Amyotrophic Lateral Sclerosis (ALS) progressively damages the nerve cells responsible for voluntary muscle movement. Over time, this leads to weakness in different muscles such as those used for movement or breathing. Breathing problems are one of the main causes of complications and reduced survival in people with ALS. This happens because the inspiratory muscles-those that help draw air into the lungs-gradually lose strength.

The study has the aim to explore the benefits of training inspiratory muscles in ALS patients in order to maintain the setrength of these muscles for as long as possible and look the impact on respiratory function.

Amyotrophic Lateral Sclerosis (ALS) is a progressive disease that damages the nerve cells responsible for voluntary muscle movement. Over time, this leads to weakness in the muscles used for movement, speech, swallowing, and breathing. Breathing problems are one of the main causes of complications and reduced survival in people with ALS. This happens because the inspiratory muscles-those that help draw air into the lungs-gradually lose strength.

Maintaining the strength of these muscles for as long as possible is very important. Specific breathing exercises, known as inspiratory muscle training, can help strengthen these muscles by making them work against a gentle resistance when breathing in. This may help people with ALS maintain their breathing capacity longer, improve comfort and quality of life, and possibly extend survival time.

This study is a prospective, controlled, randomised, double-blind, masked, multicentre clinical trial that will include 44 people diagnosed with ALS who have lived with the disease for less than two years. It will take place in several hospitals and research centres.

Participants will be randomly divided into two groups:

Experimental group (22 participants): They will follow a 12-week inspiratory muscle training programme using a device that provides resistance when breathing in. This means the muscles have to work harder, much like resistance training for other parts of the body.

Control group (22 participants): They will use the same device for the same period, but with a placebo valve that allows air to pass freely, providing no resistance.

Neither the participants nor the researchers who assess the results will know which group each person belongs to. This "double-blind" design ensures that any differences observed between the groups are due to the intervention itself and not to expectations or bias.

The main objective of the study is to find out whether inspiratory muscle resistance training helps people with ALS maintain their Maximal Inspiratory Pressure (MIP)-a measure of the strength of the breathing muscles-for a longer time compared to those who do not train with resistance.

The secondary objectives include evaluating whether this training improves tolerance to daily activities, enhances well-being and comfort in breathing, and contributes to better overall quality of life.

All participants will continue to receive their usual medical care for ALS. The breathing training is an additional, supervised activity designed to support respiratory health. Every participant will be closely monitored by a team of healthcare professionals throughout the study to ensure safety and proper follow-up.

By joining this research, patients and families are helping to improve scientific understanding of ALS and to identify non-drug interventions that may help maintain function and independence for longer. The results will contribute to the development of new recommendations for respiratory care and rehabilitation in people living with ALS.

Inclusion Criteria:

• Patients diagnosed with spinal ALS
• men and women
• diagnosis date less than two years ago, according to the "El Escorial" criteria (Appendix 2)
• PIM above the lower limit of normal
• Preserved lung function (FVC ≥ 80%, FEV1 ≥ 80%, FEV1/FVC ≥ 80%) and normal values in supine position

Exclusion Criteria:

• Patients with signs of respiratory muscle weakness (MIP and MEP below the LLN and abnormal decubitus tests19-21)
• Nocturnal hypoventilation
• Inability to perform the measurement tests
• Inability to understand and perform the exercises
• Any contraindication to the use of IMT. Severe psychiatric illness.

To ensure participant blinding, both groups will use the same training device. In the control group, the device will be modified to simulate a resistance of 5 cmH₂O (less than 5% of Maximal Inspiratory Pressure), a level that does not produce any physiological training effect. The investigator supervising the intervention will not inform participants of their group assignment at any time.

To ensure blinding of outcome measurements, a separate investigator from the research team-who will not know participants' group allocation-will conduct all evaluations.

Finally, the statistician responsible for data analysis will also be blinded to group assignment. The study database will include only coded numerical identifiers, preventing identification of the intervention or control groups. Once statistical analyses are completed, results will be returned to the principal investigator, who will then unblind the data to identify which results correspond to each group.