An Investigation of the Influence of Knee Flexion Angles on the Activity of the Triceps Surae Muscles during the Heel-Raise Test

Background: The heel-raise test (HRT) is used in clinical practice and research to assess the triceps suare (TS) muscles. The test involves repetitive unilateral heel-raises in upright stance. The test is administered with the knee straight for gastrocnemius medialis (GM) and lateralis (GL), and with the knee bent for soleus (SOL). The maximum number of heel-raises which can be performed is utilised as a clinical outcome measure and informs clinical decisions. The use of knee flexion (KF) to direct HRT assessment towards SOL or GAST currently relies on fundamental principles and has a limited evidence-base. Aims: The primary aim of this research was to investigate the influence of KF angle on SOL, GM, and GL activity and fatigue during the HRT. Secondary aims were to explore the total number of heel-raises completed, and the influence of age group on TS muscle activity and fatigue. Methods: Specific literature reviews were undertaken to contextualise the HRT within a comprehensive anatomical and biomechanical framework, and the experimental research involved a sequence of biomechanical studies. Seventeen subjects participated in a preliminary study that involved heel-raise testing to fatigue in two KF positions and generalised estimation equations were used to determine whether select KF angles were maintained. Forty-eight subjects, stratified by age and gender, participated in the main experimental study. Surface electromyography (EMG) activity of SOL, GM, and GL during maximum voluntary isometric contractions (MVIC) was collected in 0°KF, 45°KF, and 90°KF to inform EMG normalisation. Subjects then completed 10 unilateral heel-raises in 0°KF and 45°KF to investigate EMG amplitudes (% of MVIC) and heel-raises to fatigue for power spectrum analysis (median frequency and normalised slope). Mixed-effects models and stepwise regressions were used for the main analysis. Results: The preliminary study identified that select KF angles were reasonably well maintained during testing and subjects completed an average of 40 heel-raises in the two KF positions. The investigation of (EMG) muscle activity during MVIC developed a normalisation protocol specific to capturing SOL, GM, and GL peak activity. Data collected from heel-raises demonstrated higher SOL (p=.005) and lower GM (p<.001) and GL (p<.001) amplitudes in 45°KF rather than 0°KF; however, KF did not influence fatigue of the individual TS muscles. GM and GL fatigued at the same rate (p=.088), to the same extent (p=.385), and faster than SOL (p<.001) in both angles. GM (p=.008), but not GL (p=.118), fatigued more than SOL. Similar maximum numbers of heel-raises were performed in 0°KF (n=45) and 45°KF (n=48). Age did not influence any of the variables analysed. Conclusion: The research results question the common utilisation of KF to direct the HRT towards SOL or GAST assessment. Although influences of KF on EMG amplitudes were statistically significant, they may not be clinically meaningful. KF did not influence TS fatigue or the total number of heel-raises. While these findings do not imply the absence of clinical or rehabilitative benefits from HRT procedures in select KF positions, the evidence challenges the common perceptions and definitions that the test assesses SOL with the knee bent and GAST with the knee straight