Jerva Performance

Physiological Basis for Strength and Power Development

◆ ☕️☕️ 10 min read

Strength and power are popular terms in the sport performance world. Searching the web, you might find someone telling you that training at lower repetition ranges with higher loads will help you develop your maximal strength and lighter, more explosive work is good for power production, both of which are important in a sport performance setting. But what does that actually mean?

Having an understanding of the fundamental physiological mechanisms will help you put some meaning behind the words “strength” and “power”. Don’t just do what someone tells you to do. Dig deeper to understand what underlies strength and power development. This will allow you to think critically about what type of training to undertake to achieve your aim.

When we are talking about strength and power development, we are really talking about force production capabilities. For simplicity, we can broadly split the table into two – structure-related factors and neuromuscular factors that impact force production. But even beyond that, there are other crucial concepts that need to be considered.

Muscle Fiber Phenotype #

In the most basic terms, we have 2 types of muscle fibers in our bodies. We have the Type I slow-twitch muscle fibers and Type II fast-twitch muscle fibers. The names literally come from their twitch responses to repeated stimulation, with Type I having a slower twitch response.

Type I slow-twitch fibers are considered to have a high oxidative capacity and resistance to fatigue. Their contraction speed is low, and they are smaller in size. Their force production capacity is low but sustainable for long periods of time. Your postural muscles predominantly consist of Type I fibers.

Type II fast-twitch fibers can be further categorized into Type IIa and IIx. Type IIa are recognized as the “intermediate” fibers between Type I and Type IIx. They are bigger in size than Type I, have faster contraction speed and force output capacity. Type IIa have lower oxidative capacity and fatigue faster. A predominant location for Type IIa fibers in your body is your lower limbs.

Type IIx fibers are the most anaerobic fibers possessing the lowest oxidative capacity. They are the largest in size, have the highest capacity for force production and are utilized in rapid and intense movements of short duration. Type IIx fibers have the fastest contraction speed but also fatigue the fastest.

The muscle fiber distribution in your body is to a large degree determined by your genetics. Still, there is some capability of changing your muscle fiber distribution. Type I fibers tend to change very little over time. Type IIx have a tendency to take on Type IIa characteristics with higher training volumes during training phases. With detraining or peaking for performance when training volume is lowered, they drift back to Type IIx.

Cross-Sectional Area and Pennation Angle #

Muscle strength is largely correlated with the size of the muscle (cross-sectional area), with larger muscles capable of producing more force. A simple measure of the cross-sectional area, however, overlooks another crucial structural factor – the pennation angle. This is related to the angle between the longitudinal axis of the whole muscle and its fibers.

A classic measure of cross-sectional area wouldn’t reveal the actual area or volume of the muscle. Two muscles could have the same basic measure of cross-sectional area, but the pennation angle could be entirely different.

Pennation angle is an important consideration, because for a muscle with a pennation angle of 0 degrees, the direction of force will be in the same direction as the muscle’s pull on the bone. Muscles like that have sarcomeres (the basic contractile units of a muscle) lying in series. As such, they can cause large changes in movement at low force. For example, these are your semimembranosus and semitendinosus muscles of the hamstring.

For a muscle with a pennation angle of greater than 0 degrees, the direction of force and the direction of pull will not align. Muscles like that will have more parallel sarcomeres. Because of this, they will also be able to produce large force, but they won’t be causing a large range of movement. Pennate muscles are located in positions requiring small but powerful movements, such as your gluteus maximus muscle.

Training for muscle hypertrophy (muscle size) will add more sarcomeres in parallel, which appears to increase the angle of pennation and therefore force production capabilities of the muscle.

Muscle hypertrophy interventions are good for muscles with low pennation angles, such as the hamstring muscles, because they often get injured when exposed to high force. On the contrary, muscles like the rectus femoris (part of your quadriceps) and gluteus maximus rarely get injured, because they have larger pennation angles and are stronger in nature.

Neuromuscular Factors #

Motor Unit Recruitment #

A motor unit can be defined as a single motor neuron and all of the muscle fibers it innervates.

As stated by Henneman’s size principle, motor units are recruited in order of size from smallest to largest according to how much force is required to be produced. Small motor units consist of mainly slow-twitch fibers, large motor units contain mostly fast-twitch fibers.

Untrained individuals can often access the slow units but a limited amount of the fast units. This makes heavy strength training a necessity, as it will provide more access to the higher order motor units and thus develop your force production capabilities.

Neuromuscular improvements help to explain why untrained individuals might initially improve their strength quite fast as they start training despite not putting on much mass.

Motor Unit Firing Frequency #

Force production can also be increased by how often a motor unit is fired.

Firing frequency relates to the frequency at which neural impulses are transmitted from the motor neuron to the recruited muscle fibers of the motor units.

Instead of just switching the higher end motor units on, activities such as sprinting and jumping require keeping those motor units on for the duration of the movement. A combination of Henneman’s size principle and firing frequency are used to control force production. Thus, both heavy resistance and explosive training are needed to improve those qualities.

Further to this, such training can also increase synchronization, which refers to the simultaneous activation of several motor units, which can further increase force production.

Other Factors #

Length-Tension Relationship #

The length-tension relationship relates to how a muscle’s force production capabilities change as the muscle’s length is changed.

Force production is low at short muscle lengths and rises to a maximum at closer to mid-range before falling off as the muscle is stretched further.

This has got to do with the sliding filament theory that explains the mechanisms of muscle contraction. I mentioned the sarcomere above. Taking it a step further, a sarcomere consists of two main protein filaments - a thick myosin filament and a thin actin filament.

According to the sliding filament theory, the myosin and actin filaments slide past each other during a muscle contraction, which causes the shortening of the sarcomere and thus the muscle itself. The cyclic attachment and detachment of the actin and myosin filaments has been called the cross-bridge cycle, in case you want to dig deeper.

Anyways, this basic concept explains why a muscle’s force production capabilities are affected by the muscle’s length.

At both very short and very long muscle lengths, myosin and actin binding ability is suboptimal, thus resulting in a less forceful muscle contraction. At mid-range, the binding ability is close to perfect, thus force production is highest in this range.

The length-tension relationship varies between different muscles due to differences in muscle architecture and role.

It’s important to not interpret this relationship in the wrong way. The fact that muscles are strongest and thus able to produce the highest amount of force at mid-length doesn’t mean your training should only consist of training at this range.

Training only at this small mid-length range would further amplify the length-tension relationship, making the muscle even stronger at mid-length but leaving both ends incredibly weak. This can drastically increase injury risk. As such, training through the full range of motion is warranted.

Force-Velocity Relationship #

The ability to generate force varies with the velocity at which the muscle shortens.

As velocity of shortening increases, force production capabilities diminish. Higher force is always produced at lower velocities.

In case there is access to a velocity-based training device, data could be collected on how the athlete’s force production capabilities change with increases in movement velocity. Profiling that can direct training goals towards more velocity-based or more force-based focus, depending on where on the curve the athlete shows deficits.

Stretch Shortening Cycle #

When a muscle fiber is allowed to stretch and then immediately allowed to shorten, it will perform work in excess of what is possible by only shortening it.

This sequential combination of eccentric (lengthening) and concentric (shortening) muscle action is called the stretch shortening cycle (SSC).

SSC is the underlying mechanism utilized in plyometric training. Muscles are connected to bones via tendons. The function of a tendon is to transmit force from the muscle to the bone to cause movement. Tendons are capable of storing elastic energy and subsequently returning that. If executed effectively, that would increase force production.

Muscle spindles #

Our muscles have reflexive mechanisms that can be trained to enhance force production. One such mechanism is the muscle spindle.

Muscle spindles are proprioceptors that detect and respond to changes in muscle length. They are located deep in the muscle belly within non-contractile muscle fibers called the intrafusal fibers. Intrafusal fibers run parallel to extrafusal fibers that are the contractile units which bring about the changes in muscle length.

When the muscle is rapidly lengthened, the muscle spindles send an electrical impulse to the central nervous system (CNS) of potential danger to overstretch the muscle. CNS then reacts by sending back a response to shorten (contract) the muscle, which serves as a protective mechanism against overstretch. At the same time, the intrafusal fibers that contain muscle spindles also contract with the aim to dampen the signal down to get the muscle spindles to switch off.

For running and jumping, these muscle spindles can be sensitized to the eccentric contraction occurring as part of those movements.

If the stretch reflex of the muscle spindles is timed appropriately as part of a plyometric action, both the descending impulse from the CNS to contract the muscle along with the intrafusal fibers themselves contracting will amplify the force response of the muscle. This will essentially “supercharge” the muscle, taking its force production capabilities beyond its norm.

To train this mechanism, we need a high enough eccentric load to cause a rapid eccentric lengthening of the muscle. This makes the muscle spindle fire and when timed appropriately, will allow for the subsequent concentric action to be amplified. High intensity plyometrics with ground contact time of 0.25s are ideal to develop this stretch reflex mechanism.

Inhibitory Responses #

Our muscles also have inhibitory mechanisms in place. One such mechanism is the Golgi Tendon Organ (GTO).

GTOs are located at the musculotendinous junction, which is the connection between a muscle and its tendon. GTOs detect rapid changes in muscular force. They protect against excessively high force production that could damage us, but they tend to be over-sensitive. As such, they limit the amount of force we can produce, which is not a preferable characteristic for maximal force development in a performance setting.

In addition to driving adaptations in motor unit recruitment mentioned above, exposing muscles and tendons to high levels of force with maximal strength training would also dampen the response of GTOs, allowing for higher force production.

This should give you something to think about when trying to develop strength and power.
Always aim to understand the “why” behind the training that you do. It doesn’t matter if you’re an athlete yourself, or a coach, or just someone who likes to train for fun and enjoyment. Understanding why you’re doing what you’re doing will elevate your training to new heights. Don’t sleep on the power of applied knowledge.

Also, be cautious of coaches and trainers who can’t provide you with adequate reasoning as to why you should do the exercises they have prescribed for you. If they can’t explain the purpose, there is a probability that they don’t know what the fuck they’re talking about. As such, they shouldn't be coaching you in the first place.

If you want to explore some or any of the presented ideas further, I'm open to having discussions. Please don't hesitate to contact me.

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