Gymnastik- och idrottshögskolan, GIH

Achilles tendon overuse-related injuries are a frequent problem to habitual runners. Such injuries occur more often unilaterally and its etiology is associated to overloading of the tendon tissue. Inter-limb differences during running are a possible cause for overload due to eventual differences in the mechanical loading provided to each limb. Furthermore, inter-limb differences in Achilles tendon properties were found in athletes due to sport-induced differences in the mechanical loading and in non-athletes due to limb preference. Currently, inter-limb differences in the Achilles properties of habitual runners is unknown. The present thesis investigated the existence of inter-limb differences in biomechanical, neuromechanical and Achilles tendon properties in habitual runners. In Study I, thirteen triathletes performed a cycle-run simulation while vertical ground reaction force (GRFv), lower limb kinematics and triceps surae and tibialis anterior activation were evaluated bilaterally during the start, mid and end stages of the 5 km running segment. In Study II, GRFv, lower limb kinematics, triceps surae and tibialis anterior activation and Achilles tendon strain were evaluated bilaterally in habitual runners at two running speeds (2.7 m.s-1 and 4.2 m.s-1). In Study III, spatiotemporal variables, vertical (kVert) and limb (kLimb) stiffness and center of mass (COM) kinematics were evaluated bilaterally in habitual runners at the same running speeds adopted in Study II. In Study IV, maximal plantar flexion isometric force, triceps surae activation and activation ratios, and Achilles tendon morphological, mechanical and material properties were evaluated bilaterally in habitual runners. In Study I the Soleus activation was lower in the preferred limb from 53.4% to 75.89% of the stance phase (p<0.01, ES range = 0.59 to 0.80) at the end stage of running. In Study II, hip extension velocity was greater in the non-preferred limb from 71% to 93% of the stance phase (p<0.01) during running at 4.2 m.s-1 while no other inter-limb differences were observed. In Study III, no inter-limb differences were observed in spatiotemporal, kVert and kLimb at investigated running speeds. However, COM horizontal velocity was greater from 67% to 87.40% of stance the phase (p<0.05, ES >0.60) when the non-preferred limb was in contact with the ground. In Study IV, no inter-limb differences were observed in triceps surae activation or Achilles tendon properties. The activation ratios of MG and SOL, however, were observed to correlate in the preferred limb only.

In summary, neuromuscular and kinematic inter-limb differences were observed when healthy, non-injured habitual runners performed in running conditions similar to their ecological conditions. Moreover, the Achilles tendon seem to adapt similarly among limbs of habitual runners, while triceps surae activation strategies might differ between limbs. Findings of inter-limb differences occurring during running may result in overload during running and therefore might be implicated in the etiology of Achilles tendon overuse-related injuries in habitual runners. Findings of similar tendon properties among limbs suggest both limbs have similar chances of incurring in the injury process.

Coaches and clinicians might improve current preventive strategies for Achilles tendon overuse-related injuries by monitoring tendon properties and running biomechanical and neuromuscular variables bilaterally across the season.

Inter-limb differences in Achilles tendon mechanical, material and morphological properties have previously been described in non-athletes and attributed to the preferential use of a given limb. Achilles tendon overuse tendon injury generally initiate unilaterally and alters triceps surae activation and Achilles tendon properties. The investigation of inter-limb differences in muscle activation and tendon properties may provide directions for injury prevention in habitual runners. In this study triceps surae and Achilles tendon properties were investigated bilaterally in habitual runners during unilateral maximal isometric contractions. Morphological, mechanical and material Achilles tendon properties were assessed using isokinetic dynamometry, motion capture and ultrasonography while triceps surae activation strategies were assessed using electromyography. Lower limb preference was assessed for inter-limb comparisons using the Waterloo questionnaire. Zero and one-dimensional statistical analysis and Cohen’s d were employed to investigate possible inter-limb differences. Inter-limb associations in Achilles tendon properties and intra-limb associations between triceps surae activations were assessed using Pearson’s correlation coefficients. No differences were observed between the preferred and non-preferred limb in terms of triceps surae muscle activation amplitude and Achilles tendon properties. However, intra-limb association among triceps surae activation ratios were not identical between limbs. Runners and triathletes present similar Achilles tendons properties between limbs, and thus initial observations of unilateral changes in the Achilles tendon properties might be used as a strategy to prevent the onset of overuse tendon injury. The non-similar associations within activation ratios between limbs should be further explored since triceps surae activation strategies may alter loading of the Achilles tendon.

Overloading of tendon tissue may result in overuse tendon injuries in runners. One possible cause of overloading could be the occurrence of biomechanical inter-limb differences during running. However, scarce information exists concerning the simultaneous analysis of inter-limb differences in external and internal loading-related variables in habitual runners. In this study ground reaction force, joint kinematics, triceps surae and tibialis anterior activations, and medial gastrocnemius muscle-tendon junction displacement were assessed bilaterally during treadmill running at 2.7 m.s-1 and 4.2 m.s-1. Statistical parametric t-tests and effect sizes were calculated to identify eventual inter-limb differences across the stance phase and stride cycle. Hip flexion angle was 9° greater (p = 0.03, ES = 0.30) in the non-preferred limb during the flight phase at 4.2 m.s-1. Hip extension velocity was 45 deg.s-1 greater (p = 0.04, ES = 0.41) during ground contact and 25 deg.s-1 greater (p = 0.02, ES = 0.41) immediately after toe-off in the non-preferred limb at 4.2 m.s-1. Hip extension velocity was also 40 deg.s-1 greater (p = 0.01, ES = 0.46) in the non-preferred limb prior to touch-down at 4.2 m.s-1. Brief inter-limb differences in joint kinematics were not accompanied by inter-limb differences in variables associated to internal loading, suggesting they are unlikely to be underlying factors leading to tendon overloading in healthy non-injured runners.

Overuse injuries are a common problem to triathletes' population. Overuse injuries may arise from inter-limb biomechanical differences during running, but the literature lacks information regarding inter-limb differences in triathletes. In this study inter-limb differences were investigated in injury-free triathletes during the running portion of a simulated cycle-run transition. Thirteen triathletes performed a 5 km run preceded by a 20 min cycling trial at 70% of maximal power output. During the Start, Mid and End stages of running, kinetic, kinematic and muscle activation variables were compared between the preferred and non-preferred limbs across the stance phase. A statistical parametric mapping analysis showed no differences between limbs when considering kinetic and kinematic variables (p > 0.05, ES<0.60). A lower soleus activation was observed in the preferred limb (p < 0.05, ES>0.60) from 53.40-75.9% of the stance phase at the End stage of running. In conclusion, inter-limb differences in kinetic or kinematic variables may not represent a risk for overloading in triathletes. However, inter-limb differences in triceps surae activation during running after cycling may represent one potential factor leading to overuse injuries in triathletes and should be further investigated.

Knee flexion angle is used to determine saddle height during pedaling. However, it is unclear how knee flexion angle at upright standing posture affects measures and interpretation of knee flexion angle during cycling. The objective of this study was to highlight the importance of adjusting knee angle during pedaling according to the knee angle at upright posture. Seventeen cyclists performed three 10 min cycling trials at different saddle heights to induce knee flexion angles (40º, 30º or 20º when crank was at the 6 o’clock position). Knee flexion angle was determined at the sagittal plane during cycling using a 2D motion analysis system. Alteration of saddle height was performed by subtracting the knee flexion determined during an upright standing posture from the observed knee flexion during cycling. Repeatability of knee angles at upright posture in the three trials was very good (ICC=0.73). A reduction in knee flexion angle of 10.6° (95%CI [8.6, 12.6º]) during cycling was found using the adjustment for upright standing posture (p<0.01; effect size>3.0). As a result, saddle height is affected by adjustments based on knee angle measured in upright standing posture. Determining saddle height without adjusting knee angle for upright standing posture could lead to errors with possible effects on performance and/or injury risk.

OBJECTIVES: This systematic review summarises biomechanical, physiological and performance factors affecting running after cycling and explores potential effective strategies to improve performance during running after cycling.

DESIGN: Systematic review.

METHODS: The literature search included all documents available until 14th December 2021 from Medline, CINAHL, SportDiscus, and Scopus. Studies were screened against the Appraisal tool for Cross-sectional Studies to assess methodological quality and risk of bias. After screening the initial 7495 articles identified, fulltext screening was performed on 65 studies, with 39 of these included in the systematic review.

RESULTS: The majority of studies observed detrimental effects, in terms of performance, when running after cycling compared to a control run. Unclear implications were identified from a biomechanical and physiological perspective with studies presenting conflicting evidence due to varied experimental designs. Changes in cycling intensity and cadence have been tested but conflicting evidence was observed in terms of biomechanical, physiological and performance outcomes.

CONCLUSIONS: Because methods to simulate cycle to run transition varied between studies, findings were conflicting as to whether running after cycling differed compared to a form of control run. Although most studies presented were rated high to very high quality, it is not possible to state that prior cycling does affect subsequent running, from a physiological point of view, with unclear responses in terms of biomechanical outcomes. In terms of strategies to improve running after cycling, it is unclear if manipulating pedalling cadence or intensity affects subsequent running performance.