muscle Memory

OBJECTIVE

It remains unknown whether myonuclei remain elevated post anabolic-androgenic steroid (AAS) usage in humans. Limited data exist on AAS-induced changes in gene expression.

DESIGN

Cross-sectional/longitudinal.

SETTING

University.

PARTICIPANTS

Fifty-six men aged 20 to 42 years.

INDEPENDENT VARIABLES

Non-resistance-trained (C) or resistance-trained (RT), RT currently using AAS (RT-AS), of which if AAS usage ceased for ≥18 weeks resampled as Returning Participants (RP) or RT previously using AAS (PREV).

MAIN OUTCOME MEASURES

Myonuclei per fiber and cross-sectional area (CSA) of trapezius muscle fibers.

RESULTS

There were no significant differences between C (n = 5), RT (n = 15), RT-AS (n = 17), and PREV (n = 6) for myonuclei per fiber. Three of 5 returning participants (RP1-3) were biopsied twice. Before visit 1, RP1 ceased AAS usage 34 weeks before, RP2 and RP3 ceased AAS usage ≤2 weeks before, and all had 28 weeks between visits. Fiber CSA decreased for RP1 and RP2 between visits (7566 vs 6629 μm 2 ; 7854 vs 5677 μm 2 ) while myonuclei per fiber remained similar (3.5 vs 3.4; 2.5 vs 2.6). Respectively, these values increased for RP3 between visits (7167 vs 7889 μm 2 ; 2.6 vs 3.3).

CONCLUSIONS

This cohort of past AAS users did not have elevated myonuclei per fiber values, unlike previous research, but reported AAS usage was much lower. Training and AAS usage history also varied widely among participants. Comparable myonuclei per fiber numbers despite decrements in fiber CSA postexposure adheres with the muscle memory mechanism, but there is variation in usage relative to sampling date and low numbers of returning participants.

An observational human study investigating the effect of anabolic androgenic steroid use on the transcriptome of skeletal muscle and whole blood using RNA-Seq

BACKGROUND

The effects of Anabolic Androgenic Steroids (AAS) are largely illustrated through Androgen Receptor induced gene transcription, yet RNA-Seq has yet to be conducted on human whole blood and skeletal muscle. Investigating the transcriptional signature of AAS in blood may aid AAS detection and in muscle further understanding of AAS induced hypertrophy.

METHODS

Males aged 20-42 were recruited and sampled once: sedentary controls (C), resistance trained lifters (RT) and resistance trained current AAS users (RT-AS) who ceased exposure ≤ 2 or ≥ 10 weeks prior to sampling. RT-AS were sampled twice as Returning Participants (RP) if AAS usage ceased for ≥ 18 weeks. RNA was extracted from whole blood and trapezius muscle samples. RNA libraries were sequenced twice, for validation purposes, on the DNBSEQ-G400RS with either standard or CoolMPS PE100 reagents following MGI protocols. Genes were considered differentially expressed with FDR < 0.05 and a 1.2- fold change.

RESULTS

Cross-comparison of both standard reagent whole blood (N = 55: C = 7, RT = 20, RT-AS ≤ 2 = 14, RT-AS ≥ 10 = 10, RP = 4; N = 46: C = 6, RT = 17, RT-AS ≤ 2 = 12, RT-AS ≥ 10 = 8, RP = 3) sequencing datasets, showed that no genes or gene sets/pathways were differentially expressed between time points for RP or between group comparisons of RT-AS ≤ 2 vs. C, RT, or RT-AS ≥ 10. Cross-comparison of both muscle (N = 51, C = 5, RT = 17, RT-AS ≤ 2 = 15, RT-AS ≥ 10 = 11, RP = 3) sequencing (one standard & one CoolMPS reagent) datasets, showed one gene, CHRDL1, which has atrophying potential, was upregulated in RP visit two. In both muscle sequencing datasets, nine differentially expressed genes, overlapped with RT-AS ≤ 2 vs. RT and RT-AS ≤ 2 vs. C, but were not differentially expressed with RT vs. C, possibly suggesting they are from acute doping alone. No genes seemed to be differentially expressed in muscle after the long-term cessation of AAS, whereas a previous study found long term proteomic changes.

CONCLUSION

A whole blood transcriptional signature of AAS doping was not identified. However, RNA-Seq of muscle has identified numerous differentially expressed genes with known impacts on hypertrophic processes that may further our understanding on AAS induced hypertrophy. Differences in training regimens in participant groupings may have influenced results. Future studies should focus on longitudinal sampling pre, during and post-AAS exposure to better control for confounding variables.

Printed Nanocarbon Heaters for Stretchable Sport and Leisure Garments

The ability to maintain body temperature has been shown to bring about improvements in sporting performance. However, current solutions are limited with regards to flexibility, heating uniformity and robustness. An innovative screen-printed Nanocarbon heater is demonstrated which is robust to bending, folding, tensile extensions of up to 20% and machine washing. This combination of ink and substrate enables the heated garments to safely flex without impeding the wearer. It is capable of producing uniform heating over a 15 × 4 cm area using a conductive ink based on a blend of Graphite Nanoplatelets and Carbon Black. This can be attributed to the low roughness of the conductive carbon coating, the uniform distribution and good interconnection of the carbon particles. The heaters have a low thermal inertia, producing a rapid temperature response at low voltages, reaching equilibrium temperatures within 120 s of being switched on. The heaters reached the 40 °C required for wearable heating applications within 20 s at 12 Volts. Screen printing was demonstrated to be an effective method of controlling the printed layer thickness with good interlayer adhesion and contact for multiple printed layers. This can be used to regulate their electrical properties and hence adjust the heater performance.

Repeated Ischemic Preconditioning Effects on Physiological Responses to Hypoxic Exercise

INTRODUCTION: Repeated ischemic preconditioning (IPC) can improve muscle and pulmonary oxygen on-kinetics, blood flow, and exercise efficiency, but these effects have not been investigated in severe hypoxia. The aim of the current study was to evaluate the effects of 7 d of IPC on resting and exercising muscle and cardio-pulmonary responses to severe hypoxia.METHODS: A total of 14 subjects received either: 1) 7 d of repeated lower-limb occlusion (4 × 5 min, 217 ± 30 mmHg) at limb occlusive pressure (IPC) or SHAM (4 × 5 min, 20 mmHg). Subjects were tested for resting limb blood flow, relative microvascular deoxyhemoglobin concentration ([HHB]), and pulmonary oxygen (Vo_2p) responses to steady state and incremental exercise to exhaustion in hypoxia (fractional inspired O₂ = 0.103), which was followed by 7 d of IPC or SHAM and retesting 72 h post-intervention.RESULTS: There were no effects of IPC on maximal oxygen consumption, time to exhaustion during the incremental test, or minute ventilation and arterial oxygen saturation. However, the IPC group had higher delta efficiency based on pooled results and lower steady state Δ[HHB] (IPC ∼24% vs. SHAM ∼6% pre to post), as well as slowing the [HHB] time constant (IPC ∼26% vs. SHAM ∼3% pre to post) and reducing the overshoot in [HHB]: Vo₂ ratio during exercise onset.CONCLUSIONS: Collectively, these results demonstrate that muscle O₂ efficiency and microvascular O₂ distribution can be improved by repeated IPC, but there are no effects on maximal exercise capacity in severe hypoxia.Chopra K, Jeffries O, Tallent J, Heffernan S, Kilduff L, Gray A, Waldron M. Repeated ischemic preconditioning effects on physiological responses to hypoxic exercise. Aerosp Med Hum Perform. 2022; 93(1):13-21.

Stretchable Carbon and Silver Inks for Wearable Applications

For wearable electronic devices to be fully integrated into garments, without restricting or impeding movement, requires flexible and stretchable inks and coatings, which must have consistent performance and recover from mechanical strain. Combining Carbon Black (CB) and ammonia plasma functionalized Graphite Nanoplatelets (GNPs) in a Thermoplastic Polyurethane (TPU) resin created a conductive ink that could stretch to substrate failure (>300% nominal strain) and cyclic strains of up to 100% while maintaining an electrical network. This highly stretchable, conductive screen-printable ink was developed using relatively low-cost carbon materials and scalable processes making it a candidate for future wearable developments. The electromechanical performance of the carbon ink for wearable technology is compared to a screen-printable silver as a control. After initial plastic deformation and the alignment of the nano carbons in the matrix, the electrical performance was consistent under cycling to 100% nominal strain. Although the GNP flakes are pulled further apart a consistent, but less conductive path remains through the CB/TPU matrix. In contrast to the nano carbon ink, a more conductive ink made using silver flakes lost conductivity at 166% nominal strain falling short of the substrate failure strain. This was attributed to the failure of direct contact between the silver flakes.

Concurrent adaptations in maximal aerobic capacity, heat tolerance, microvascular blood flow and oxygen extraction following heat acclimation and ischemic preconditioning

We investigated the effects of: 1) Ischemic pre-conditioning (IPC) plus a concurrent five-day heat acclimation + IPC (IPC + HA), 2) five-day HA with sham IPC (HA), or 3) control (CON) on thermoneutral measurements of endurance performance, resting measures of skeletal muscle oxygenation and blood flow. Twenty-nine participants were randomly allocated to three groups, which included: 1) five-days of repeated leg occlusion (4 x 5-min) IPC at limb occlusive pressure, plus fixed-intensity (55% V˙ O_2max) cycling HA at ~36 °C/40% humidity; 2) HA plus sham IPC (20 mmHg) or 3) or CON (thermoneutral 55% V˙ O_2max plus sham IPC). In IPC + HA and HA, there were increases in maximal oxygen consumption (O_2max) (7.8% and 5.4%, respectively; P < 0.05), ventilatory threshold (V_T) (5.6% and 2.4%, respectively, P < 0.05), delta efficiency (DE) (2.0% and 1.4%, respectively; P < 0.05) and maximum oxygen pulse (O₂pulse-Max) (7.0% and 6.9%, respectively; P < 0.05) during an exhaustive incremental test. There were no changes for CON (P > 0.05). Changes (P < 0.05) in resting core temperature (T_C), muscle oxygen consumption (m V˙ O₂), and limb blood flow (LBF) were also found pre-to-post intervention among the HA and IPC + HA groups, but not in CON (P > 0.05). Five-days of either HA or IPC + HA can enhance markers of endurance performance in cooler environments, alongside improved muscle oxygen extraction, blood flow, exercising muscle efficiency and O₂ pulse at higher intensities, thus suggesting the occurrence of peripheral adaptation. Both HA and IPC + HA enhance the adaptation of endurance capacity, which might partly relate to peripheral changes.

No thermoregulatory or ergogenic effect of dietary nitrate among physically inactive males, exercising above gas exchange threshold in hot and dry conditions

The aim of this study was to determine the effect of five days dietary nitrate (NO₃^-) consumption on exercise tolerance and thermoregulation during cycling in hot, dry conditions. In a double-blind, randomised crossover design, 11 healthy males participated in an exercise tolerance test (T_lim) in the heat (35°C, 28% relative humidity), cycling above the thermoneutral gas exchange threshold, after five days of dietary supplementation, with either NO₃^-rich beetroot juice (BR; ∼ 9.2 mmol NO₃^-) or placebo (PLA). Changes in plasma [NO₃^-] and nitrite [NO₂^-], core and mean skin temperatures, mean local and whole-body sweat rates, heart rate, perceptual ratings and pulmonary gas exchange were measured during exercise, alongside calorimetric estimations of thermal balance. Mean arterial pressures (MAP) were recorded pre-T_lim. There were no differences in T_lim between conditions (BR = 22.8 ± 8.1 min; Placebo = 20.7 ± 7.9 min) (P = 0.184), despite increases in plasma [NO₃^-] and [NO₂^-] (P < 0.001) and a 3.8% reduction in resting MAP (P = 0.004) in the BR condition. There were no other differences in thermoregulatory, cardio-metabolic, perceptual or calorimetric responses to the T_lim between conditions (P > 0.05). Dietary NO₃^- supplementation had no effect on exercise tolerance or thermoregulation in hot, dry conditions, despite reductions in resting MAP and increases in plasma [NO₃^-] and [NO₂^-]. Healthy, yet physically inactive individuals with no known impairments in vasodilatory and sudomotor function do not appear to require BR for ergogenic or thermolytic effects during exercise in the heat.

Digit ratio (2D:4D) and salivary testosterone, oestradiol and cortisol levels under challenge: Evidence for prenatal effects on adult endocrine responses

BACKGROUND

Digit ratio (2D:4D) is a marker for prenatal sex steroids and a correlate of sporting performance. This association may exist because low 2D:4D is linked to high prenatal levels of testosterone (T) and low oestrogens (E). It was recently suggested that low 2D:4D, and particularly low right-left 2D:4D (or Dr-l), is a marker for T changes in response to physical and aggressive challenges. If correct, this link may in part explain the association between 2D:4D and sports performance.

AIMS

We tested this hypothesis in adults.

STUDY DESIGN

Three experimental treatments were completed using a randomised, cross-over design; (i) cycle sprints plus an aggressive video (S+V), (ii) aggressive video plus cycle sprints (V+S), and (iii) a control session.

SUBJECTS

24 healthy adults (12 men and 12 women).

OUTCOME MEASURES

Salivary T, oestradiol (E2) and cortisol (C) levels were measured on six occasions across each session and pooled for analysis.

RESULTS

The S+V treatment was associated with a rise in T and C levels, and Dr-l was significantly and negatively correlated with T and E2 with these effects confined to men. The right 2D:4D and Dr-l were also negatively correlated with the T/C ratio and Dr-l negatively related to the E2/C ratio in men during the S+V treatment.

CONCLUSIONS

We suggest that the hormonal responses to a challenge are programmed by prenatal levels of T and E with possible links to sporting performance in adulthood.

Simulated games activity vs continuous running exercise: a novel comparison of the glycemic and metabolic responses in T1DM patients

To compare the glycemic and metabolic responses to simulated intermittent games activity and continuous running exercise in type 1 diabetes. Nine patients (seven male, two female; 35 ± 4 years; HbA1c 8.1 ± 0.2%/65 ± 2 mmol/mol) treated on a basal-bolus regimen completed two main trials, a continuous treadmill run (CON) or an intermittent running protocol (INT). Patients arrived to the laboratory fasted at ∼ 08:00 h, replicating their usual pre-exercise meal and administering a 50% reduced dose of rapid-acting insulin before exercising. Blood glucose (BG), K(+) , Na(++) , pH, triglycerides, serum cortisol and NEFA were measured at baseline and for 60 min post-exercise. Interstitial glucose was measured for a further 23 h under free-living conditions. Following exercise, BG declined under both conditions but was less under INT (INT -1.1 ± 1.4 vs CON -5.3 ± 0.4 mmol/L, P = 0.037), meaning more patients experienced hypoglycemia (BG ≤ 3.5 mmol/L; CON n = 3 vs INT n = 2) but less hyperglycemia (BG ≥ 10.9 mmol/L; CON n = 0 vs INT n = 6) under CON. Blood lactate was significantly greater, and pH lower, with a temporal delay in K(+) under INT (P < 0.05). No conditional differences were observed in other measures during this time, or in interstitial glucose concentrations during the remaining 23 h after exercise. Simulated games activity carries a lower risk of early, but not late-onset hypoglycemia than continuous running exercise in type 1 diabetes.

Digit Ratio (2D:4D): A Biomarker for Prenatal Sex Steroids and Adult Sex Steroids in Challenge Situations

Digit ratio (2D:4D) denotes the relative length of the second and fourth digits. This ratio is considered to be a biomarker of the balance between fetal testosterone (T) and estrogen (E) in a narrow window of early ontogeny. Evidence for this assertion is derived from direct and indirect measures of prenatal hormonal exposure (in experimental animals, via amniotic fluid samples and in the study of sex-typical traits) in relation to 2D:4D. In contrast, the relationships between 2D:4D and levels of sex steroids in adults are less clear, as many correlational studies of 2D:4D and adult sex steroids have concluded that this association is statistically non-significant. Here, we suggest that in order to understand the link between 2D:4D and sex hormones, one must consider both fetal organizing and adult activating effects of T and E. In particular, we hypothesize that 2D:4D correlates with organizing effects on the endocrine system that moderate activating effects in adulthood. We argue that this is particularly evident in "challenging" conditions such as aggressive and sexual encounters, in which individuals show increased levels of T. We discuss this refinement of the 2D:4D paradigm in relation to the links between 2D:4D and sports performance, and aggression.

Effects of caffeine and carbohydrate mouth rinses on repeated sprint performance

Our purpose was to examine the effectiveness of carbohydrate and caffeine mouth rinses in enhancing repeated sprint ability. Previously, studies have shown that a carbohydrate mouth rinse (without ingestion) has beneficial effects on endurance performance that are related to changes in brain activity. Caffeine ingestion has also demonstrated positive effects on sprint performance. However, the effects of carbohydrate or caffeine mouth rinses on intermittent sprints have not been examined previously. Twelve males performed 5 × 6-s sprints interspersed with 24 s of active recovery on a cycle ergometer. Twenty-five milliliters of either a noncaloric placebo, a 6% glucose, or a 1.2% caffeine solution was rinsed in the mouth for 5 s prior to each sprint in a double-blinded and balanced cross-over design. Postexercise maximal heart rate and perceived exertion were recorded, along with power measures. A second experiment compared a combined caffeine-carbohydrate rinse with carbohydrate only. Compared with the placebo mouth rinse, carbohydrate substantially increased peak power in sprint 1 (22.1 ± 19.5 W; Cohen's effect size (ES), 0.81), and both caffeine (26.9 ± 26.9 W; ES, 0.71) and carbohydrate (39.1 ± 25.8 W; ES, 1.08) improved mean power in sprint 1. Experiment 2 demonstrated that a combination of caffeine and carbohydrate improved sprint 1 power production compared with carbohydrate alone (36.0 ± 37.3 W; ES, 0.81). We conclude that carbohydrate and (or) caffeine mouth rinses may rapidly enhance power production, which could have benefits for specific short sprint exercise performance. The ability of a mouth-rinse intervention to rapidly improve maximal exercise performance in the absence of fatigue suggests a central mechanism.

Influence of active recovery on professional rugby union player's ability to harness postactivation potentiation

AIM

After postactivation potentiation (PAP) has been induced, current research recommends that, on average, an 8 minute passive recovery period is applied before engaging in subsequent dynamic exercise. However, given the importance of maximizing time usage during the warm-up of elite athletes, it is likely that further exercise would be incorporated into this time frame. This study aimed to examine the effects of passive and active recovery on the ability to utilize PAP.

METHODS

In a randomised and counter balanced design, 36 professional rugby union players completed two experimental trials involving a baseline countermovement jump (CMJ), followed by a PAP stimulus (3 x 3 repetitions at 87% of 1-RM back squat) and CMJ retesting after 8 minutes of passive or active recovery. The active recovery involved subjects performing ballistic bench throws (1 x 3 repetitions at 30% 1-RM bench press) 4 minutes after the lower body PAP stimulus. Data presented as mean±SD.

RESULTS

Baseline peak power output (PPO) was not different between conditions (P=0.61). CMJ PPO increased from baseline under both conditions, however the delta (mean±SD; passive +161±127 vs. active +116±44 W; P=0.03) and % change (passive 3.3±2.8 vs. active 2.3±0.9 %; P=0.03) in PPO was greater after the passive recovery, when compared to the active recovery.

CONCLUSION

In conclusion, the passive and active recovery periods both led to increases in lower-body PPO, nevertheless, the passive recovery elicited the greatest performance changes. However, the active recovery is a more practical option for athletes, as it maximizes time usage during warm-up.

Intermittent lower-limb occlusion enhances recovery after strenuous exercise

Repeated cycles of vascular occlusion followed by reperfusion initiate a protective mechanism that acts to mitigate future cell injury. Such ischemic episodes are known to improve vasodilation, oxygen utilization, muscle function, and have been demonstrated to enhance exercise performance. Thus, the use of occlusion cuffs represents a novel intervention that may improve subsequent exercise performance. Fourteen participants performed an exercise protocol that involved lower-body strength and power tests followed by repeated sprints. Occlusion cuffs were then applied unilaterally (2 × 3-min per leg) with a pressure of either 220 (intervention) or 15 mm Hg (control). Participants immediately repeated the exercise protocol, and then again 24 h later. The intervention elicited delayed beneficial effects (24 h post-intervention) in the countermovement jump test with concentric (effect size (ES) = 0.36) and eccentric (ES = 0.26) velocity recovering more rapidly compared with the control. There were also small beneficial effects on 10- and 40-m sprint times. In the squat jump test there were delayed beneficial effects of occlusion on eccentric power (ES = 1.38), acceleration (ES = 1.24), and an immediate positive effect on jump height (ES = 0.61). Thus, specific beneficial effects on recovery of power production and sprint performance were observed both immediately and 24 h after intermittent unilateral occlusion was applied to each leg.

Circadian disruption and remedial interventions: effects and interventions for jet lag for athletic peak performance

Jet lag has potentially serious deleterious effects on performance in athletes following transmeridian travel, where time zones are crossed eastwards or westwards; as such, travel causes specific effects related to desynchronization of the athlete's internal body clock or circadian clock. Athletes are particularly sensitive to the effects of jet lag, as many intrinsic aspects of sporting performance show a circadian rhythm, and optimum competitive results require all aspects of the athlete's mind and body to be working in tandem at their peak efficiency. International competition often requires transmeridian travel, and competition timings cannot be adjusted to suit individual athletes. It is therefore in the interest of the individual athlete and team to understand the effects of jet lag and the potential adaptation strategies that can be adopted. In this review, we describe the underlying genetic and physiological mechanisms controlling the circadian clock and its inherent ability to adapt to external conditions on a daily basis. We then examine the fundamentals of the various adaptation stimuli, such as light, chronobiotics (e.g. melatonin), exercise, and diet and meal timing, with particular emphasis on their suitability as strategies for competing athletes on the international circuit. These stimuli can be artificially manipulated to produce phase shifts in the circadian rhythm to promote adaptation in the optimum direction, but care must be taken to apply them at the correct time and dose, as the effects produced on the circadian rhythm follow a phase-response curve, with pronounced shifts in direction at different times. Light is the strongest realigning stimulus and careful timing of light exposure and avoidance can promote adjustment. Chronobiotics such as melatonin can also be used to realign the circadian clock but, as well as timing and dosage issues, there are also concerns as to its legal status in different countries and with the World Anti-Doping Agency. Experimental data concerning the effects of food intake and exercise timing on jet lag is limited to date in humans, and more research is required before firm guidelines can be stated. All these stimuli can also be used in pre-flight adaptation strategies to promote adjustment in the required direction, and implementation of these is described. In addition, the effects of individual variability at the behavioural and genetic levels are also discussed, along with the current limitations in assessment of these factors, and we then put forward three case studies, as examples of practical applications of these strategies, focusing on adaptations to travel involving competition in the Rugby Sevens World Cup and the 2016 Summer Olympics in Rio de Janeiro, Brazil. Finally, we provide a list of practice points for optimal adaptation of athletes to jet lag.

Right-left digit ratio (2D:4D) and maximal oxygen uptake

A low digit ratio (2D:4D) and low 2D:4D in the right compared with the left hand (right-left 2D:4D) are thought to be determined by high in utero concentrations of testosterone, and are related to "masculine" traits such as aggression and performance in sports like running and rugby. Low right-left 2D:4D is also related to sensitivity to testosterone as measured by the number of cytosine-adenine-guanine triplet repeats in exon 1 of the androgen receptor gene. Here we show that low right-left 2D:4D is associated with high maximal oxygen uptake (VO2(max)), high velocity at VO2(max), and high maximum lactate concentration in a sample of teenage boys. We suggest that low right-left 2D:4D is linked to performance in some sports because it is a proxy of high sensitivity to prenatal and maybe also circulating testosterone and high VO2(max).