Effect of Upper Limb Isometric Training on Hamstring Activation among Healthy Adults

This crossover trial will take place at the Clinical Examination Laboratory of Hospital Canselor Tuanku Muhriz (HCTM) in Malaysia. Participants will be recruited via convenience sampling through advertisements on campus bulletin boards, social media groups, and the university staff email directory. Eligibility will be determined by a health questionnaire that assesses exclusion criteria, footedness, and dominant leg. Participants who meet the inclusion criteria will sign a consent form after receiving detailed information about the study. Potential participants will be recruited to complete two sessions of bilateral upper limb isometric training (ULIT): (1) submaximal contraction and (2) maximal contraction. Each session will include three bilateral ULIT exercises performed in a standing position: Exercise 1: Isometric Wall Push-Up, Exercise 2: Isometric Shoulder Extension, and Exercise 3: Isometric Shoulder External Rotation. These exercises aim to target muscle groups across the posterior myofascial kinetic chain, considering possible interconnections with the hamstrings. The Nicolet EDX system will assess changes in hamstring muscle activity using surface electrodes placed across the semitendinosus, biceps femoris, rectus abdominis, and lower trapezius muscles of the dominant limb. Electrodes are positioned parallel to the muscle fibers in accordance with the SENIAM (Surface ElectroMyoGraphy for the Non-Invasive Assessment of Muscles) guidelines. Muscle activation will be measured using the root mean square (RMS) of peak amplitude. To improve interpretability, RMS values are expressed as percentages relative to a baseline and averaged over 5-second isometric contractions. Muscle activity ratios for each exercise are calculated using the following formula: Ratio (%) = (muscle activity during ULIT exercise/muscle activity during two-foot quiet standing) × 100. Familiarisation Session: Prior to the experimental trials, all participants will undergo a familiarisation session at least three days before the first intervention session to familiarise themselves with the ULIT protocol and the equipment used for hamstring activation measurements. During this session, participants will practice the isometric exercises and apply surface electromyography (sEMG) sensors to ensure proper placement and comfort. The final analysis will not utilise any data from the familiarisation session. The familiarisation session will last approximately 30 minutes. Intervention Session: The interventions will be conducted by a physiotherapist with more than five years of clinical experience. Following the attachment of the electrodes, there is a 15-minute interval before the intervention begins. An outcome evaluator, blinded to the intervention sequence and intensity, will assess outcome measurements at three time points: pre-intervention (baseline measurement), during the intervention exercise (primary outcome measure), and post-intervention (secondary outcome measure). Participants' baseline assessment is taken for the hamstring flexibility of the dominant leg, the maximal voluntary isometric contraction (MVIC) of hamstring peak force and muscle activation during two-foot quiet standing (anatomical position stance width). Each intervention exercise series will be conducted in a similar order, starting with a 5-second maximal effort trial for each exercise. Next, the participants will be verbally ordered to exert either 50% (submaximal ULIT session) or 100% (maximal contraction ULIT session) of their maximum effort, maintaining it for 5 seconds, followed by 5 seconds of rest between each repetition, for a total of 5 repetitions. To minimise muscular fatigue, a one-minute break is given between each exercise. During ULIT exercises, participants will have a hand-held dynamometer attached to their distal hand to monitor force production and ensure consistent effort. An additional effort will be performed if any measured strength exceeds or is less than 10% of the target. The total intervention session will last approximately 60 minutes.