This is a very common scenario found particularly by individuals who are just starting off doing resistance training in the gym on a weight loss program. To provide clarity on why this has occurred, it is important to understand the mechanisms in place in our body to promote an increase in muscle strength and hypertrophy. There is also a need for understanding the training and nutritional requirements needed to elicit a hypertrophic response.
There are two main types of adaptations that occur in the body to increase our level of strength. These are morphological and neurological adaptations. Morphological adaptations involve changes to the muscle such as an increase in size, cross sectional area, hyperplasia and angle that the muscle fibres run (pennation angle). However, majority of these adaptations occur over a prolonged period of time and would not primarily account for the increases in strength in the short term. These short-term changes in strength are primarily due to neurological adaptations, such as an increased motor unit recruitment and firing rate. An increase in motor unit recruitment occurs when you start training, as motor units that did not need to be previously activated now need to be activated. The new physical demands being placed on your body cause this adaptation. This is similar to why an increase in the firing rate of motor units occurs as the body is needing to keep up with the stresses placed upon it. To do this the speed of the signal from the brain to the spinal cord to the muscle is increased to be able to forcefully contract the muscle more rapidly. This allows for progressions in strength to take place without having an increase in muscle size. Thus, strength can be increased through neurological adaptations particularly as a beginner to weight training.
Now that you have an understanding of the mechanisms behind why you were able to get stronger, I am now going to explain why you may not have been able to put on muscle. To be able to put on muscle it is crucial to look at your calorie intake and training frequency. For an individual who is on a weight loss program their calorie intake will be at a deficit. Meaning the individual will be burning more calories than they are consuming. This is ideal for someone wanting to lose weight but not put on muscle. This is due to a calorie surplus being required to put on muscle, where more calories need to be consumed than what is burnt throughout the day. The body requires this increase in calories to be able to keep up with the increased training frequency and to be able to rebuild the muscle. Protein also plays a key role in the muscle building process as it lays down the foundations for recovery and growth to occur. Which is why protein is primarily increased to create a calorie surplus. Thus, if you are not eating in a calorie surplus and consuming enough protein it will be quite challenging to be able to put on muscle.
An individual’s frequency of their weight sessions also differs for someone wanting to lose weight and someone wanting to put on muscle. The main goal for a weight loss program is to be able to maintain muscle mass while in a calorie deficit, which is why only 60 minutes a week is prescribed. This varies for a muscle gain program as each muscle group needs to be optimally loaded at least twice a week. This places the muscle under the necessary stress to cause an adaption. To achieve this, 180 minutes of weight training is required every week, which is 3x the amount performed in comparison to a weight loss program. Therefore, this is why you may not be able to increase your levels of muscle mass on a weight loss program.
In summary, adaptations to the nervous system instead of the muscle can cause early increases in strength. Also, to be able to increase muscle size you need to eat in a calorie surplus and perform weight training for at least 180 minutes a week. This is why weight loss and muscle gain are characterised as two separate programs as they require completely different prescriptions.