Level 3 Gym Instructor – Muscle: Fibre Recruitment And Types Of Work

This Level 3 Gym Instructor article is about fibre recruitment and the different types of work, or contraction, that muscles do.

Man Olympic Clean - Level 3 Gym InstructorWhen a muscle needs to overcome a force (such as lifting an object) the recruitment of muscle fibres is not random. Small motor units (type 1) are preferred, then type 2a, then if this is not sufficient type 2b. This can’t be maintained for long however, as lactic acid accumulation causes fatigue and with strong forces or heavy weights, exhaustion and damage to the myosin cross bridges.

This means a muscle reaches the limits of its contractile power when the maximum recruitment of motor units is reached. At rest the muscle will repair and compensate by adding more myosin and actin filaments ready for next time. This is known as adaptation. Without sufficient rest, or required nutrients, there will be insufficient adaptation or even insufficient repair. In other words, no increase in muscle size or strength.

Force Production And Muscle Length

If you have read any of my articles on fitness and muscle building you’ll know that I always stress the importance of working every muscle through a full range of movement, and if you’ve followed this advice you’ll know that the hardest part of any exercise is at the start and the end phase, where the muscle is at its longest and shortest points. This is especially the case for exercises that put the muscle into its very shortest or very longest position at some point.

The reason is that when a muscle is in a long or stretched position, such as the start of a bicep curl, there is not much overlap between the myosin cross bridges and the actin filaments, which means force production is low and the perceived effort is therefore high.

During the middle of the range of movement a lot more of the cross bridges can make contact, so force production is high and perceived effort is lower.

Towards the end of the bicep curl the muscle is in a short position so considerably less force can be generated, which is why this is always the hardest phase of the lift.

The temptation for many people is to lift a bigger weight and reduce the range of movement, concentrating on the mid-range of the exercise where force production is highest and perceived effort is lowest. However, whatever the goal, far more benefit will come from reducing the weight until it can be lifted through a full range of movement. It will also reduce the risk of damage not only to muscle but to tendons, joints and ligaments.

Types Of Muscular Work

A muscle that is working is contracting, but a muscle that is contracting is not necessarily moving. There are several different types of contraction and including a variety in a workout program adds to its effectiveness and keeps it interesting, with the overall goal dictating which type is used the most and which the least. It also helps to prevent over-training, which can come from too much prolonged emphasis on one type of contraction or exercise.

A muscle that contracts as it shortens against a force that remains the same is an isotonic contraction, but due to different motor units being recruited throughout the move this is almost impossible. A dynamic contraction means movement of a joint through its full range while against a constant external load, which is a lot more likely.

If the dynamic contraction is getting shorter as it produces tension it is a concentric contraction, in other words the acceleration phase, such as the lifting phase of a bicep curl. If it is working dynamically under tension but getting longer it is an eccentric contraction, in other words the deceleration phase, such as the lower phase of a bicep curl.

Eccentric contractions control or decelerate the rate of lowering, as in a bench press, and a lift that has more emphasis than normal on the eccentric is known as a negative rep. But this does not mean there is not considerable effort involved. Because the myosin cross bridges are pulled back during an eccentric contraction they cannot break contact with the actin and recycle. This means that more cross bridging is needed which involves more contractile power from the muscle, which in turn creates higher tension than a concentric contraction with the same load.

So if a dynamic movement is controlled in both phases, or with the help of spotter concentrates almost totally of the eccentric (negative, lowering) phase, the greater tension can increase gains compared to purely concentric.

One thing it will definitely do is create greater stress on the cross bridges, which can cause microtears in the muscle, and like any tissue damage this results in a natural inflammatory response. Pain and swelling can be felt within 12 hours but it is usually worse over the next 48 hours, which is why it is referred to as Delayed Onset Muscle Soreness (DOMS). Repairing the muscle begins immediately and could involve adaptation in the form of more actin and myosin filaments, preparing for an encounter with the same amount of force in the short term. But recovery from the pain could take several days, during which time the muscle should not be exercised.

Compensation and adaptation means that if the same exercise with the same weight is repeated routinely, DOMS should no longer occur. But as with any effective exercise regime, programs should be altered and updated regularly, which means that even an advanced athlete confronted with a new or more intense activity can wake up sore the next day!

Isokinetic work is where the speed of the contraction is kept constant throughout a dynamic move, so the concentric and eccentric phases are done at the same rate. However, the speed of a contraction changes as the force changes at different angles, plus gravity is always acting to speed up the eccentric phase. In a gym, Isokinematic is a more accurate term and involves equipment such as nautilus machines.

Woman Doing Plank - Level 3 Gym InstructorIsometric means “same length”, so an isometric contraction is one in which tension is being created in a muscle from cross bridge formation and cycling, but the length remains the same. The cross bridges attach and reattach to the same point. The plank exercise is a good example of this. As the effect of the tension built up by contracting muscle will in turn effect connective tissue there will be some change in muscle length however, so a more accurate term would be Static, which means “not moving”.

As always, any questions or feedback leave a comment below.

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