Cornell University Ergonomics Web

DEA 3250/6510 CLASS NOTES

Hand Tool Design and Musculoskeletal Disorders

Introduction - Hand tools are anything that can be manipulated by the hand. The economic and political stability of early cultures often depended directly on the sophistication of available hand tools, e.g. weapons, instruments. The use of hand tools is ubiquitous and poorly designed hand tools in an industrial plant may affect more than 10% of workers per year.

Poor Design - may cause decreases in productivity with slower work and more errors. Increases in injuries to the wrist, forearm, and shoulders may also occur. Illnesses may also increase due to the effects of long term injury effects. Accidents may also increase as will compensation costs.

Biomechanical Considerations in Hand Tool Design - Forceful grip exertions of hand rely on muscle contractions in forearm, and muscle forces are transferred to fingers via tendons.

• Grip Configuration - determines level of muscle exertion and tendon tension, and there is some effect of hand and wrist anthropometry.
• Wrist Angle - during grip-type exertions directly affects the amount of intra-wrist supporting forces acting normal to the direction of tendons and synovia (the lubricating sheaths around tendons).

Shape and Size Considerations for Better Performance -

• Shape the tools to avoid extremes of wrist deviation - Allowing hand and forearm to remain in alignment during forceful grip exertion often requires special handle design such as:
  • Bend in the Tool Handle - This has been effective in reducing wrist-related disorders in users of pliers and knives.
  • Pistol grip vs Cylindrical grip designs - Especially applicable to motorized hand tools. Here driving the torque of tool creates tendency for the tool to rotate in worker's hand unless firmly gripped. Any wrist deviation leads to significantly increased risk of injuries. Pistol-shaped tools allow for greater control with less force and wrist deviation.
• Shape the tool to avoid shoulder abduction - If the tool requires extreme wrist deviation, the usual reaction is to raise the arm to decrease wrist stress. This biomechanical tradeoff increases stresses on the shoulder joint. Abduction up to 20 reduces excessive load on the shoulder, but as angles increase beyond 20° so the increase in shoulder load leads to an increase in muscle fatigue. If shoulder abduction exceeds 30° then you get a rapid increase in fatigue. If shoulder abduction is 60° , then muscles fatigue 3X as fast. If shoulder abduction is 60°, then muscles fatigue 6X as fast. Keeping arms down as close as possible to the body can minimize fatigue.
  
• Shape the tool to assist the grip - Slight contouring of the grip or flared handles can increase comfort and reduce slippage in sweaty hands. Handles should be at least 4 - 5" long for power grip. Longer handles help distribute forces on fingers. Smooth handles for tools requiring wrist rotation should be avoided because of the increased risk of slippage and rotational wrist damage. Padding handles reduces the force needed to grip the tool.
  • For Forceful Squeezing - ensure that tool can be gripped by men and women, starting grip distance isn't too great, and that forces aren't concentrated on a few fingers or the center of the palm.
    
  • Anatomical Limitations - are the locations of the median nerve, arteries, synovium for finger flexor tendons directly under skin of palm.
• Hand tool weight - Effects of tool weight can aggravate muscle actions necessary to precisely position and stabilize the tool during operation. Tool balancers which counterbalance a tool may be effective depending on how the tool is to be used, how often and how long. Use of rests, supports, two hand grips, etc. can all help to decrease the effort required to use heavy tools.
• Right vs Left-handed Tools - Try to design tools for operation with both hands. When only right-handed users are considered, left-handers may be at an increased risk of injury. Left-handed people are 5 times more likely to suffer injuries trying to cope with right handed products.

Musculoskeletal Disorders (MSDs)

Introduction - MSDs are the major work place injury. There are many different names for these injuries:
CTD - Cumulative Trauma Disorders
OOD - Occupational Overuse Disorder
RMI - Repetitive Motion Injury
RMD - Repetitive Motion Disorder
RSI - Repetitive Strain Injury
UECTD - Upper Extremity Cumulative Trauma Disorder

Etiology of MSDs 
Four risk factors increase the likelihood of an MSD:

1. Posture of hand and body - Posture deviated from "neutral position" increases the likelihood of injury. This can include hand deviations, wrist extensions, or poor seated posture.
2. Number of Repetitions - High frequency of performing a motion can increase the likelihood of injury. Insufficient micro breaks between motions (such as keystrokes) and the resulting muscle fatigue contribute to this process.
3. Amount of Force - A higher force with which the motion is performed can increase the likelihood of injury. A common problem is that keys on a keyboard are hit with excessive force.
4. Level of Stress - Stress increases muscle tension which in turn increases the forces on the tendons.

Tendon and Nerve Disorders
Minor disorders of tendons and their sheaths are quite common. Tendon disorders often occur at or near the joints where the tendons rub nearby on ligaments and bones. The most frequently noted symptoms are a dull aching sensation over the tendon, discomfort with specific movements, and tenderness to touch. Seldom is there noticeable redness or local heat. Recovery is usually slow and the condition may easily become chronic if the cause is not eliminated.

Anatomical Review - The hand is cupped to protect the inside of the palm and the wrist is contoured away from the body. These factors help to protect the median nerve located in the center of the forearm and hand which controls the thumb, first two digits and half of next digit. It also protects the radial artery, the blood supply to this area. The ulnar nerve and artery serve the area from the outside of the wrist to the last to fingers. However, with hand tools we put hands in contact with the world, repetitively, which can lead to several types of disorders.

Tendon disorders - Some examples are:

1. Tendinitis - is a form of tendon inflammation that occurs when a muscle/tendon unit is repeatedly tensed. With further exertion, some of the fibers that make up the tendon can actually fray or tear apart. The tendon becomes thickened, bumpy and irregular. In tendons without sheaths, such as in the shoulder, the injured area may calcify. Without rest and sufficient time for the tissues to heal, the tendon may be permanently weakened.
2. Tenosynovitis - is a general term for a repetitive-induced tendon injury involving the synovial sheath. The sheath may be stimulated to produce excessive amounts of synovial fluid which accumulates causing the sheath to become swollen and painful. If the tendon surface becomes irritated and rough, and if the sheath becomes inflamed and continues to press on the tendon, a condition called stenosing tenosynovitis may occur. Two types of this condition are De Quervain's disease and trigger finger.
3. Ganglionic Cyst - The affected sheath swells up with synovial fluid and causes a bump under the skin, often on the wrist. At one time ganglions were called Bible bumps because the Bible bumps because the Bible (then the most available book), was used to pound and rupture the ganglion. Ganglions are treated surgically today.
4. Epicondylitis - tendons at the elbow are unsheathed. The median nerve passes through the inside of the elbow and the ulnar nerve passes over the outside of the elbow. When the elbow is flexed nerve compression can occur. Injuries occurring at the elbow include:

• medial epicondylitis (golfer's elbow) - irritation of the finger flexor muscle attachments at the elbow
• lateral epicondylitis (tennis elbow) - irritation of the finger extensor muscle attachments at the elbow. Symptoms are most common at the outer elbow.

Nerve disorders - Nerve injuries occur when repeated or sustained work activities expose the nerves to pressure from hard, sharp edges of the work surface, tools, or nearby bones, ligaments, and tendons. Carpal tunnel syndrome is an increasingly common nerve disorder.

• Carpal Tunnel Syndrome (CTS) - The tendons for flexing the fingers, the median nerve and blood vessels pass through the carpal tunnel, under the carpal ligament, from the forearm to the hand. If any of the tendon sheaths become swollen in the cramped carpal tunnel, the median nerve may be pinched. This is one cause of the syndrome.
  • Symptoms of CTS - include pain, numbness, and tingling of the hands. These sensations usually are felt in the areas of the skin connected to the median nerve - the first three fingers and the base of the thumb.
  • Warning Signs and Causes for Carpal Tunnel Syndrome - In general when staring work, move hands slowly. Movement programs can help to reduce risks.
  • Sensory Fibers - Though there is poor sensory feedback from synovial sheath, early warning signs might include aching muscles. The median nerve is a mixed nerve with sensory fibers and motor fibers. Sensory fibers die first. Warning signs may include: tingling in finger tips, numbness in finger tips, blanching in finger tips, peripheral swelling, and pain at night time (up arm).
  • Nerve Compression - Nerve which leads to the hand from the spine may be compressed at the shoulder from wearing a heavy back pack, carrying heavy handbags with thin straps, or body building - increasing muscle mass in shoulder area.
  • Wrist deviation and Intracarpal pressure - Intracarpal pressure increases significantly at >20° ulnar and > 10° radial deviation (lateral deviation), and at more than 15° Flexion (down) or Extension (up - vertical deviation).

Neurovasular disorders - Some MSDs involve both the nerves and adjacent blood vessels. Examples include: thoracic outlet syndrome and vibration-induced white finger.

• Thoracic Outlet Syndrome - is a general term for compression of the nerves and blood vessels between the neck and shoulder (the neurovascular bundle). If the circulation is impeded by activities or postures that put excessive pressure on these blood vessels, the adjacent tendons, ligaments, and muscles are deprived of oxygen and nutrients. This produces an ischemic condition, which slows muscle recovery and limits the duration of muscle activity.
  • Symptoms of TOS - are similar to those of CTS, namely numbness in the fingers of the hand. The pulse at the wrist may be weakened.
• Vibration-Induced White Finger (Raynaud's disease) - recurrent episodes of finger blanching due to complete closure of the digital arteries. Exposure to cold may serve to trigger vasospasm in the fingers. Forceful gripping and prolonged use of vibrating tools can cause this problem.
  • Symptoms of VIWF - are progressive. They may begin with intermittent numbness and tingling, leading to whitening of the tips of the fingers and pain and skin that turns pale and cold as whole fingers start to blanch. Eventually there is a loss of sensation and control in the fingers and hands. Because fingers are receiving no blood, gangrene can set in.
  • More information on VIWF

Non-occupational MSD Injury Risk Factors -

• Gender - Women are 3 times more likely to get CTS partly because of hormonal status. 30% of pregnant women CTS (temporarily) because the basal pressure of the tunnel increases with increases in fluid retention.
• Size of wrist - The size of the wrist channel may play a role in CTS by influencing whether or not the inflamed sheath irritates the nerves.
• Wrist Shape - Broad, rounder wrists generally have less CTS than more square wrists.
• Body Weight - Heavier people have higher risk for CTS.
• Age - Older people have a higher risk for CTS.
• Sports - Repetitive actions involved in sports such as volleyball can increase risk for CTS.
•  Speed - with which the hands are used can influence risk. Flapping the hand quickly such as with the "royal wave" may be detrimental.

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