Optimizing Hamstring Activation Through EMG-Feedback and the Hamshell Device

Hamstring injuries represent a significant burden in both recreational and competitive sports. Conventional hamstring training programmes primarily target muscle strength and hypertrophy. Despite generally positive outcomes, individual responses to strength-focused interventions vary considerably, suggesting that improvements in strength alone may not fully account for reductions in injury risk. Neuromuscular control and movement quality - factors likely relevant to hamstring injury risk - are not explicitly addressed in most traditional training protocols.

The Nordic hamstring exercise (NHE) is frequently used in clinical and sports practice, but presents biomechanical limitations, particularly with regard to loading at extended knee angles. The Hamshell device has been developed to address these limitations by enabling hamstring-specific loading across a broader range of knee angles.

Electromyographic (EMG) biofeedback is a technique in which real-time information about muscle electrical activity is provided to the user during exercise. This approach has been shown to enhance voluntary muscle activation and neuromuscular control in various rehabilitation and training contexts. Combining EMG biofeedback with Hamshell-based training may therefore optimise neuromuscular adaptations beyond what can be achieved with resistance training alone.

Study Design

This is a single-centre, prospective, randomised controlled intervention study with a parallel-group design, conducted at the Praxisklinik Rennbahn AG in Muttenz, Switzerland.

Participants

50 recreationally active adults (aged 18-40 years), defined as engaging in at least 150 minutes per week of moderate-intensity physical activity. Individuals with a current or recent (≤ 6 months) musculoskeletal injury of the lower extremities, or who are unable to follow study procedures, are excluded. Participants are randomly assigned in a 1:1 ratio to the intervention or control group (n = 25 per group).

Intervention

Both groups complete 10 training sessions over 5 weeks (twice per week), each lasting approximately 30 minutes, at the study centre. Sessions consist of hamstring and gluteus maximus exercises performed at extended knee angles using the Hamshell device. The first and middle sessions are supervised to ensure correct exercise technique and appropriate progression; EMG is recorded during these supervised sessions.

The intervention group receives real-time visual EMG biofeedback during all training sessions, aimed at optimising biceps femoris and gluteus maximus activation. The control group performs the identical training programme without any biofeedback.

Assessments

Participants undergo baseline (T0) and post-intervention (T1) assessments, each comprising:

• Hamstring flexibility testing
• Countermovement jumps (CMJ) with simultaneous EMG recording of the biceps femoris and gluteus maximus
• Three maximal voluntary contractions (MVCs) of the same muscles on the Hamshell device
• Isokinetic strength testing of knee flexors and extensors in concentric and eccentric modes at 30°/s and 150°/s, and isometrically (0°/s), with concurrent EMG recording

Training adherence and session ratings of perceived exertion (RPE) are recorded for all sessions.

Primary Endpoint

Integrated EMG (iEMG) activity of the biceps femoris during the modified Nordic hamstring exercise, assessed at post-intervention (T1).

Secondary Endpoints

• Hamstring and quadriceps strength (isokinetic dynamometry, concentric and eccentric modes at 0°/s, 30°/s, and 150°/s) with concurrent EMG recording of the biceps femoris and gluteus maximus
• Hamstring flexibility
• EMG activity of the biceps femoris and gluteus maximus during the eccentric phase of the CMJ
• Functional performance (CMJ vertical impulse)
• Training adherence and session RPE

Statistical Analysis

Primary and secondary endpoints will be analysed using analysis of covariance (ANCOVA), with post-intervention values as the dependent variable, group allocation as the fixed factor, and corresponding baseline values as covariates.

Sample Size

A sample size of 25 participants per group (50 total) provides ≥ 80% power to detect a standardised mean difference of d = 0.81 (based on between-group iEMG differences reported by Croce et al.), at a two-sided significance level of α = 0.05.

Study Duration

The estimated duration of the main investigational period is approximately 12 months (First-Participant-In: June 2026; Last-Participant-Out: May 2027).

Inclusion criteria

• Age between 18 and 40 years
• Recreationally active, defined as engaging in at least 150 minutes per week of moderate-intensity physical activity

Exclusion criteria

• Current or recent (≤6 months) musculoskeletal injury of the lower extremities
• Inability to follow study procedures or instructions