As aging leads to a decline in muscle function and overall physical performance, interventions targeting muscle strength and neuromechanical properties are critical for maintaining functional independence in older adults. Long-length eccentric training has shown promise in enhancing muscle performance, but its effects in older adults, particularly in a home-based setting, remain underexplored.

The aim of this trial is to assess whether a targeted home-based training intervention incorporating long-length eccentric contractions can enhance functional capacity and neuromechanical properties in healthy older adults. Participants will be semi-randomly allocated to one of two groups: an experimental group incorporating long-length eccentric contractions (Group 1) or a conventional resistance training group (Group 2), with group assignment stratified to ensure an equal number of men and women in each group. Each participant will complete three laboratory-based experimental sessions (i.e., two Pre-training sessions and one Post-training session) , during which five key evaluations will be conducted: (i) the 5-Time Sit to Stand test, serving as the primary outcome measure; (ii) the Timed Up and Go test; (iii) assessments of isometric and dynamic muscle strength in the knee extensors and plantar flexors; (iv) measurements of the cross-sectional areas of the vastus lateralis, rectus femoris, gastrocnemius medialis, and soleus muscles; and (v) evaluation of tendon stiffness.

Following the pre-evaluation, each participant will engage in a 24-session, semi-supervised, home-based training protocol over a period of 8 to 11 weeks. The training program will include eight exercises, with four targeting the upper limbs and four focused on the lower limbs. Group 1 will perform the lower limb exercises at long muscle lengths, while Group 2 will perform the same exercises in a conventional manner (i.e., at neutral or shorter muscle lengths). The total training volume (calculated as repetitions × sets, in arbitrary units) will be matched across both groups, ensuring that any observed differences in outcomes are attributable to the specific training modality rather than differences in workload.

Home-based resistance training has emerged as a practical alternative for individuals who lack access to traditional, facility-based exercise programs. This is particularly relevant for older adults, who often face multiple barriers to participating in supervised center-based training-the current gold standard-including reduced mobility, transportation difficulties, and financial constraints. As a result, home-based programs can offer a more accessible option to support engagement in resistance training. Evidence suggests that home-based resistance training can lead to improvements in strength and functional capacity among healthy older individuals; however, these gains tend to be modest. One key limitation is that exercise intensity in home settings may not progress adequately over time, often due to the absence of supervision or limited motivation to increase effort.

Besides training localization and level of supervision, the modality of contraction appears to be of importance. Previous evidences in young but also elderly adults suggest that eccentric training should be performed at long muscle length to obtain the greatest improvements in neuromuscular and physical functions. At the muscle level, the force-length relationship indicates that muscle tension is greater at long vs short muscle length. This is an obvious but important point since mechanical tension during training exercises is a key parameter for muscle plasticity and strength gains. It has been reported positive effects of a short-term (i.e. 3-week) eccentric training performed in a lengthened position on architectural and functional characteristics of the hamstrings in young adults. However, they found no difference between long and short muscle lengths, likely because of the short training duration. It has also been reported significant neuromuscular (e.g. fascicle length, pennation angle, voluntary activation level) and functional (e.g. muscle force) improvements after a 6-week eccentric training at long muscle length in the hamstrings of young adults. Resistance training interventions focusing on eccentric contractions seem particularly interesting for older individuals, as they are less metabolically demanding than concentric contractions for comparable workloads.

Although home-based resistance training programs emphasizing eccentric contractions have been shown to be more effective than those focusing on concentric contractions for improving lower limb strength, mobility, and postural stability in healthy older adults, the integration of eccentric exercises performed at long muscle lengths within home-based protocols has yet to be explored. This represents a significant challenge, as both contraction intensity and muscle length are difficult to monitor and control in real-world, home-based settings compared to tightly controlled laboratory environments.

The aim of this trial is to assess whether a home-based resistance training program incorporating long-length eccentric contractions of the lower limbs can lead to greater improvements in functional capacities and neuromechanical properties in healthy older adults compared to a conventional resistance training program. Participants will be randomly assigned to one of two groups: (i) a control group performing a conventional home-based resistance training program, (ii) an experimental group following the same program, with the addition of specific long-length eccentric contractions integrated into lower limb exercises.

This trial is designed to address two primary research questions:

(i) Does the integration of long-length eccentric contractions enhance the functional benefits of a conventional resistance training program?

(ii) Are any observed functional improvements associated with specific neuromechanical adaptations?

The corresponding hypotheses are:

1. A home-based resistance training incorporating specific long-length eccentric contractions is more effective to improve functional abilities of healthy old adults.
2. These greater functional improvements result from specific neuromechanical adaptations.

Thirty healthy older adults will be recruited to participate in the study. Initial screening will be conducted via telephone to assess eligibility based on inclusion and exclusion criteria. If concerns arise during this call-specifically, if the participant answers "yes" to any item from the ICOPE Step 1 questionnaire-they will undergo further evaluation by a geriatrist to confirm eligibility. Written informed consent will be obtained from all participants prior to enrollment. Participants will then be semi-randomly allocated to one of two groups: a conventional resistance training group or an experimental group incorporating long-length eccentric contractions, with group assignment stratified to ensure an equal number of men and women in each group. Each participant will complete three laboratory-based experimental sessions and 24 semi-supervised, home-based training sessions over a period of 8 to 11 weeks.

Three experimental sessions will be conducted in the laboratory at the following time points: (i) four weeks prior to training (Pre1), (ii) one week before training (Pre2), and (iii) within one week following the final training session (Post). The two pre-training sessions (Pre1 and Pre2) will establish baseline values for all outcome measures and allow assessment of natural variability in the absence of an intervention. These baseline variations will be compared between groups to ensure initial comparability and to serve as internal controls.

To evaluate training effectiveness, post-training outcomes will be compared between groups using ANCOVA, with Pre2 values as covariates. This approach will directly address the primary research question: Does the addition of long-length eccentric contractions to a conventional training program enhance functional improvements in older adults?

The sessions will consist of 5 evaluations/tests:

5 Time Sit to Stand test, which is the main outcome Timed Up and Go test Measurements of isometric and dynamic muscle strength of the knee extensors and plantar flexors Measurement of cross-sectional areas of vastus lateralis and rectus femoris, as well as gastrocnemius medialis and soleus muscles Measurement of tendon stiffness

At the end of the second experimental session (Pre2), immediately prior to the start of the training intervention, each participant will complete a familiarization session with a researcher, during which they will be guided through the exercises specific to their assigned group (conventional or long-length eccentric program). This session ensures that participants can perform the exercises safely and correctly at home.

Participants will be provided with a training logbook to document their sessions throughout the intervention. They will also receive all necessary equipment, including a weighted vest and a lifting strap, and for those in the long-length eccentric group, an adjustable step platform. To further support autonomy and adherence, participants will be given instructional materials, including video demonstrations and a printed exercise manual.

The intervention will consist of 24 training sessions over a period of 8 to 11 weeks. Of these, 20 sessions will be performed independently at home, while 4 sessions will take place in the laboratory in small groups, supervised by a qualified adapted physical activity instructor and/or physiotherapist.

To maintain engagement and provide support, a researcher will conduct weekly follow-up phone calls to monitor progress, address any questions, and ensure adherence to the training protocol.

Eight different exercises will be included in the training program of both groups, 4 centered around the upper limbs and 4 for the lower limbs:

biceps curls military press triceps extension rowing heel drop: performed on the floor for the conventional group, and performed on the edge of a step for the other group chair squat drop: performed on the floor for the conventional group, and performed with the feet propped up on a step for the other group band dorsiflexion stair descent

Each training session will start with a warm-up and end with a cool-down consisting of stretching and active mobilization (neck, shoulders, elbows, wrists, hips, knee and ankles). The four lower limbs exercises and two upper limbs exercises will be performed each training session, the lower limbs exercises alternating each session between the four given exercises. The intensity of the exercises will be progressively increased by either modifying the execution of the movement (e.g., adjusting step height to alter muscle length) or adding weights, up to a maximum of 20% of the participant' bodyweight. This progression in intensity will be carefully controlled and managed using the CR10 Rating of Perceived Exertion scale. Training volume (calculated as the number of repetitions × number of sets per exercise, in arbitrary units) will gradually increase throughout the training protocol.

During the protocol, participants will be asked not to alter their usual daily activities.

Inclusion Criteria:

• Men or women 60 years and older
• Individuals eligible for social security coverage
• Individuals without health issues (e.g., musculoskeletal injuries, chronic conditions)
• Individuals who have given their informed consent

Exclusion Criteria:

• Accustomed to resistance exercise (i.e., no participation in a resistance training program within the 6 months prior to enrollment, or not engaging in lower-limb resistance training more than once per week as part of their current routine).
• Pathologies affecting muscle strength in the lower limbs (e.g., neurological or neuromuscular conditions, history of ankle or knee trauma)
• Individuals residing in a healthcare or social institution
• Individuals receiving psychiatric care
• Individuals deprived of their liberty
• Adults under legal protection (guardianship, curatorship, or legal safeguard)
• Participation in another ongoing biomedical research study
• Individuals in an emergency situation
• Individuals not covered by social security
• BMI over 30
• Minimal Mental State Examination (MMSE) score of 25 or lower
• Significant visual impairment

The Individual Participant Data (IPD) will be made available to other researchers upon request, under specific conditions. A data-sharing plan will be developed in accordance with the relevant ethical guidelines, ensuring participant confidentiality and data protection. We will collaborate with other researchers interested in neuromechanical adaptations and aging-related functional capacities, sharing anonymized data via a secure platform. Access to the IPD will be granted only to those who meet the criteria for research use and will be subject to institutional review board approval. Our intention is to promote transparency and foster further exploration in this field while respecting participant privacy and the integrity of the data.

We are committed to making the following data available: Study Protocol, Statistical Analysis Plan, Informed Consent Form, and Analytic Code. This will ensure transparency, reproducibility, and facilitate further research in the field

At the end of study completion.