The sarcomere is the fundamental force-generating unit of muscle. It follows a predictable length-tension relationship, and this is fairly well-understood by many who study muscle, train individuals, and rehabilitate patients. If I were to ask a personal trainer, physical therapist, or exercise science student, I am sure (hopeful?) that they could tell me that this relationship consists of an ascending limb at short lengths, a plateau in the middle, and a descending limb at long lengths. However, things may start to become unclear when asking people what the plateau in the middle – where sarcomeres produce the most force – is called. The purpose of this post is hash out some confusion regarding nomenclature and the point at which sarcomeres can produce the most force.
Terms and Definitions
To ensure that we are on the same page, it seems prudent to describe terms and definitions that are commonly used, how they are described in the literature, and how I will be using them in this piece. The following are taken from Fridén & Lieber1.
Optimal length – Length at which myofilament overlap is optimal and force is maximal (2.6-2.8 µm in human muscle, 2.0-2.2 µm in frog muscle)
Slack length – Length at which muscle force equals zero; this length is unknown for most human muscles but is the retracted length that a muscle becomes after the tendinous insertion is cut
Resting length – A clear definition of this length is not possible, since passive tension is variable between muscles and the resting condition is not well defined; should not be used to describe an absolute length
In situ length – Muscle length under a specified joint angle configuration; should not be used to describe an absolute length
In vivo length – Muscle length under a specified joint angle configuration; should not be used to describe an absolute length
Clarification on Resting Length
Personally, I do not find the definition of resting length provided to be very informative, so I will expand on it. It is common to see the length-tension relationship illustrated with an active and passive component. Here, the point on the x-axis where the passive component begins to contribute tension is defined as the resting length. This is well-illustrated in Figure 1, below. For the remainder of this article, I will refer to this definition as restingproper. This definition differs greatly from a more lay definition or understanding, in that one may think resting length is the length of a muscle when the joint(s) it crosses is in the "neutral" position. (How one chooses to define neutral is a different discussion altogether.) I will refer to this definition as restinglay.
Figure 1. Muscles with different passive length-tension relationships, and thus restingproper lengths, are depicted. Curves #1, #2, and #3 would have restingproper lengths of 1.5, 2.5, and 3.5 µm, respectively. Figure from Fridén and Lieber1.
Optimal vs. Resting Length
In multiple classes, I was taught that muscles, and thus their sarcomeres, produce the most amount of force while at resting length. (Note that no subscript is associated with this definition, because it is presented with ambiguity; therefore, I will cover this topic as it applies to both restingproper and restinglay.) This idea appears to be fairly rife, as it is suggested by a number of textbooks2-4 and authoritative organizations5-7. Unfortunately, it is incorrect.
From the definitions presented above, one can clearly see that optimal length is the appropriate term to describe the length at which sarcomeres produce the most force, especially when compared to restingproper, as the passive length-tension curve may shift horizontally in a muscle-specific manner (Figure 1). In other words, it is possible for the passive length-tension curve to begin at lengths that are different than optimal length. While this does not preclude restinglay from also representing optimal length, other work does.
The operating range and restinglay length of sarcomeres within a muscle appear to be heterogeneous and muscle-dependent. Both in vivo8 and cadaver9–10 work suggest this. See, for example, Figure 2, which shows that different muscles not only have different restinglay sarcomere lengths, but also different operating ranges. The heterogeneity of restinglay lengths is further evidenced by a cadaver study by Ward and colleagues9, who reported sarcomere lengths of every major lower extremity muscle (Table 1).
Figure 2. A) In vivo length-tension relationship of wrist flexors and extensors. The wrist flexors tend to reside on the ascending limb, while the extensors reside closer to optimal length and cross into both the ascending and descending limbs. FCU = flexor carpi ulnaris; FCR = flexor carpi radialis; ECRB = extensor carpi radialis brevis; ECRL = extensor carpi radialis longus; and ECU = extensor carpi ulnaris. B) The length-tension relationship of the lumbar multifidus muscle. Together, these data provide strong evidence that optimal length does not occur at restinglay length. From Lieber and Ward8.
Table 1. Sarcomere lengths in the lower extremity muscles of cadavers.
|Muscle||Sarcomere length (µm)|
|Psoas||3.11 ± 0.28|
|Iliacus||3.02 ± 0.18|
|Gluteus maximus||2.60 ± 0.36|
|Gluteus medius||2.40 ± 0.18|
|Sartorius||3.11 ± 0.19|
|Rectus femoris||2.42 ± 0.30|
|Vastus lateralis||2.14 ± 0.29|
|Vastus intermedius||2.17 ± 0.42|
|Vastus medialis||2.24 ± 0.46|
|Gracilis||3.24 ± 0.21|
|Adductor longus||3.00 ± 0.37|
|Adductor brevis||2.91 ± 0.25|
|Adductor magnus||2.19 ± 0.32|
|Biceps femoris long head||2.35 ± 0.28|
|Biceps femoris short head||3.31 ± 0.17|
|Semitendinosus||2.89 ± 0.28|
|Semimembranosus||2.61 ± 0.25|
|Tibialis anterior||3.14 ± 0.16|
|Extensor hallucis longus||3.24 ± 0.11|
|Extensor digitorum longus||3.12 ± 0.20|
|Peroneus longus||2.72 ± 0.25|
|Peroneus brevis||2.76 ± 0.19|
|Gastrocnemius medial head||2.59 ± 0.26|
|Gastrocnemius lateral head||2.71 ± 0.24|
|Soleus||2.12 ± 0.24|
|Flexor hallucis longus||2.37 ± 0.24|
|Flexor digitorum longus||2.56 ± 0.25|
|Tibialis posterior||2.56 ± 0.32|