SUMMARY

Background: A vital tool in military action, the grenade is a tactical weapon for use at a distance. To provide for optimal throwing range and accuracy, the mass of the grenade must be examined. Historically, only the physical distance and accuracy to target is measured to determine the most advantageous mass of a thrown object. This neglects the effects each object has on the throwing soldier. Resultant fatigue and injury can therefore only be understood through direct measurement.

Methods: This study examined 3 soldiers as they threw 5 grenades of different mass. Each soldier utilized 3 throwing methods including; crouching before a standing throw for distance, crouching before a standing throw for accuracy, and running to a standing throw for maximum distance possible. Electromyography (EMG) was collected on 4 muscle groups and a full body 3D inertial measurement unit (IMU) system was worn to capture kinematic throwing data including anatomical angle ranges, rotational velocities, and sequence of body movements.

Results: 1. Overall muscle effort was directly proportional to the mass. 2. Shoulder range of motion was found to decrease with increased mass. 3. Specifically, external shoulder rotation was found to decrease with increasing grenade mass. 4. Compared to throwing for accuracy, when soldiers threw for distance they had higher maximum muscle activity but lower overall muscle effort spent as the high amplitude was over a short duration. 5. Throws were found to be appropriate for the open kinetic chain (OKC) principle. 6. The AFG grenade was found to have the lowest potential for shoulder injury, but further study in needed. 7. Throwing grenades with 16 ounces mass for maximum distance was found to require the least amount of total work.

Biomechanical Analysis: Optimal Mass for Overhand Grenade Throwing

December 2014

STUDY INFORMATION The data collection was carried out over the course of 2 days by Zach Scarano,

biomedical engineering expert and graduate of Arizona State University’s Biomedical Engineering program. Analysis of the data was conducted by Michael Sykes, MS

Biomedical Engineering under the guidance of Dr. Peter Konrad, PhD Sports Science.

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Kinematic Analysis

INTRODUCTION To better evaluate the impact each grenade had on the throwing soldiers, the kinematic sequence was analyzed. This kinematic sequence itself or the underlying kinetic mechanism is often called the “proximal-to-distal sequence,” “kinetic chain,” or “whip-like effect” (Atwater, 1979; Feltner, 1989;Fleisig et al., 1996a; Kibler, 1995). While data was collected on the entire body, only those arm segments involved in the Open Kinetic Chain (OKC) of throwing were considered, see (figure 1). The OKC model suggests that an overhand throw takes place similar to a whip unfurling with the proximal body sections moving first followed by the distal elements as seen in figure 1. This sequence of motion predicts the shoulder to rotate followed by the forearm with the hand moving last.

FIGURE 1: Open Kinetic Chain (OKC) of the throwing motion, showing 6 positions from windup to follow though. The sequence of applied torques follow from (a) to (h) in this model, whereby the final rotational torque (h) should be about the wrist. The lower figures show the amount of trunk rotation during each sequence of the OKC throwing motion.

METHODS Subjects More than 40 soldiers were asked to perform the same grenade throwing activities. Of these, 3 were chosen to wear the additional EMG and IMU sensors for kinematic data capture. Differences in height, weight and muscle mass were sought after when deciding the 3 participants to better represent the entire soldier population.

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Apparatus Four muscle groups known to be used during the overhand throwing motion were analyzed using the Noraxon myoMUSCLE EMG system at a sample rate of 3000Hz. Each of the 3 participants were right hand dominant and therefore the right external oblique, right lower trapezius, right infraspinatus, and left external oblique were studied. A 16 sensor wireless Noraxon myoMOTION IMU system, sampling at 200Hz, was worn by each participant to collect the 3D kinematic parameters of the throwing motion and the myoRESEARCH biomechanical analysis software was used to analyze them synchronously. Set-up Each of the two Noraxon systems used provide for wireless sensor communication and the ability to synchronize the data collection without additional equipment. The wireless EMG sensors were affixed to the skin of each subject using Noraxon double sided sensor tape and self adhering electrodes were used. Additionally, Noraxon neoprene body straps were provided to adorn the 16 wireless IMU sensors. The software used, myoRESEARCH 3.6, required only a portable laptop computer and allows for control of Noraxon hardware, data collection, analyses and processing. All of this equipment was operated outside on a training field using only the battery power of the laptop. Procedure While the formal 40+ person study was being conducted, 3 individual soldiers were selected and asked to wear the EMG and IMU sensors. One research assistant was able to adorn and calibrate the wireless sensors as each soldier waited for their respective turn to throw. Each of the 5 different grenades were thrown using 3 separate stations. Additionally, each throw was repeated to allow for a higher confidence in the collected data. Therefore each soldier threw a total of 30 grenades, 3 stations * 5 grenade styles * 2 throws. It was found that the high rotational velocities and linear accelerations experienced about the throwing arm resulted in the IMU sensors to saturate and therefore become unreliable after 2 consecutive throws. Calibration of the IMU sensors was therefore completed after each set of identical throws and was found to correct the problem. The EMG signals captured were processed using the myoRESEARCH 3.6 software by rectification and applying an 8th order Butterworth high pass filter with cutoff frequency of 0.1 Hz. Overhand Throwing Biomechanics To begin analysis we defined the phases of interest during the grenade throw as seen in figure 6. Both acceleration and position data was used to define both the phases and their transitions. PHASES

1. Address 2. Arm Cocking 3. Arm Acceleration 4. Arm Deceleration 5. Follow Through

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Address phase was defined as the moment showing no accelerations and before the throwing arm began to rotate. The arm cocking phase began at the first sample showing shoulder rotations and ending when the arm began to accelerate in the forward direction. Arm acceleration phase starts just after the shoulder reaches maximum external rotation and ends when the ball is released. The first sample that measured a deceleration from the maximum, defined the beginning of the arm deceleration phase. Once the throwing arm began to drop and cross the chest, the follow through phase began and ends once the rotational velocity about the throwing wrist measures zero.

Using these phases we calculated the total and average muscle activity, range of motion (ROM) of the shoulder, and accelerations about each the shoulder, elbow and wrist.

arm cocking arm acceleration arm deceleration follow through

FIGURE 2: The above illustrates phases 2-5 of the overhand throwing motion used in the grenade throws. It is easy to see from this profile view the importance of shoulder range of motion during this motion.

Definition of analysis periods When using multiple data collection systems it can be difficult to ensure proper sample time alignment and process the raw signals. The myoRESEARCH 3.6 software allowed for easy data viewing and manipulation as seen in figures 3-5 while giving robust signal processing options.

FIGURE 3: The above computer screenshot shows the 4 EMG signals and accompanying 3D avatar data showing the six phases of overhand throwing; Address (1), Arm Cocking (2), Arm Acceleration (4-5), Arm Deceleration(6), Follow Through (7-8).

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Each record had markers placed at 8 unique locations defining the 5 phases as seen in figure 3. From these markers the myoRESEARCH 3.6 software was able to give analytic results for EMG activities, arm accelerations and ROM reports as seen in figures 4-5. FIGURE 4-5: Above left shows an example report giving arm segment rotations including minimum/maximum angles achieved and total ROM. Above right is an example of the EMG report provided by myoResearch 3.6.

RESULTS & DISCUSSION Study Aims The aims of this study were: 1. To investigate the throwing experience, and the types of injuries associated with

overhand throwing motions. 2. To characterize 5 grenades of different mass, specifically their respective impact on the

human body. 3. To analyze at least a small percentage of possible biomechanical colorations present

within the collected data. 4. To provide practical suggestions based on the findings from above. Open Kinetic Chain It was determined that the Open Kinetic Chain model was most appropriate when discussing throwing grenades. The OKC was used to describe the pattern of the throwing motion 3-4. In the OKC pattern, body segments work like a linkage, with the distal end free to move and the proximal end fixed. The grenade throwing movement is an example of OKC motion, with the proximal segment accelerating the distal segment until the release. Flesig et al.6 and Atwater7 also indicated that the forces are accumulated for proximal to distal segments in the human throwing motion. Atwater7 and Flesig et al.8 also point out that in the human upper body throwing motion the force is transferred according to the kinetic chain principle, which proceeds from the proximal end to the distal end.

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First, the trunk forward motion and leftward rotation are accelerated by respective joint torques produced by relatively large muscles located at the lower extremity and trunk. The shoulder horizontal flexion torque and internal rotation torque during this phase prevent the upper arm from lagging behind relative to the trunk. As a result, the angular velocity of the upper arm also increases with that of the trunk. Thus, the motions of the trunk and upper arm in the early phase are produced by the instantaneous direct effect from large proximal muscles. The angular velocities of the trunk and upper arm produced by the above mechanism are the sources of the velocity-dependent torque acting for the elbow extension. As a result, the elbow joint angular velocity increases, and concurrently, the forearm angular velocity relative to the ground also increases. The forearm angular velocity subsequently accelerates the elbow extension FIGURE 6: The Open Kinetic Sequence between the shoulder (yellow), elbow (red) and wrist (green) showing the most proximal segment moving first followed by the second and lastly the most distal body segment.

Grenade Comparison

FIGURE 7: The above chart shows the EMG and 3D IMU Motion analysis of the total throwing motion, including all 5 predefined phases of the distance throw. Note the EMG activity increases with increased mass while the external shoulder rotation decreases with increasing mass. The OKC model predicts the kinetic energy percentage should be largest for shoulder and smallest for wrist, which corresponds with experimental findings.

GRENADE COMPARISON

[ Distance Throwing ]

General Details Grenade 1 Grenade 2 Grenade 3 Grenade 4 Grenade 5

Mass 16 oz 18 oz 22 oz 24 oz 18 oz

Shape Round Round Round Round Ellipsoid

EMG Analysis (mV) Grenade 1 Grenade 2 Grenade 3 Grenade 4 Grenade 5

Right External Oblique (Max/Mean) 249/497 314/668 381/562 456/689 417/702

Right Lower Trapezius (Max/Mean) 416/669 373/611 144/287 192/291 293/558

Right Infraspinatus (Max/Mean) 187/250 153/235 218/273 203/350 243/362

Left External Oblique (Max/Mean) 235/347 367/492 382/1059 1034/1842 944/1143

3D Motion Analysis (degrees) Grenade 1 Grenade 2 Grenade 3 Grenade 4 Grenade 5

Backward trunk tilt angle 8.9 8.6 8.7 8.1 9.1

Forward trunk tilt angle 26.2 27.1 26.9 22.9 25.6

Knee angle of lead leg (at release) 41.1 40 39.3 37.4 41.4

Sholder internal rotation 49 na 52 50.8 49.7

Shoulder external rotation 129 122 114 107 133

Total Shoulder rotation 178 na 166 157.8 182.7

Elbow flex angle (max) 78.5 77.4 74.4 72.3 82.4

Elbow extend angle (max) -28.8 -28.8 -24.1 -25.9 -21.6

Wrist flex angle (max) 16.9 13.2 14 13.3 9.9

Wrist extend angle (max) 15.4 16.1 14.8 11.9 12.7

Kinetic energy percent of shoulder 61.2 57.4 58.9 54.1 69.2

Kinetic energy percent of elbow 30.7 32.8 33.9 31.1 27.4

Kinetic energy percent of wrist 8.1 9.8 7.2 14.6 3.4

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FIGURE 8: The chart above shows the data for only the accuracy throws of this study. Clear differences appear when compared to the distance throw including a higher average EMG activity over a longer period of time and measurably smaller ranges of motion for the throwing arm.

System Errors The EMG data from 4 of the 90 total grenade throws could not be properly analyzed as the gear worn by the participants pulled on the lead wires resulting in a poor signal. This problem was recognized by the research assistant and the EMG sensors were repositioned with good resultant signal quality. The rotational velocities measured about the arm during an overhand throw has been demonstrated as the single fastest motion capable by the human body (6-7). During this study participants exceeded 5000 degrees per second about the shoulder, elbow and wrist which resulted in the myoMOTION IMU sensors to saturate. After the follow through phase the avatar displayed some discontinuity visibly with the participants. This was mitigated by recalibrating the myoMOTION system between each pair of like throws. The data analyzed for this study was not affected by the IMU data saturating during the Arm Deceleration phase as all external shoulder rotation occurs before this time. Video was collected during the study but was not considered during analysis due to privacy concerns. The addition of this video data would allow for a second method to measure both velocities and angle changes giving a higher level of total confidence to the report.

GRENADE COMPARISON

[ Accuracy Throwing ]

General Details Grenade 1 Grenade 2 Grenade 3 Grenade 4 Grenade 5

Mass 16 oz 18 oz 22 oz 24 oz 16 oz

Shape Round Round Round Round Ellipsoid

EMG Analysis Grenade 1 Grenade 2 Grenade 3 Grenade 4 Grenade 5

Right External Oblique (Max/Mean) 187/285 199/268 202/275 257/287 183/241

Right Lower Trapezius (Max/Mean) 274/317 289/448 233/395 322/440 226/299

Right Infraspinatus (Max/Mean) 293/457 359/469 196/493 261/479 302/413

Left External Oblique (Max/Mean) 234/288 292/341 238/355 277/361 232/374

3D Motion Analysis Grenade 1 Grenade 2 Grenade 3 Grenade 4 Grenade 5

Backward trunk tilt angle 6.6 9.3 6.9 7.2 12.1

Forward trunk tilt angle 19.4 17.1 12.5 13.6 13.5

Knee angle of lead leg (at release) 33.1 37.9 34 30.8 39.4

Sholder internal rotation na na 40.7 41.4 44.5

Shoulder external rotation 106 99.2 102.6 98.3 113.6

Total Shoulder rotation na na 143.3 139.7 158.1

Elbow flex angle (max) 80.1 66.5 72.1 72.3 80.4

Elbow extend angle (max) -27 -26.4 -27.2 -24.9 -20.9

Wrist flex angle (max) 17.4 16.6 16.6 15.1 8.8

Wrist extend angle (max) 11.7 13.8 9.9 11 12.5

Kinetic energy percent of shoulder 49.6 50.9 52.1 53.2 56.3

Kinetic energy percent of elbow 36.6 30.2 31.7 35.9 35.1

Kinetic energy percent of wrist 13.8 18.9 16.2 10.9 8.6

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CONCLUSIONS As grenade throwing remains an important training requirement for soldiers, this study intended to examine the physical stresses on the body. Injuries related to the overhand throwing motion have been investigated and found to have a high correlation to reduced external shoulder rotation and torque about all joints of the throwing arm. The overhand throwing motion produces some of the fastest movements capable by humans. In this study, for example, each participant exceeded 5000 degrees per second of rotational velocity about both the elbow and wrist while accelerating at over 16G's. Applying the OKC to our 16 segment rigid body model allowed for work and energy to be estimated for each throw measured. This method shows that an increase in grenade mass over the range tested requires more work and produced higher torques about the throwing arm. Collection of muscle activity from dominant muscle groups also supported this finding. The AFG, prolate spheroid shaped grenade, was found to be a different throwing motion than that characterized for the round style. As these two movements have unique torque requirements the measured muscle activity could not be directly compared for this study. Analyzing only the round grenade shows an increase in duration and maximum amplitude for increased object mass. From these two findings it is recommended that the 16 ounce grenade be used to reduce chance of injury and fatigue. Beyond its relevance to understanding injury, rotational range of motion at the shoulder may also have significant performance consequences. It is therefore recommended that further investigation take place to determine the impact of grenade mass on the soldier's throwing proficiency over time.

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REFERENCES 1. Lu LC, Shaw KP. A correlative study of hand-grenadethrower’s fracture and ody characteristics. J Med Sci 1997;18:11-17. 2. Shaw KP, Shaw KY, Liu JC. A biomechanical study of hand-grenade thrower’s fracture. J Med Sci 1988;9: 91-102. 3. The arm as a whole. In: Steindler A, ed. Kinesiology of the Human Body under Normal and Pathological Conditions. Springfield: Thomas, 1955:556-568. 4. Newton H. Ask the experts. Strength and Condit 1994; 16:74. 5. Throw-like and push-like movement patterns. In: Kreighbaum E, Katharine MB, eds. Biomechanics: A Qualitative Approach for Studying Human Movement. Boston: Allyn and Bacon, 1996:335-354. 6. Flesig GS, Steven WB, Rafael FE, Andrews JR. Biomechanics of overhand throwing with implication for injuries. J Sports Med 1996;21:421-437. 7. Atwater AE. Biomechanics of over-arm throwing movements and throwing injuries. Exerc Sport Sci Rev 1979;7:43-85. 8. Flesig GS, Escamilla RF, Andrews JR. Kinematic and kinetic comparison between baseball pitching and football passing. J Appl Biomech 1996;12:207-224. 9. Feltner M, Jesus D. Dynamics of the shoulder and elbow joint of the throwing arm during a pass-ball pitch. Int J Sport Biomech 1986;2:235-259. 10. Matsuo T, Escamilla RF, Flesig GS, Barrentine SW, Andrews JR. Comparison of kinematic and temporal parameters between different pitch velocity groups. Journal of Applied Biomechanics 2001;17:1-13. 11. Stodden DF, Flesig GS, McLean SP, Lyman SP, Andrews JR. Relationship of pelvis and upper torso Kinematic to pitched baseball velocity. J Appl Biomech 2001;17:164-172. 12. Kibler WB. Biomechanical analysis of the shoulder during tennis activities. Clin Sports Med 1995;14:79-85. 13. Elliott BC, Marshall RN, Noffal GJ. Contributions of upper limb segment rotations during the power serve in tennis. J Appl Biomech 1995;11:433-442. 14. Elliott BC, Grove JR. A three-dimensional cinematographic analysis of the fastball and curveball pitches in baseball. J Appl Biomech 1986;2:20-28.

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APPENDIX A (additional figures)

FIGURE 8: Accelerations experienced on the dominant throwing upper arm, in all three axis. Markers again indicate the phases of throwing considered within this report.

FIGURE 9: The EMG of all 4 muscle groups measured after the signals have been rectified and filtered.

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APPENDIX B (sample of participant B’s EMG analysis)

FIGURE 10: The EMG of throwing 4 different grenades for accuracy. The similarities in waveforms between measurements gives a good indication of well working equipment and much more to learn from this already collected data set.

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APPENDIX B continued (sample of participant B’s EMG analysis)

FIGURE 10: The EMG of throwing 4 different grenades for maximum distance. Before using more advanced analysis techniques, it is possible to see the correlation of increased mass to increased muscle activity.