The different role of each head of the triceps brachii muscle in elbow extension

Abstract

Objective

The aim of this study was to investigate the functional role of each head of the triceps brachii muscle, depending on the angle of shoulder elevation, and to compare each muscle force and activity by using a virtual biomechanical simulator and surface electromyography.

Methods

Ten healthy participants (8 males and 2 females) were included in this study. The mean age was 29.2 years (23–45). Each participant performed elbow extension tasks in five different degrees (0, 45, 90, 135, and 180°) of shoulder elevation with three repetitions. Kinematics data and surface electromyography signal of each head of the triceps brachii were recorded. Recorded kinematics data were then applied to an inverse kinematics musculoskeletal modeling software function (OpenSim) to analyze the triceps brachii's muscle force. Correlation between muscle force, muscle activity, elbow extension, and shoulder elevation angle were compared and analyzed for each head of triceps brachii.

Results

At 0° shoulder elevation, the long head of the triceps brachii generates a significantly higher muscle force and muscle activation than the lateral and medial heads (p < 0.05). While at 90°, 135° and 180° shoulder elevation, the medial head of the triceps brachii showed a significantly higher muscle force than the long and the lateral heads (p < 0.05).

Conclusions

Each head of the triceps brachii has a different pattern of force and activity during different shoulder elevations. The long head contributes to elbow extension more at shoulder elevation and the medial head takes over at 90° and above of shoulder elevation. This study provides further understanding of triceps brachii's for clinicians and health trainers who need to investigate the functional role of the triceps brachii in detail.

Introduction

The triceps Brachii (TB) is a powerful extensor muscle of the upper extremity.¹ It has been described as a single muscle unit with three heads (medial, lateral, and long heads),² and a cadaver study found that the medial head of the TB was attached to the olecranon by a deeper separated tendon than the other heads.³ Different fatigue rates between each head were also observed in a hand grip task during an elbow extension.⁴ Hence, it is important to define the role of each TB head.

Biomechanical simulation allows us to investigate the function of a muscle by observing its properties during a particular movement. OpenSim is an open-source inverse kinematics musculoskeletal modeling software for both the development and analysis of dynamic simulations of human movement (Stanford, California, USA). OpenSim has been widely used to analyze muscle force of both the upper and lower limbs, musculoskeletal geometry (such as muscle length) and muscle-tendon properties on these forces5, 6, 7, 8, 9, 10, 11; it has been validated for biomechanical simulator purposes.¹² Muscle force always produces electrical activity, which can be recorded with an sEMG and serve as an objective parameter to support biomechanical simulation.13, 14, 15

One study for triceps muscle and its sEMG activity has been published previously,¹⁶ but the authors only analyzed sEMG activity for repetitive isometric contractions on the heads of the TB and showed the electrical activity of the muscle for its physiological state without kinematics data for its functionality. However, by using inverse kinematics and the sEMG parameter concurrently, we can observe both physiology (muscle activation) and kinematics (muscle force and length) as a unit entity. To our knowledge, no study has assessed the role of each of the heads of the TB using both parameters simultaneously. Therefore, the aim of the present study was to investigate the functional role of each head of the TB by comparing its muscle force and activity during various shoulder elevations. We hypothesized that each head of the TB will have a different force and activity pattern during various shoulder elevation angles. Hence, the muscle's integrity and functional insufficiency can be assessed during physical examination. In addition, this method would also be helpful for athletes to optimize TB training.

Materials and methods

Ten young healthy volunteers with no history of elbow and shoulder pain or disabilities (8 males and 2 females, age range: 23–45, mean: 29.2 years) participated in this study. There was no range of motion (ROM) limitation for any participant. Each participant was required to perform active elbow extension tasks at five different angles of shoulder elevation; 0°, 45°, 90°, 135°, and 180° (Fig. 1). Participants performed 3 repetitions of elbow extension with their dominant right arm for each task, with a minute interval between repetitions to reduce muscle fatigue. The data recording sessions were started after synchronizing the participants' motion to the metronome. Elbow kinematics and triceps head muscle activation were recorded simultaneously.

Fig. 1.

Elbow extension task simulation on OpenSim interface for A. 0°; B. 45°; C. 90°; D. 135°; and E. 180° shoulder elevation.

Muscle force and length measurement

The participant's kinematics data were measured using BioNomadix® wireless Tri-axial Accelerometer (BN-ACCL3 Receiver + Transmitter, Biopac Systems Inc., CA) prior to muscle force measurement. An accelerometer sensor with a 2000 Hz sampling rate was attached to the participant's wrist with adjustable straps. Elbow angles were derived from the angle of vector between the reference position (maximum elbow extension at each shoulder elevation) and the relative position of the accelerometer during each task.

Measured elbow kinematics were then applied to an adapted OpenSim model of the upper limb, which was derived from the Stanford VA Upper Extremity Dynamic Model¹⁷ (Fig. 1). The model consists of rigid bodies representing the trunk, upper arm, forearm, and hand and it was constrained to allow trunk lean, three degrees of freedom at the glenohumeral joint, and flexion/extension at the elbow joint. The actuator set comprised 29 muscles crossing the glenohumeral and elbow joints. Muscle attachments sites are determined from digitized muscle insertions, which were derived from its moment arm calculation,¹⁸ and its anatomical descriptions.¹⁹ The model was manually scaled to participant characteristics. The muscle forces for each triceps head were then analyzed using the OpenSim static optimization function.

Using the same-scaled model, each muscle length was also recorded. The starting position was full elbow flexion in order to calculate muscle length. The muscle length for each triceps head was then analyzed using the OpenSim Muscle Analysis function. Muscle lengths were then normalized to the 0° shoulder flexion as the baseline. Changes in the muscle length during different shoulder flexions were then compared for each triceps head.

Muscle activation measurement

Muscle activation of the TB long head, lateral head, and medial head were recorded with Surface Electromyography (Biopac MP150A-CE Data Acquisition System, Biopac System Inc., CA). Ports of the digital channels (HLT100C) were connected to three recording electrodes (TSD150B, 2 cm inter-electrode spacing, Biopac System Inc., CA) and a ground reference electrode (Kendall 100 Series Foam Electrodes, Medtronic, MN). Electrodes were then positioned according to the European recommendations for Surface Electromyography for Non-Invasive Assessment of Muscles (SENIAM)²⁰ (Fig. 2) on each triceps head. Intra-session reliability of the triceps sEMG recording with similar electrode positioning with a dynamic contraction was shown to have a good relative reliability (ICC = 0.94–0.99) and sufficient absolute reliability (CV (%) = 10.75/10.69).²¹

Fig. 2.

Surface Electromyography Electrode and Accelerometer placement on the three heads of the TB muscle for posterior right upper extremity; A. Long Head; B. Lateral Head; C. Medial Head; D. Ground Reference; E. Accelerometer.

Categorization of processed data and statistical analysis

Calculated muscle force and normalized muscle activation were categorized based on the corresponding elbow angle intervals (which were 135°–110°, 110°–85°, 85°–60°, 60°–35°, 35°–10° and 10°–0°, with 0° as elbow full extension) and its corresponding shoulder elevation angles (which were 0°, 45°, 90°, 135° and 180°).

Statistical analysis

All statistical analysis was performed with SPSS (SPSS Inc., Chicago, IL). In order to analyze the interaction between each muscle head, a series of repeated measures ANOVA were run. Three-way repeated measures ANOVA was conducted to determine the effects of TB muscle heads, elbow joint angle, and shoulder elevation angle on the muscle force and muscle activity. Subsequently, for each muscle force and muscle activation, a series of two-way repeated ANOVA of the TB muscle heads and the elbow joint angle for each shoulder elevation angle separately. The level of statistical significance for all analyses was set to P = 0.05.

Results

Muscle force and activation

We found a difference in the muscle force and muscle activity of each head of TB (Fig. 3). A statistically significant interaction was observed between TB muscle heads and shoulder elevation angle on the muscle force (F = 155.368, p < 0.001) and muscle activation (F = 12.593, p < 0.001). This result indicates a different function for each head during different shoulder elevations.

Fig. 3.

Muscle force and muscle activation, from left to right, of the long head, lateral head, and medial head in active elbow extension movement. The muscle force and activation of the long head and medial head differed from each other depending on the elbow joint angle and shoulder elevation angle. Muscle force and activation of the lateral head was similar to the medial head, albeit significantly lower.

In 0° shoulder elevation, the long head was shown to generate a significantly higher muscle force and muscle activity than the lateral head (p = 0.000 and p = 0.017, respectively) and also than the medial head (p = 0.000 and p = 0.000, respectively); however, no significant difference of muscle force and muscle activity were observed between lateral and medial heads (p = 0.059 and p = 0.070, respectively).

On the other hand, in 90° shoulder elevation, the medial head muscle was shown to increase its generation of muscle force and muscle activity, which were significantly higher than those of the long head (p = 0.000 and p = 0.000, respectively) and the lateral head (p = 0.000 and p = 0.000, respectively). No significant differences of muscle force and muscle activity were observed between the long and lateral head (p = 0.359 and p = 0.670, respectively).

Furthermore, at 180° shoulder elevation, the lateral head muscle increased its generation of muscle force and muscle activity, which were significantly higher than those of the long head (p = 0.000 and p = 0.005 respectively), but still significantly lower than the muscle force and muscle activity of the medial head (p = 0.000 and p = 0.001, respectively).

The long head showed uniform distribution of muscle activity throughout all elbow extension and shoulder elevation angles; however, the lateral and medial head had uneven distribution of muscle activity showing a tendency to be activated more at the terminal shoulder elevation.

With regard to muscle force, all heads of the TB showed a maximum point at 110° and showed a downward trend when the elbow was extended more than 85°. The medial head showed more uniform muscle force distribution throughout shoulder elevations particularly after reaching 45°. The muscle force pattern changes were observed to be more acute in the long and lateral heads.

Muscle length

The long head of the TB demonstrated significantly increased muscle length during all shoulder elevations (45°, p = 0.000; 90°, p = 0.000; and 135°, p = 0.000 respectively). There was no significant difference in muscle length when shoulder elevation was between 135° to its maximal elevation (p = 0.137). The lateral and medial heads of the TB did not show a significant difference in muscle length during all shoulder elevations (45°, p = 0.142; 90°, p = 0.174; 135°, p = 0.337; 180°, p = 0.190 and 45°, p = 0.145; 90°, p = 0.176; 135°, p = 0.373; 180°, p = 0.195 respectively).

Discussion

We evaluated the change of muscle force, activation, and length on each head of the TB at different shoulder flexion angles. The most notable finding was that the long head and medial head contributed to elbow extension at a different shoulder elevation level. In low shoulder elevations, the long head played the main role in extending the elbow, but by increasing the shoulder elevation angle, the medial head takes over as the major muscle force for extending the elbow. However, the lateral head has a similar pattern of muscle force to the medial head, but with a significantly lower force. From the clinical point of view, the integrity of the long head of the TB can be properly assessed in a fully extended shoulder position. In this position, the long head of the TB will be the most dominant muscle which contributes to elbow extension, and the role of both lateral and medial head are as an adjunct synergic support. This clinical implication is pertinent in terms of triceps insufficiency cases after elbow-replacement surgery. Physical examination is often misleading and results in wrong diagnosis. Furthermore, different shoulder elevation angles should also be considered in terms of sports-related or athletic training.

We also found that all heads of TB presented higher muscle force and muscle activation, when the elbow was flexed beyond 90°. We observed that the medial head showed the highest force generation during the elbow flexion between 85° and 110° regardless of the shoulder elevation. This result will help the clinician to easily assess an isolated TB tendon-tear case, in which the patient has weakness only when he or she attempts an elbow extension in the arc from full flexion to 90° of flexion due to the missing force of the TB. This result was also supported by a previous study in which the triceps has its greatest lever arm when the elbow is fully flexed.²²

This study demonstrated the muscle force for the medial head as uniformly distributed compared to other heads, and this may explain the low incidence of medial head injury.²³ Both muscle force and muscle activity for all heads declined when the elbow reached its terminal extension. This finding suggests that during the last degree of elbow extension, the elbow joint can withstand a minimal force load but would be prone to injury if eccentric loading was applied.

Regarding the muscle length, the long head had a significant increase in its length during shoulder flexion, while the lateral and medial muscle length stayed the same. Although this observation was expected, due to the anatomical characteristics of the long head (the only bi-articular muscle of the triceps complex), it reveals an explanation for why the primary muscle of elbow extension shifts from the long head to medial head during shoulder flexion. When the long head muscle stretches to its maximal length, the generation of active tension decreases in concordance with Blix curve.²⁴ To compensate for this loss of tension, the medial and lateral heads generate more force to keep the elbow extension motion steady.

This study has several limitations, the most notable being the small number of participants. Second, the regulation of elbow extension speed depends on the rhythmic ability of the subject, due to the nature of the metronome. Nevertheless, our study is the first to analyze the function of each head of the triceps brachii during different shoulder elevations using the OpenSim function.

Conclusions

Each head of the TB has a different pattern of force and activity during different shoulder elevations. At low shoulder elevations, the long head of the TB has the major role in elbow extension. While in higher shoulder elevation angles, the medial head takes over as the major muscle. This study provides further understanding of TBs for clinicians and health trainers who need to investigate the functional role of the triceps in detail. Further studies using biomechanical simulation of upper-limb muscles is suggested.

Key points

Based on our study, we conclude that each head of the TB has a different role for extending the elbow at a different shoulder elevation. At low shoulder elevation, the long head generates the necessary force to extend the elbow. While in high shoulder elevation angles (at least 90°), the medial head takes over as the major muscle for extending the elbow. We believe that this study contributes to providing an insight in understanding the detailed function of the triceps and may be useful for clinicians examining the integrity of the TB and determining the functional insufficiency of each head.

Grant support

This study is funded by the Ministry of Science and ICT (NRF-2012M3A6A3056425).

Conflicts of interest

No benefits in any form have been received or will be received from a commercial party related directly or indirectly to the subject of this article.

Institutional review board

IRB approval from Asan Medical Center, no: S2016-1712-0002.