Gymnastik- och idrottshögskolan, GIH

The current thesis is a joint venture between the Swedish School of Sport and Health Sciences and Ski Team Sweden Alpine and Skicross, with the aim of improving return to sport (RTS) procedures.

High-force lower limb dominant sports movements (e.g., alpine skiing) place substantial demands on the lower limbs and their associated tissues. When these loads exceed the tissue’s tolerance, injuries occur. RTS after a knee injury requires risk assessment and acceptance of the risk. To enable informed decision-making, data regarding tissue healing (tissue strength), task-specific tissue loading (tissue stress), and risk-modifying factors are essential. The main knowledge gap concerns how different movements affect joint tissue loading and how to assess joint readiness for high-force, sports-specific activities. This thesis investigates how different tests may be applied to quantify joint readiness for RTS.

Study I demonstrated that isokinetic knee extension tests exhibited low concurrent validity with asymmetries observed in explosive movements (jumps). Study II shows that lower limb stiffness is a valid method for assessing stiffness and proposes a novel approach to quantifying joint stiffness. Study III reveals that asymmetries based on ground reaction forces greatly underestimate asymmetries in knee joint loading. Study IV shows that this discrepancy between ground reaction force and knee joint reaction force is primarily due to the contribution of muscle forces.

The findings suggest that baseline testing is crucial in sports and that jump tests are reliable, accurate, and feasible to include in such assessments. Lower limb stiffness should be analysed during these tests, alongside asymmetries in stiffness and force. Symmetry should be considered a universal goal, as even minor asymmetries can have significant consequences for joint loading. Muscle forces and recruitment strategies greatly influence joint loading; thus, optimising muscle balance and neuromuscular coordination should be prioritised in training and rehabilitation. Musculoskeletal modelling provides a valuable method for understanding load distribution, and its integration into testing protocols is strongly recommended. This is especially important within alpine skiing due to the potentially severe consequences if an incidence occur, therefore lower tolerance of asymmetries, compensations and performance insufficiencies should be allowed for alpine skiers. Application of baseline jump tests collecting both kinetics with dual force plates and kinematics with markerless motion-capture together with increased analysis could significantly improve RTS procedures and maybe decrease adverse incidences during RTS.

Doktorandprojektet är ett samarbetsprojekt mellan GIH och STSA, med syftet att förbättra rutinerna kring återgång till idrott efter knäskada (RTS).

Idrotter som belastar nedre extremitet såsom utförsåkning, utsätter leder och ledvävnader i nere extremitet för hög stress. När applicerad stress är högre än vävnadens stresstolerans uppstår en skada. Beslutsfattandeprocessen för RTS bör vara baserat både på relevant informationsbild och strategiska perspektiv, vilket kräver information om vävnadsläkning (vävnadsstyrka) samt hur olika idrottsrelaterade och idrottsspecifika rörelser belastar vävnaden (vävnadsstress). Utöver detta tillkommer andra faktorer som påverkar risk och risktolerans. Avhandlingen undersöker hur olika tester kan användas för att kvantifiera individens beredskap för olika belastningar.

Studie I: Utvärderade konceptet höger-vänster symmetri (ILS) och jämförde reliabilitet och precision för ILS beroende på testtyp. Resultatet indikerade att ILS är förekommande hos oskadade deltagare, att ILS har stor variation beroende på uppgift och test, samt att ILS från isokinetisk knäextension uppvisade låg validitet jämfört med idrottsrelaterade rörelser såsom hopp. Studie II: Utvärderade konceptet stiffness, och jämförde olika metoder för att beräkna stiffness. Studien visade att metoden lower limb stiffness (LLS) är en giltig metod för att undersöka styvhet samt föreslår en ny metod för att analysera joint stiffness. Studie III: Utvärderade sambandet mellan asymmetrier i markreaktionskrafter (GRF) och knäledsreaktionskrafter (KJRF). Studien visade att GRF kraftigt underskattar KJRF vilket i sin tur leder till en felaktig riskbedömning om beslut enbart baseras på GRF. Studie IV: Utvärderade metoder för att beräkna KJRF, genom att jämföra traditionell inversdynamik (V3D) med muskuloskeletal modulering (MSM). Studien visade att V3D kraftigt underskattar KJRF och att muskelkrafter till stor del förklarar skillnaderna mellan metoderna vilket innebär att MSM bör användas vid bedömning av vävnadsbelastning. 

Slutsatser från avhandling: Förmåga till symmetrisk rörelse är viktig för att fördela laster inom kroppen. Studie I betonar vikten av kontinuerlig testning för att skapa en baslinje som kan användas efter skada för att utvärdera beredskap. Studie III visar dessutom att små asymmetrier i GRF kan leda till betydelsefulla skillnader i ledbelastning som orsakas av muskelrekrytering och muskelkrafter. Utifrån resultat från studie I och IV så bör acceptans av asymmetri vara låg samt att idrottarens muskelrekryteringsstrategier bör optimeras för att minska negativ effekt av yttre belastning. Studie II visade att LLS är tillämpbar, och kan ge värdefull information kring individens neuromuskulära funktion samt sen-muskelsamspel och bör därför inkluderas vid testning. Studie IV visar att data kring både kinetik (krafter) och kinematik (rörelser) bör analyseras vid testning samt att MSM erbjuder ett kraftfullt verktyg för att förstå belastningsfördelning i kroppen och utvecklingsarbete bör genomföras för att möjliggöra användandet av MSM inom idrott och idrottsmedicin.

Inter-limb asymmetry in function or performance is an external measure used in sports science and medicine assessing readiness. How asymmetries propagate and affect internal joint loading is, however, poorly understood. This study presents the first investigation of asymmetries in ground reaction forces and knee joint reaction forces during a countermovement jump. Eleven uninjured male participants performed three countermovement jumps. Forces and motion were analysed in the Anybody Modelling System. Total resultant knee joint reaction force (KJRF[R]) was six times higher than the total resultant ground reaction force (GRF[R]) and the magnitude of the interlimb asymmetry was 6.6 times higher for KJRF[R] than GRF[R] indicating a significantly (p < 0.05) higher load in the joint structures of the dominant knee than the non-dominant knee. KJRF[AP] reached 1743 +/- 556 (877-2910) N in the posterior direction at a knee angle of 105 degrees, representing a load carried solely by the passive structures of the knee together with the articular contact surfaces. Even a small asymmetry determined through external measures such as GRF, therefore had a high impact on joint loading. Previously used thresholds (10 % deficit) for return to sport may therefore be questioned, and lower thresholds may be relevant to prevent injuries in the dominant limb.

INTRODUCTION: Stiffness (k) describes a material's resistance to deformation and is useful for understanding neuromuscular function, performance, and injury risk. The aim of this study is to compare the lower limb stiffness method (k_LLS), which uses only force plate data, with methods combining force plate and motion capture data to calculate stiffness during the eccentric phase of a countermovement.

MATERIAL AND METHODS: Twelve resistance-trained males: age 24.9 (4.4) years, height 1.81 (0.05) m, weight 88.2 (14) kg) performed three maximal effort countermovement jumps (CMJ). Data were collected synchronously using three-dimensional (3D) kinematic and kinetic data (dual force plate setup). Lower limb stiffness (z), joint stiffness (x, y, and z), and leg stiffness (linear, sagittal plane, and 3D) were calculated for the eccentric phase of all CMs.

RESULTS: k_LLS showed high concurrent validity with strong correlations to kinetic-kinematic methods (r = 0.90-0.97, p < 0.05). A linear mixed model revealed no significant differences in k-values between k_LLS and leg stiffness, indicating high concurrent validity.

DISCUSSION: k_LLS offers valid and valuable information affecting performance, injury risk, and return-to-sport decisions.

CONCLUSION: The findings suggest that k_LLS is a valid method for calculating stiffness in CMJs and equal to 3D leg stiffness.

This investigation was designed to evaluate changes in plasma and muscle levels of free amino acids during an ultra-endurance exercise and following recovery. Nine male ultra-endurance trained athletes participated in a 24-h standardized endurance trial with controlled energy intake. The participants performed 12 sessions of running, kayaking and cycling (4 x each discipline). Blood samples were collected before, during and after exercise, as well as after 28 h of recovery. Muscle biopsies were taken 1 week before the test and after exercise, as well as after 28 h of recovery. During the 24-h exercise, plasma levels of branched-chain (BCAA), essential amino acids (EAA) and glutamine fell 13%, 14% and 19% (P<0.05) respectively, whereas their concentrations in muscle were unaltered. Simultaneously, tyrosine and phenylalanine levels rose 38% and 50% (P<0.05) in the plasma and 66% and 46% (P<0.05) in muscle, respectively. After the 24-h exercise, plasma levels of BCAA were positively correlated with muscle levels of glycogen (r2=0.73, P<0.05), as was the combined concentrations of muscle tyrosine and phenylalanine with plasma creatine kinase (r2=0.55, P<0.05). Following 28-h of recovery, plasma and muscle levels of amino acids had either returned to their initial levels or were elevated. In conclusion, ultra-endurance exercise caused significant changes elevations in plasma and muscle levels of tyrosine and phenylalanine, which suggest an increase in net muscle protein breakdown during exercise. There was a reduction in plasma concentrations of EAA and glutamine during exercise, whereas no changes were detected in their muscle concentration after exercise.

Interleukin 6 (IL-6) response was studied during two ultra endurance events – one laboratory 24 h protocol (9 men) with exercise intensity set to 60 % of VO_2max and one Adventure Race over 6 days (12 men/6 women) with a self-selected race pace, including rests, of about 38 % of VO_2max. In the 24 h protocol IL-6 level was elevated from 0.76 ± 0.48 pg mL^-1 at rest to 7.16 ± 2.70 pg mL^-1 at 6 h, and increased further to 10.58 ± 1.04 pg mL^-1 at 12 h, but remained thereafter unchanged at 24 h, (10.89±0.36 pg mL^-1). All participants had nearly identical values at 12 and 24 h, supporting intensity as main determinant in the IL-6 response since exercise duration did not increase IL-6 level after 12 h. Possible confounding factors do not seem to influence the IL-6 concentration during the longer races (>12h), but might very well do so during shorter exercise bouts. In the 6-day race IL-6 increased from rest to 24 h, but thereafter there was no change in plasma IL-6 value until the end of the race (140 h). There was no elevation of TNF-α in any of the protocols, suggesting that the competitors were free from systemic inflammation. During endurance exercise lasting >12 h intensity and not duration is the main determinant of the IL-6 response, while during shorter exercise bouts both intensity and duration contribute to the accumulation of IL-6 in plasma.

Energy turnover was assessed in two conditions of mixed ultra-endurance exercise. In Study 1, energy expenditure and intake were measured in nine males in a laboratory over 24 h. In Study 2, energy expenditure was assessed in six males during an 800-km Adventure race (mean race time 152.5 h). Individual correlations between heart rate and oxygen uptake ([Vdot]O(2)) were established during pre-tests when kayaking, cycling, and running. During exercise, energy expenditure was estimated from continuous heart rate recordings. Heart rate and [Vdot]O(2) were measured regularly during fixed cycling work rates to correct energy expenditure for drift in oxygen pulse. Mean energy expenditure was 18,050 +/- 2,390 kcal (750 +/- 100 kcal . h(-1)) and 80,000 +/- 18,000 kcal (500 +/- 100 kcal . h(-1)) in Study 1 and Study 2 respectively, which is higher than previously reported. Energy intake in Study 1 was 8,450 +/- 1,160 kcal, resulting in an energy deficit of 9,590 +/- 770 kcal. Body mass decreased in Study 1 (-2.3 +/- 0.8 kg) but was unchanged in Study 2. Fat mass decreased in Study 2 (-2.3 +/- 1.5 kg). In Study 1, muscle glycogen content decreased by only 60%. Adventure racing requires a high energy expenditure, with large inter-individual variation. A large energy deficit is caused by inadequate energy intake, possibly due to suppressed appetite and gastrointestinal problems. The oxygen pulse, comparing start to 12 h of exercise and beyond, increased by 10% and 5% in Study 1 and Study 2 respectively. Hence, estimations of energy expenditure from heart rate recordings should be corrected according to this drift.

Introduction: In clinical medicine, natriuretic peptides, including N-terminal B-type natriuretic peptide (NT-proBNP), are used to detect increased myocardial wall tension in conditions such as heart failure. Tachycardia, arrhythmias and physical exercise may also increase these peptides. The clinical reference value is <100 ng/l, and in clinical practice values >300 ng/l are a strong indicators of heart failure, and values >5000 ng/l highly significant for mortality within 3 months.

Methods: We examined the levels of NT-proBNP after ultra-endurance exercise, and also made an attempt to relate NT-proBNP to performance. The subjects (12 males and 3 females) participated in the Adventure Racing World Championship, a 5-6 days non-stop competition open for mixed gender team of four athletes. They were all healthy, well-trained athletes with experience from several years of competitions at international elite level. Blood samples for determination of NT-proBNP were drawn before exercise (Pre-Ex), at the end of the race (End-Ex) and 24 hours after exercise (Post-Ex). Each athlete rated his/her own performance at the end of the race (i.e. the last 12 hours) on a scale from 1 (good, stronger than teammates), 2 (intermediate) and 3 (poor, got towed in, needed help from teammates). In addition, each athlete was also rated according to the same scale by the other three members of his/her team. Thereafter the four rates were pooled. Note that the rating is in relation to the team rather than the result of the competition.

Results: The average exercise duration was approx. 150 hours, and the calculated average work intensity was 40 % (in percent of respective VO2peak), including time for rest, change of equipment, and food intake. The levels of NT-proBNP increased from 31 ± 14 (10-56) [mean ± SD (min-max)] at Pre-Ex to 487 ± 648 (52-2480) at End-Ex. At Post-Ex the corresponding levels were 224 ± 219 (12-634). At End-Ex seven subjects had NT-proBNP below the reference value. The rated performance for four of them was 1, and the remaining three were rated as 2. Three of the subjects had markedly higher levels than previously reported (>900 ng/l) and they were rated 3, 3 and 2, respectively.

Discussion: Extreme levels of NT-proBNP, up to 2500 ng/l, are present after ultra-endurance exercise in healthy athletes without any clinical signs or symptoms of heart failure. On the other hand, these extreme values may be an indicator of cardiac fatigue, previously described after endurance exercise. Furthermore, in this study high levels of NT-proBNP seem to be associated with decreased exercise performance.

Purpose: The aim of this study was to investigate if extreme workload would induce signs of cardiac fatigue similar to that in skeletal muscle, e.g. decreased velocity of contraction.

Methods: The subjects were 12 men and 3 women who participated in the Adventure Racing World Championship, a 5-7 days non-stop competition open for mixed gender teams of four. All subjects were healthy, well-trained ultra-endurance athletes with experince from several years of training and competition at international elite level. Measurements of the heart's contraction velocities were conducted using tissue Doppler imaging (VIVID7) in a resting situation at baseline, immediately after the race, and after 24 hours of recovery.

Results: Characteristics for the subjects were at baseline (mean ± SD, for men and women): age 30±3 and 27±4; interventricular septal thickness 10.5±0.7 and 8.0±0.0 mm; left ventricular end-diastolic diameter 54.4±3.4 and 45.0±3.0 mm; posterior wall thickness 10.4±0.9 and 8.0±1.0 mm; early to late diastolic filling velocity (E/A) 2.3±0.6 and 2.2±0.2. Exercise duration was approx. 150 hours, and the calculated average work intensity was 40% of respective VO2peak, including time for rest, change of equipment, and food intake. Values of contraction velocities are presented in the table.

Conclusions: All athletes had normally sized hearts. Based on contraction velocities we found no evidence of cardiac fatigue after ultra-endurance exercise. A difference compared to studies that found cardiac fatigue in other sports (e.g. marathon, triathlon) is that even though our population exercised for an extreme duration the average intensity was low. This might point towards that exercise intensity, not duration, is the primary source for cardiac fatigue.

In this paper we report a reversed drift in heart rate (HR) but increased oxygen uptake (VO(2)) during ultra-endurance exercise. Nine well-trained male athletes performed 24-h exercise in a controlled laboratory setting, with alternating blocks of kayaking, running and cycling. Each block included 110 min of exercise and 10 min of rest, with an average work intensity of approximately 55% of respective VO(2peak). Blood samples were taken and HR and VO(2) measured every 6th hour during steady-state cycling at fixed work rate. As assumed HR was increased at 6 h by 15 +/- 6 beats/min compared with initial level (0 h). Thereafter the drift did not progress continuously, but instead unexpectedly returned toward initial values, although the plasma levels of catecholamines increased continuously during exercise. VO(2) was increased by 0.22 +/- 0.15 L/min (10%) at 6 h and 0.37 +/- 0.18 L/min (17%) at 12 h compared with 0 h, and thereafter remained stable. This implies an increased oxygen pulse (VO(2)/HR) by approximately 10% at the last half of the 24-h exercise compared with 0 h. Consequently, sole use of HR would give inaccurate estimates of exercise intensity and energy expenditure during endurance exercise lasting more than 6 h, and different patterns of cardiovascular drift need to be taken into account.