Isometric Contraction

Currently there is no consensus on the most appropriate testing positions for HHD use, with a recent systematic review demonstrating a variety of methodologies used for lower limb assessment in previous research [25 (link)]. Based on prior research and our own pilot work of assessments in a variety of different positions, we implemented those shown in Fig 1. These testing positions have shown strong reliability for the measurement of isometric strength in previous studies for the hip [36 (link)], knee [37 (link)], and ankle [37 (link)] muscle groups.
Assessment of isometric muscle strength and power was performed with the participants in three positions (seated, supine, and prone); hip flexors, knee extensors, and knee flexors were assessed in a seated position; ankle plantarflexors, ankle dorsiflexors, hip abductors, and hip adductors in a supine position; hip extensors in a prone position. These positions were chosen to minimise changes in position by the participant to enhance the feasibility of testing in a clinical setting. All tests involved maximal voluntary isometric contractions. Assessment using the HHDs was conducted first. The order was randomised for assessor and HHD, however the order of the muscle groups tested was kept consistent as shown in Fig 1; for example if HHD1 was randomly assigned first, all seated muscle groups would be assessed, followed by HHD2 assessing seated muscle groups, with the same order of HHDs for supine and then prone muscle groups. Following a rest period of five minutes, the same protocol was repeated by the second assessor. During pilot testing, problems arose in the assessment of very strong muscle groups, namely the knee extensors and ankle plantarflexors. To assist the assessor in overcoming the force produced by the participant, the plinth was placed close to a wall, which aided the assessors in their resistance of the participants’ contractions for these two muscle groups (see Fig 1B and 1D).
Following HHD testing, the isometric strength and power of participants was then assessed using the KinCom dynamometer utilising the positions described for the HHDs. In order to minimise position changes and reduce time requirements, the order of muscles tested was different during the assessment with the KinCom dynamometer. The order for the KinCom was as follows: knee extensors, knee flexors, hip flexors, hip abductors, hip adductors, hip extensors, ankle plantarflexors, and ankle dorsifexors. Instructions provided to participants for all trials were ‘at the count of three, push/pull as hard and as fast as you can and hold that contraction until I say relax’. Each test lasted between three to five seconds and ended after a steady maximal force was produced by the participant. Participants were instructed to hold the side of the plinth for stabilization (see Fig 1). Constant verbal encouragement was provided throughout the testing. Only the right limb of each participant was assessed to reduce fatigue and the time demands of the testing session. A submaximal practice trial was given for each muscle group on both HHDs and the fixed dynamometer to ensure the participant understood the contraction required. Two trials were recorded for each muscle group, again to minimise the time requirements of testing.

Participants were comfortably seated in a chair with their right forearm and hand supinated and supported by a custom built device (Figure 1). The elbow was positioned in 90 degrees flexion. The device restricted any movement of the upper arm, forearm, wrist, and fingers. Bipolar EMG-recordings were obtained using 2 pairs of self-adhesive surface electrodes (Ag-AgCl, solid gel, foam electrodes (35 × 22 millimeters)) placed in a standard tendon belly montage. EMG-signals were recorded using a CED (Cambridge Electronic Design Ltd) data acquisition and amplifier system with a bandpass filter of 20 to 3000 Hz at a display sensitivity of 0.5 microvolt/division (amplifier range 100 millivolt), using a recording time from 150 milliseconds before until 850 milliseconds after each stimulus. The sampling rate was 20.000 samples/second. The EMG data were collected using Spike2 laboratory software (Cambridge Electronic Design Ltd).
First, EMG-activity was recorded from the biceps brachii muscle (BB). The forearm and hand were supinated and EMG-activity of the BB was recorded while participants performed elbow flexion against a fixed frame. Secondly, to obtain isometric abductor digiti minimi (ADM) muscle contractions, right digit V abduction was performed against a fixed frame, while digits II-IV were immobilized by Velcro straps. For measurements of both proximal and distal muscles, participants were instructed to exert maximum force for 3 seconds during 3 trials. The maximal voluntary EMG-activity was defined as the mean EMG-amplitude achieved during these 3 trials.

The composition of all solutions in this study is summarized as follows. Normal Krebs solution contained the following (in mM): 120 NaCl, 5.9 KCl, 25 NaHCO₃, 1.2 NaH₂PO₄, 11.5 dextrose, 2.5 CaCl₂, 1.2 MgCl₂ (Biosharp, China). High KCl solution (96 mM) was prepared as normal Krebs but with equimolar substitution of NaCl with KCl. Oxytocin (MedChemExpress, China) was dissolved in deionized water to prepare 10^-11 to 10^-6 M concentration for isometric contractions. Digestion solution was prepared: 2 mg/ml type II collagenase, 1 mg/ml BSA, and 0.5 mg/ml deoxyribonuclease I was dissolved in Dulbecco’s Modified Eagle Medium (DMEM) (Sigma-Aldrich, American). The normal glucose medium consisted of DMEM with 5.5 mmol/L glucose, 10% fetal bovine serum (FBS) (Sigma-Aldrich, American), and 1% penicillin/streptomycin (Gibco, American) solution. The high glucose medium consisted of DMEM with 25 mmol/L glucose, 10% FBS, and 1% penicillin/streptomycin solution. CoCl₂ medium was prepared in normal glucose containing 200 µmol/L CoCl₂ (Sigma-Aldrich, American). High glucose with echinomycin medium was prepared in high glucose containing 10 nmol/L echinomycin (MedChemExpress, China). CoCl₂ with echinomycin medium was prepared in CoCl₂ medium containing 10 nmol/L echinomycin. CoCl₂ with L-methionine medium was 10 µmol/L L-methionine (Sigma-Aldrich, American) dissolved in CoCl₂ medium. High glucose with L-methionine medium was 10 µmol/L of L-methionine dissolved in high glucose medium. High KCl (96 mM) and oxytocin (10^-7 M) were dissolved in the respective medium for cell contractions.