Quo Vadis Nordic Hamstring Exercise-Related Research?-A Scoping Review Revealing the Need for Improved Methodology and Reporting

Abstract

The objective of this scoping review is to assess Nordic Hamstring Exercise quality (ANHEQ) of assessments and interventions according to the ANHEQ rating scales and to present practical recommendations for the expedient design and reporting of future studies. A total of 71 Nordic Hamstring Exercise (NHE) assessments and 83 NHE interventions were selected from the data sources PubMed, Scopus, and SPORTDiscus. Research studies which were presented in peer-reviewed academic journals and implemented the NHE during laboratory-based assessments or multi-week interventions met the eligibility criteria. NHE assessments analyzed force (51%), muscle activation (41%), knee angle kinematics (38%), and bilateral symmetry (37%). NHE interventions lasted 4-8 weeks (56%) and implied an exercise volume of two sessions per week (66%) with two sets per session (41%) and ≥8 repetitions per set (39%). The total ANHEQ scores of the included NHE assessments and interventions were 5.0 ± 2.0 and 2.0 ± 2.0 (median ± interquartile range), respectively. The largest deficits became apparent for consequences of impaired technique (87% 0-point-scores for assessments) and kneeling height (94% 0-point-scores for interventions). The 0-point-scores were generally higher for interventions compared to assessments for rigid fixation (87% vs. 34%), knee position (83% vs. 48%), kneeling height (94% vs. 63%), and separate familiarization (75% vs. 61%). The single ANHEQ criteria, which received the highest score most frequently, were rigid fixation (66% of assessments) and compliance (33% of interventions). The quality of NHE assessments and interventions was generally 'below average' or rather 'poor'. Both NHE assessments and interventions suffered from imprecise reporting or lacking information regarding NHE execution modalities and subsequent analyses. Based on the findings, this scoping review aggregates practical guidelines how to improve the design and reporting of future NHE-related research.

Keywords: Nordic Curls; eccentric resistance training; execution quality; hamstring injury prevention; hamstring lowers; knee flexor strength; neuromuscular adaptations.

Figure 1

Flow chart of study identification, screening, and selection for the quantitative analysis of the quality of Nordic Hamstring Exercise (NHE) assessments and interventions.

Figure 2

Percentage distributions of (a) diagnostic tools and parameters of NHE assessments (pooled according to the corresponding biomechanical measuring procedure) and (b) stimulus characteristics of NHE interventions. Total ANHEQ scores (c) and 0-point-ANHEQ scores (d) are presented for shared ANHEQ items 1–4 of assessments (grey bars) and interventions (black bars). Footnote (abbreviations): NHE, Nordic Hamstring Exercise; F_max, peak force; LSI, limb symmetry index; M_max, peak moment; φ_knee, knee angle; load_ROM, load across entire range of motion; DWA, downward acceleration; ROM_knee, knee range of motion; ω_knee, mean knee angular velocity; φ_hip, hip angle; TUT, time under tension; EMG, electromyography; not specif, not specified; ANHEQ, Assessing Nordic Hamstring Exercise Quality.

Figure 3

Representative Nordic Hamstring Exercise (NHE) training setups which provide fixed resistance to the heels, reasonable knee position, and expedient kneeling height: (a) appropriately weighted gym bench with cushion at the heels and at the shins (e.g., foam pads), (b) appropriately weighted bar, (c) doorway pull-up bar, and (d) a custom-made NHE device with modifiable shank inclination.