Jens Jung Nielsen

The Yo-Yo IR2 Test: Physiological Response, Reliability, and Application to Elite Soccer

PURPOSE To examine the physiological response, reliability, and validity of the Yo-Yo intermittent recovery level 2 test (Yo-Yo IR2). METHODS Thirteen normally trained male subjects carried out four Yo-Yo IR2 tests, an incremental treadmill test (ITT), and various sprint tests. Muscle biopsies and blood samples were obtained, and heart rate was measured before, during, and after the Yo-Yo IR2 test. Additionally, 119 Scandinavian elite soccer players carried out the Yo-Yo IR2 test on two to four occasions. RESULTS Yo-Yo IR2 performance was 591 +/- 43 (320-920) m or 4.3 (2.6-7.9) min. Test-retest coefficient of variation in distance covered was 9.6% (N = 29). Heart rate (HR) at exhaustion was 191 +/- 3 bpm, or 98 +/- 1% HRmax. Muscle lactate was 41.7 +/- 5.4 and 68.5 +/- 7.6 mmol x kg(-1) d.w. at 85 and 100% of exhaustion time, respectively, with corresponding muscle CP values of 40.4 +/- 5.2 and 29.4 +/- 4.7 mmol x kg(-1) d.w. Peak blood lactate was 13.6 +/- 0.5 mM. Yo-Yo IR2 performance was correlated to ITT performance (r = 0.74, P < 0.05) and VO2max (r = 0.56, P < 0.05) but not to 30- and 50-m sprint performance. Yo-Yo IR2 performance was better (P < 0.05) for international elite soccer players than for moderate elite players (1059 +/- 35 vs 771 +/- 26 m) and better (P < 0.05) for central defenders (N = 21), fullbacks (N = 20), and midfielders (N = 48) than for goalkeepers (N = 6) and attackers (N = 24). Fifteen elite soccer players improved (P < 0.05) Yo-Yo IR2 performance by 42 +/- 8% during 8 wk of preseasonal training. CONCLUSION This study demonstrates that the Yo-Yo IR2 test is reproducible and can be used to evaluate an athletes ability to perform intense intermittent exercise with a high rate of aerobic and anaerobic energy turnover. Specifically, the Yo-Yo IR2 test was shown to be a sensitive tool to differentiate between intermittent exercise performance of soccer players in different seasonal periods and at different competitive levels and playing positions.

Muscle temperature and sprint performance during soccer matches – beneficial effect of re‐warm‐up at half‐time

The relationship between quadriceps muscle temperature (Tm) and sprint performance was evaluated during soccer matches in 25 competitive players. In one game, Tm was determined frequently (n=9). In another game, eight players performed low‐intensity activities at half‐time (re‐warm‐up, (RW), whereas another eight players recovered passively (CON). Tm was 36.0±0.2 °C at rest and increased (P<0.05) to 39.4±0.2 °C before the game and remained unaltered during the first half. At half‐time, Tm decreased (P<0.05) to 37.4±0.2 °C, but increased (P<0.05) to 39.2±°C during the second half. In CON and RW, Tm and core temperature (Tc) were similar before and after the first half, but 2.1±0.1 and 0.9±0.1 °C higher (P<0.05), respectively, in RW prior to the second half. At the onset of the second half, the sprint performance was reduced (P<0.05) by 2.4% in CON, but unchanged in RW. The decrease in Tm was correlated to the decrease in performance (r=0.60, P<0.05, n=16). This study demonstrates that in soccer, the decline in Tm and Tc during half‐time is associated with a lowered sprint capacity at the onset of the second half, whereas sprint performance is maintained when low‐intensity activities preserve muscle temperature.

Recreational soccer is an effective health-promoting activity for untrained men

To examine the effects of regular participation in recreational soccer on health profile, 36 healthy untrained Danish men aged 20–43 years were randomised into a soccer group (SO; n = 13), a running group (RU; n = 12) and a control group (CO; n = 11). Training was performed for 1 h two or three times per week for 12 weeks; at an average heart rate of 82% (SEM 2%) and 82% (1%) of HRmax for SO and RU, respectively. During the 12 week period, maximal oxygen uptake increased (p<0.05) by 13% (3%) and 8% (3%) in SO and RU, respectively. In SO, systolic and diastolic blood pressure were reduced (p<0.05) from 130 (2) to 122 (2) mm Hg and from 77 (2) to 72 (2) mm Hg, respectively, after 12 weeks, with similar decreases observed for RU. After the 12 weeks of training, fat mass was 3.0% (2.7 (0.6) kg) and 1.8% (1.8 (0.4) kg) lower (p<0.05) for SO and RU, respectively. Only SO had an increase in lean body mass (1.7 (0.4) kg, p<0.05), an increase in lower extremity bone mass (41 (8) g, p<0.05), a decrease in LDL-cholesterol (2.7 (0.2) to 2.3 (0.2) mmol/l; p<0.05) and an increase (p<0.05) in fat oxidation during running at 9.5 km/h. The number of capillaries per muscle fibre was 23% (4%) and 16% (7%) higher (p<0.05) in SO and RU, respectively, after 12 weeks. No changes in any of the measured variables were observed for CO. In conclusion, participation in regular recreational soccer training, organised as small-sided drills, has significant beneficial effects on health profile and physical capacity for untrained men, and in some aspects it is superior to frequent moderate-intensity running.

Four weeks of speed endurance training reduces energy expenditure during exercise and maintains muscle oxidative capacity despite a reduction in training volume

We studied the effect of an alteration from regular endurance to speed endurance training on muscle oxidative capacity, capillarization, as well as energy expenditure during submaximal exercise and its relationship to mitochondrial uncoupling protein 3 (UCP3) in humans. Seventeen endurance-trained runners were assigned to either a speed endurance training (SET; n = 9) or a control (Con; n = 8) group. For a 4-wk intervention (IT) period, SET replaced the ordinary training ( approximately 45 km/wk) with frequent high-intensity sessions each consisting of 8-12 30-s sprint runs separated by 3 min of rest (5.7 +/- 0.1 km/wk) with additional 9.9 +/- 0.3 km/wk at low running speed, whereas Con continued the endurance training. After the IT period, oxygen uptake was 6.6, 7.6, 5.7, and 6.4% lower (P < 0.05) at running speeds of 11, 13, 14.5, and 16 km/h, respectively, in SET, whereas remained the same in Con. No changes in blood lactate during submaximal running were observed. After the IT period, the protein expression of skeletal muscle UCP3 tended to be higher in SET (34 +/- 6 vs. 47 +/- 7 arbitrary units; P = 0.06). Activity of muscle citrate synthase and 3-hydroxyacyl-CoA dehydrogenase, as well as maximal oxygen uptake and 10-km performance time, remained unaltered in both groups. In SET, the capillary-to-fiber ratio was the same before and after the IT period. The present study showed that speed endurance training reduces energy expenditure during submaximal exercise, which is not mediated by lowered mitochondrial UCP3 expression. Furthermore, speed endurance training can maintain muscle oxidative capacity, capillarization, and endurance performance in already trained individuals despite significant reduction in the amount of training.

Exercise but not Prostanoids Enhance Levels of Vascular Endothelial Growth Factor and other Proliferative Agents in Human Skeletal Muscle Interstitium

In the present study we examined whether exercise and prostanoids have an effect on the muscle interstitial concentration of vascular endothelial growth factor (VEGF) and on the proliferative effect of muscle interstitial fluid. Dialysate from resting and exercising human skeletal muscle, obtained either during control conditions or during cyclooxygenase inhibition, was examined for its content of VEGF and for its effect on endothelial cell proliferation. Microdialysis probes with high (960 kDa) and low (5 kDa) molecular‐mass cut‐off membranes were placed in the vastus lateralis muscle of healthy young males. The subjects performed one‐legged knee extensions (20 W). The concentration of VEGF in the 960 kDa dialysate was greater (P < 0.05) during exercise compared to at rest (67 ± 28 vs. 230 ± 22 pg ml−1). The rate of endothelial cell proliferation was 2.7‐fold higher (P < 0.05) with the 960 kDa dialysate from resting muscle than with perfusate and was 5.8‐fold higher (P < 0.05) than the perfusate value with dialysate from exercising muscle. VEGF was not enhanced with exercise in the 5 kDa dialysate, yet the exercise dialysate induced a 1.9‐fold higher (P < 0.05) proliferation than the resting dialysate. Cyclooxygenase inhibition did not affect the VEGF concentration or the proliferating effect of the dialysates (P > 0.05). This study demonstrates for the first time that VEGF is present in the interstitium of human skeletal muscle and that exercise enhances the interstitial concentration of VEGF and of other, as yet unidentified, angiogenic compounds. Products of cyclooxygenase do not appear to have an effect on the release of VEGF or other proliferative agents in human skeletal muscle.

Activity profile and physiological response to football training for untrained males and females, elderly and youngsters: influence of the number of players.

The present study examined the activity profile, heart rate and metabolic response of small‐sided football games for untrained males (UM, n=26) and females (UF, n=21) and investigated the influence of the number of players (UM: 1v1, 3v3, 7v7; UF: 2v2, 4v4 and 7v7). Moreover, heart rate response to small‐sided games was studied for children aged 9 and 12 years (C9+C12, n=75), as well as homeless (HM, n=15), middle‐aged (MM, n=9) and elderly (EM, n=11) men. During 7v7, muscle glycogen decreased more for UM than UF (28 ± 6 vs 11 ± 5%; P<0.05) and lactate increased more (18.4 ± 3.6 vs 10.8 ± 2.1 mmol kg−1 d.w.; P<0.05). For UM, glycogen decreased in all fiber types and blood lactate, glucose and plasma FFA was elevated (P<0.05). The mean heart rate (HRmean) and time >90% of HRmax ranged from 147 ± 4 (EM) to 162 ± 2 (UM) b.p.m. and 10.8 ± 1.5 (UF) to 47.8 ± 5.8% (EM). Time >90% of HRmax (UM: 16–17%; UF: 8–13%) and time spent with high speed running (4.1–5.1%) was similar for training with 2–14 players, but more high‐intensity runs were performed with few players (UM 1v1: 140 ± 17; UM 7v7: 97 ± 5; P<0.05): Small‐sided games were shown to elucidate high heart rates for all player groups, independently of age, sex, social background and number of players, and a high number of intense actions both for men and women. Thus, small‐sided football games appear to have the potential to create physiological adaptations and improve performance with regular training for a variety of study groups.

Positive performance and health effects of a football training program over 12 weeks can be maintained over a 1-year period with reduced training frequency

We examined whether improvements in the performance and health profile of an intensive 12‐week football intervention could be maintained with a reduced training frequency. Seventeen healthy untrained males completed the study. Ten subjects trained 2.4 times/week for 12 weeks and another 52 weeks with 1.3 sessions/week [football group (FG)] and seven subjects acted as controls [control group (CG)]. For FG, fat mass (3.2 kg) and systolic blood pressure (8 mmHg) were lower (P<0.05) after 64 than 0 weeks, and VO2max (8%) and Yo‐Yo intermittent endurance level 2 test performance (49%) were higher (P<0.05), with no difference between 64 and 12 weeks. After 64 weeks, quadriceps muscle mass (11%), mean fiber area (10%) and citrate synthase activity (18%) were higher (P<0.05) than those at 0 weeks. Leg bone mass (3.5%) and density (2.0%) were higher (P<0.05) after 64 than 0 weeks, but not different between 12 and 0 weeks. Plantar jump force (17–18%), 30‐m sprinting velocity (1.3–3.0%) and muscle glycogen concentration (19–21%) were higher (P<0.05) and blood lactate during submaximal exercise was lower (27–72%, P<0.05) after 64 than after 12 and 0 weeks. The above‐mentioned variables were unaltered for CG. In conclusion, positive adaptations in cardiovascular fitness obtained over 12 weeks of regular recreational football training can be maintained over a 1‐year period with a reduced training frequency, with further development in musculo‐skeletal fitness.

Caffeine intake improves intense intermittent exercise performance and reduces muscle interstitial potassium accumulation

The effect of oral caffeine ingestion on intense intermittent exercise performance and muscle interstitial ion concentrations was examined. The study consists of two studies (S1 and S2). In S1, 12 subjects completed the Yo-Yo intermittent recovery level 2 (Yo-Yo IR2) test with prior caffeine (6 mg/kg body wt; CAF) or placebo (PLA) intake. In S2, 6 subjects performed one low-intensity (20 W) and three intense (50 W) 3-min (separated by 5 min) one-legged knee-extension exercise bouts with (CAF) and without (CON) prior caffeine supplementation for determination of muscle interstitial K(+) and Na(+) with microdialysis. In S1 Yo-Yo IR2 performance was 16% better (P < 0.05) in CAF compared with PLA. In CAF, plasma K(+) at the end of the Yo-Yo IR2 test was 5.2 ± 0.1 mmol/l with no difference between the trials. Plasma free fatty acids (FFA) were higher (P < 0.05) in CAF than PLA at rest and remained higher (P < 0.05) during exercise. Peak blood glucose (8.0 ± 0.6 vs. 6.2 ± 0.4 mmol/l) and plasma NH(3) (137.2 ± 10.8 vs. 113.4 ± 13.3 μmol/l) were also higher (P < 0.05) in CAF compared with PLA. In S2 interstitial K(+) was 5.5 ± 0.3, 5.7 ± 0.3, 5.8 ± 0.5, and 5.5 ± 0.3 mmol/l at the end of the 20-W and three 50-W periods, respectively, in CAF, which were lower (P < 0.001) than in CON (7.0 ± 0.6, 7.5 ± 0.7, 7.5 ± 0.4, and 7.0 ± 0.6 mmol/l, respectively). No differences in interstitial Na(+) were observed between CAF and CON. In conclusion, caffeine intake enhances fatigue resistance and reduces muscle interstitial K(+) during intense intermittent exercise.

Performance enhancements and muscular adaptations of a 16‐week recreational football intervention for untrained women

The present study investigated the performance effects and physiological adaptations over 16 weeks of recreational football training and continuous running for healthy untrained premenopausal women in comparison with an inactive control group [Football group (FG): n=21; running group (RG): n=18; CO: n=14]. Two weekly 1‐h training sessions were performed in FG and RG. After 4 and 16 weeks of training VO2max was elevated (P<0.05) by 7% and 15%, respectively, in FG, and by 6% and 10%, respectively, in RG. After 16 weeks, Yo‐Yo intermittent endurance level 2 performance was 33% and 19% better (P<0.05) for FG and 29% and 21% better (P<0.05) for RG than after 4 and 0 weeks, respectively. Peak sprinting speed was 12% higher (21.0 ± 0.6 vs 18.8 ± 0.7 km/h; P<0.05) for FG after the training period, whereas no difference was observed for RG. After 4 weeks citrate synthase (CS) and 3‐hydroxyacyl‐CoA dehydrogenase (HAD) activity was 9% and 8%, respectively, higher (P<0.05) than before training in FG with no further changes during the last 12 weeks. In RG, CS increased (P<0.05) by 12% after 4 weeks and no significant increase was observed for HAD. In FG, the number of capillaries per fiber was 18% higher (P<0.05) after 16 weeks (2.44 ± 0.15 vs 2.07 ± 0.05 cap/fiber), with no significant difference for RG. No differences were observed between 0 and 16 weeks for CO. In conclusion, recreational womens football leads to significant increases in VO2max, performance and muscular adaptations throughout a 16‐week training period. Thus, football can be used as an activity to elevate the physical capacity of untrained women.

Exercise training normalizes skeletal muscle vascular endothelial growth factor levels in patients with essential hypertension.

Methods Vascular endothelial growth factor (VEGF) protein and capillarization were determined in muscle vastus lateralis biopsy samples in individuals with essential hypertension (n = 10) and normotensive controls (n = 10). The hypertensive individuals performed exercise training for 16 weeks. Muscle samples as well as muscle microdialysis fluid samples were obtained at rest, during and after an acute exercise bout, performed prior to and after the training period, for the determination of muscle VEGF levels, VEGF release, endothelial cell proliferative effect and capillarization. Results Prior to training, the hypertensive individuals had 36% lower levels of VEGF protein and 22% lower capillary density in the muscle compared to controls. Training in the hypertensive group reduced (P < 0.01) mean arterial blood pressure by 7.1 ± 0.8 mmHg, enhanced (P < 0.01) the capillary-to-fiber ratio by 17% and elevated (P < 0.05) muscle VEGF protein by 67%. Before training, acute exercise did not induce an increase in muscle interstitial VEGF levels above resting levels, but a five-fold increase (P < 0.05) was observed after the training period. Acute exercise induced an elevated (P < 0.05) endothelial cell proliferative effect of muscle dialysate after, but not before, training. Conclusion In summary, exercise training markedly elevates VEGF protein levels in muscle tissue, increases exercise-induced VEGF release from muscle and the cell proliferative effect of muscle dialysate. These alterations are paralleled by a lowering of blood pressure and an increased capillary-per-fiber ratio, but unaltered capillary density.