Background and Purpose. Frequency and duration of static stretching have not been extensively examined. Additionally, the effect of multiple stretches per day has not been evaluated. The purpose of this study was to determine the optimal time and frequency of static stretching to increase flexibility of the hamstring muscles, as measured by knee extension range of motion (ROM). Subjects. Ninety-three subjects (61 men, 32 women) ranging in age from 21 to 39 years and who had limited hamstring muscle flexibility were randomly assigned to one of five groups. The four stretching groups stretched 5 days per week for 6 weeks. The fifth group, which served as a control, did not stretch. Methods. Data were analyzed with a 5 × 2 (group × test) two-way analysis of variance for repeated measures on one variable (test). Results. The change in flexibility appeared to be dependent on the duration and frequency of stretching. Further statistical analysis of the data indicated that the groups that stretched had more ROM than did the control group, but no differences were found among the stretching groups. Conclusion and Discussion. The results of this study suggest that a 30-second duration is an effective amount of time to sustain a hamstring muscle stretch in order to increase ROM. No increase in flexibility occurred when the duration of stretching was increased from 30 to 60 seconds or when the frequency of stretching was increased from one to three times per day.

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The Effect of Time and Frequency of Static Stretching on Flexibility of the Hamstring Muscles

THACKER, S. B., J. GILCHRIST, D. F. STROUP, and C. D. KIMSEY, JR. The Impact of Stretching on Sports Injury Risk: A Systematic Review of the Literature. Med. Sci. Sports Exerc., Vol. 36, No. 3, pp. 371–378, 2004. Purpose: We conducted a systematic review to assess the evidence for the effectiveness of stretching as a tool to prevent injuries in sports and to make recommendations for research and prevention. Methods: Without language limitations, we searched electronic data bases, including MEDLINE (1966 –2002), Current Contents (1997–2002), Biomedical Collection (1993–1999), the Cochrane Library, and SPORTDiscus, and then identified citations from papers retrieved and contacted experts in the field. Meta-analysis was limited to randomized trials or cohort studies for interventions that included stretching. Studies were excluded that lacked controls, in which stretching could not be assessed independently, or where studies did not include subjects in sporting or fitness activities. All articles were screened initially by one author. Six of 361 identified articles compared stretching with other methods to prevent injury. Data were abstracted by one author and then reviewed independently by three others. Data quality was assessed independently by three authors using a previously standardized instrument, and reviewers met to reconcile substantive differences in interpretation. We calculated weighted pooled odds ratios based on an intention-to-treat analysis as well as subgroup analyses by quality score and study design. Results: Stretching was not significantly associated with a reduction in total injuries (OR 0.93, CI 0.78 –1.11) and similar findings were seen in the subgroup analyses. Conclusion: There is not sufficient evidence to endorse or discontinue routine stretching before or after exercise to prevent injury among competitive or recreational athletes. Further research, especially well-conducted randomized controlled trials, is urgently needed to determine the proper role of stretching in sports. Key Words: ATHLETES, CONDITIONING, META-ANALYSIS, SYNTHESIS

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The impact of stretching on sports injury risk: a systematic review of the literature.

POPE, R. P., R. D. HERBERT, J. D. KIRWAN, and B. J. GRAHAM. A randomized trial of preexercise stretching for prevention of lower-limb injury. Med. Sci. Sports Exerc.,Vol. 32, No. 2, pp. 271‐277, 2000. Purpose: This study investigated the effect of muscle stretching during warm-up on the risk of exercise-related injury. Methods: 1538 male army recruits were randomly allocated to stretch or control groups. During the ensuing 12 wk of training, both groups performed active warm-up exercises before physical training sessions. In addition, the stretch group performed one 20-s static stretch under supervision for each of six major leg muscle groups during every warm-up. The control group did not stretch. Results: 333 lower-limb injuries were recorded during the training period, including 214 soft-tissue injuries. There were 158 injuries in the stretch group and 175 in the control group. There was no significant effect of preexercise stretching on all-injuries risk (hazard ratio [HR] 5 0.95, 95% CI 0.77‐1.18), soft-tissue injury risk (HR 5 0.83, 95% CI 0.63‐1.09), or bone injury risk (HR 5 1.22, 95% CI 0.86 ‐1.76). Fitness (20-m progressive shuttle run test score), age, and enlistment date all significantly predicted injury risk ( P , 0.01 for each), but height, weight, and body mass index did not. Conclusion: A typical muscle stretching protocol performed during preexercise warm-ups does not produce clinically meaningful reductions in risk

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A randomized trial of preexercise stretching for prevention of lower-limb injury.

Despite limited scientific knowledge, stretching of human skeletal muscle to improve flexibility is a widespread practice among athletes. This article reviews recent findings regarding passive properties of the hamstring muscle group during stretch based on a model that was developed which could synchronously and continuously measure passive hamstring resistance and electromyographic activity, while the velocity and angle of stretch was controlled. Resistance to stretch was defined as passive torque (Nm) offered by the hamstring muscle group during passive knee extension using an isokinetic dynamometer with a modified thigh pad. To simulate a clinical static stretch, the knee was passively extended to a pre-determined final position (0.0875 rad/s, dynamic phase) where it remained stationary for 90 s (static phase). Alternatively, the knee was extended to the point of discomfort (stretch tolerance). From the torque-angle curve of the dynamic phase of the static stretch, and in the stretch tolerance protocol, passive energy and stiffness were calculated. Torque decline in the static phase was considered to represent viscoelastic stress relaxation. Using the model, studies were conducted which demonstrated that a single static stretch resulted in a 30% viscoelastic stress relaxation. With repeated stretches muscle stiffness declined, but returned to baseline values within 1 h. Long-term stretching (3 weeks) increased joint range of motion as a result of a change in stretch tolerance rather than in the passive properties. Strength training resulted in increased muscle stiffness, which was unaffected by daily stretching. The effectiveness of different stretching techniques was attributed to a change in stretch tolerance rather than passive properties. Inflexible and older subjects have increased muscle stiffness, but a lower stretch tolerance compared to subjects with normal flexibility and younger subjects, respectively. Although far from all questions regarding the passive properties of humans skeletal muscle have been answered in these studies, the measurement technique permitted some initial important examinations of vicoelastic behavior of human skeletal muscle.

Passive properties of human skeletal muscle during stretch maneuvers : A review

Following a request from the European Commission, the EFSA Panel on Dietetic Products, Nutrition and Allergies was asked to deliver a scientific opinion on the safety of caffeine, providing advice on caffeine intakes, from all dietary sources that do not give rise to concerns about adverse health effects for the general healthy population and subgroups thereof. Possible interactions between caffeine and other constituents of so-called “energy drinks”, alcohol, p-synephrine and physical exercise should also be addressed. Single doses of caffeine up to 200 mg (about 3 mg/kg bw for a 70-kg adult) do not give rise to safety concerns. The same amount does not give rise to safety concerns when consumed < 2 hours prior to intense physical exercise under normal environmental conditions. Other constituents of “energy drinks” at typical concentrations in such beverages (about 300–320, 4 000 and 2 400 mg/L of caffeine, taurine and D-glucurono-γ-lactone, respectively), as well as alcohol at doses up to about 0.65 g/kg bw, would not affect the safety of single doses of caffeine up to 200 mg. Habitual caffeine consumption up to 400 mg per day does not give rise to safety concerns for non-pregnant adults. Habitual caffeine consumption up to 200 mg per day by pregnant women does not give rise to safety concerns for the fetus. Single doses of caffeine and habitual caffeine intakes up to 200 mg consumed by lactating women do not give rise to safety concerns for breastfed infants. For children and adolescents, the information available is insufficient to derive a safe caffeine intake. The Panel considers that caffeine intakes of no concern derived for acute caffeine consumption by adults (3 mg/kg bw per day) may serve as a basis to derive single doses of caffeine and daily caffeine intakes of no concern for these population subgroups. © European Food Safety Authority, 2015

https://orbit.dtu.dk/files/118543075/4102.pdf

Scientific Opinion on the safety of caffeine

Scientific, medical, and practical aspects of stretching.

Proper fit of the bicycle.

Confusion and controversy exist regarding the cardiovascular effects of dietary supplements containing caffeine and Citrus aurantium (bitter orange) extract. The primary protoalkaloidal ingredient in bitter orange extract is p-synephrine which has some structural similarities to ephedrine and nor-epinephrine, but exhibits markedly different pharmacokinetic and receptor binding properties. The goal of this study was to investigate the cardiovascular effects of a product containing caffeine, bitter orange extract (p-synephrine) and green tea extract in mildly overweight individuals. Fourteen female and nine male subjects (age 24.7 ±7.4 yrs, BMI: 26.6 ±3.8) volunteered in this randomized, placebo-controlled, crossover, double-blind designed study. On day one, subjects entered the laboratory following an overnight fast. Heart rate and blood pressure were recorded at 60 min. Expired air was analyzed for the next 10 min of the session. At each of three meals, subjects ingested one capsule that was either a non-caloric placebo or a dietary supplement that contained 13 mg p-synephrine and 176 mg caffeine. On the following day, the subjects returned and repeated the protocol for data collection beginning 60 min after consuming one capsule of the placebo or the dietary supplement. No effects of the dietary supplement on heart rate, systolic and diastolic blood pressure or mean arterial pressure were observed. No between or within group differences were observed when data were analyzed for gender and caffeine usage. A small but significant decrease in resting respiratory exchange ratio was observed for the low caffeine user group in response to the product containing caffeine and p-synephrine. The results of this study indicate that ingestion of a product containing bitter orange extract, caffeine and green tea extract does not lead to increased cardiovascular stress and that fat oxidation may increase in certain populations.

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Effect of Acute Administration of an Herbal Preparation on Blood Pressure and Heart Rate in Humans

The aim of this study was to assess the influence of ad libitum water ingestion, using a back-mounted hydration system (BMHS), on fluid balance during alpine skiing. Fourteen skiers skied on two different days. On one day, seven skiers ingested water during skiing via the BMHS and the other seven skiers refrained from fluid ingestion during skiing until the midday break (NW trial). On the second day, the trials were reversed. Results indicated that when skiers used the BMHS they drank significantly more water than during the NW trials (2.0 ± 0.9 vs. 0.78 ± 0.4 litres). However, skiers drank significantly more at the midday break during the NW trials than during the BMHS trials (0.78 ± 0.4 vs. 0.4 ± 0.2 litres). Percent change in plasma volume was less during the BMHS trials than during the NW trials (−0.1 ± 5.3 vs. −4.9 ± 5.2%), urine osmolality was maintained in the BMHS trials but rose from 295 ± 80 to 818 ± 168 mOsm · kg−1 at midday during the NW trials, and body mass loss was minimized during...

Edmund R. Burke

Papers

Scientific, medical, and practical aspects of stretching.

Proper fit of the bicycle.

Effect of Acute Administration of an Herbal Preparation on Blood Pressure and Heart Rate in Humans

The effects of ad libitum fluid ingestion on fluid balance during alpine skiing in recreational skiers

Getting in Shape: Workout Programs for Men and Women