Load-carrying during stair ascent: a demanding functional test

Gait & Posture 1993; 1: 35-44

Load-carrying during stair demanding functional test

ascent: a

H Moffet PhD1, C L Richards PhD1, F Malouin PhD1, G Bravo PhD2

‘Physiotherapy Department, Faculty of Medicine Lava1 University, and Neurobiology Research Center, Hopital de I’Enfant-Jesus, Quebec; 2Geriatric Research Center, Youville Hospital, Sherbrooke

Summary

The functional demand of a new test, stair ascent with a 22-kg load, was estimated by comparing the limb muscle activations and movements during stair ascent with 22 kg (EXP test) and without load (CTL test) in 15 normal men. During tests, the sagittal plane movements of the hip, knee, and ankle were measured with a TRIAX electrogoniometer and the EMG activity of six leg extensor muscles was picked up with surface electrodes. Temporal parameters of the stair ascent cycle were determined from footswitch closures. Angle, EMG, and footswitch signals were simultaneously recorded (100 Hz), then fed to an IBM PC for further analysis. After a IO-min rest period the maximal activation levels (MALs) of the knee extensor muscles were recorded during maximal voluntary knee extensions (MVC) at 30 and 180 deg s-l using a computer-controlled Kin-Corn dynamometer. Load-carrying induced a significant increase in EMG levels of the six leg extensor muscles studied during the stance phase. This increase ranged from 16% for the medial gastrocnemius and medial hamstrings to 28% for the gluteus maximus. In contrast, load-carrying resulted in only minor changes in the movement profiles and temporal parameters of stair ascent. The functional demand placed on the knee extensors during the CTL and EXP tests was further estimated by comparing the percent muscle activation levels (PMALs). The PMALs, which compared the MALs during the stair ascent tests to the MALs produced during the MVCs, provide an indirect measure of the force required to perform a locomotor task. The mean peak PMALs for the vastus medialis and the vastus lateralis were 64% and 72% respectively for the CTL test and 82% for the EXP test. Thus, comparison of the PMALs indicates that the knee extensor demand increased considerably with load and that a near-maximal knee extensor muscle activation is needed to perform the loaded stair test. These results suggest that stair ascent with a 22-kg load is a more sensitive functional test than stair ascent unloaded to evaluate the final stage of recovery of the leg extensors post-injury when only small knee extensor strength deficits persist. Such a test is of particular importance when evaluating the capacity of individuals to return to a job that places heavy demands on the knee.

Key words: Functional evaluation, gait, stair ascent, EMG

Introduction

The quantitative evaluation of functional capacity of normal or physically impaired subjects is difficult for many reasons. First the requirements of a chosen task, the test task, must be known. Secondly the performance of normal subjects skilled at the particular task must be dissected to yield biomechanical measures that are rep-

Received; 1 I October 1992 Accepted: 11 January 1993 Correspondence and reprint requests to: Dr Carol L. Richards, Centre de recherche en neurobiologie, H6pital de I’Enfant-Jesus, 1401. 18e Rue Quebec, P.Q. Canada, GlJ 124

S 1993 Butterworth-Heinemann Ltd 096&6362/93/01035-10

resentative of the global motor skill. Then the intra- and intersubject variability of these measures must also be known. Fourthly the motor activity chosen as a test of functional capacity must be closely related to activities performed in real-life work or leisure activities. Finally, if this task is used to define a residual motor deficit, the requirement of the task must be demanding enough to reveal the deficit.

In sports and work-related activities, the ability to walk with a normal pattern on level ground does not test the capacity to succeed in more demanding tasks. Indeed, we know from calculations of forces’ and moments during walking2J and stair ascent and des- cent2.4.s, that walking requires muscle moments in the lower extremity extensor muscles (support moment)

36 Gair & Posture 1993; 1: No 1

Table 1. Characteristics of 15 normal men

Age (years) Weight (kg) Height (cm)

Range

21-35 64-93

166-l 90

Mean SD

27.0 4.4 74.7 9.6

177.0 6.9

equal to about 33% of that required to ascend stairs. Moreover, estimates of the functional demand placed on the knee extensor muscles by comparison of muscle activation levels during various locomotor activities have shown walking to require about 25% and stair ascent about 65% of maximal levels obtained during isokinetic contractions6*7. The use of stair ascent as a test of maximal capacity thus seems more appropriate than walking. In comparison to walking, however, few reports on movements, muscle activationsq-6+14 or muscle moments2q4,s during stair ascent in normal subjects have been published. A few studies have reported comparisons of normal and pathological stair ascent pat- terns6J0J5J6. Although stair ascent as a functional test has been used to estimate recovery post-meniscectomp17, recent work in our laboratory’8 has shown that stair ascent alone is not demanding enough to reveal adaptations related to knee extension strength deficits of 5-- 30% (compared to sound leg). These findings suggest the need for a more sensitive functional test to reveal these deficits.

The main purpose of this study is thus to evaluate the functional demand of stair ascent with load in normal subjects by comparing muscle activations in six extensor muscles of the lower extremity during stair ascent unloaded and loaded with 22 kg. A secondary aim is to examine the feasibility of using stair ascent with load as a test of near-maximal functional capacity for the knee extensor muscles.

Subjects and methods

Subjects

Muscle activation and movement patterns of the right leg were studied during stair ascent in fifteen healthy men. In eleven of these (cases l-l I), maximal activation levels (MALs) of the right knee extensors were measured during maximal voluntary isokinetic knee extension movements. All were volunteers with no musculoskeletal disorders of the lower extremities. Subject characteristics are given in Table 1.

Methods Loading apparatus

In a single laboratory session each subject performed two different stair ascent tests: (1) stair ascent without an external load, the control test (CTL); and (2) stair ascent with an external load of 22 kg, the experimental test (EXP). Each test was carried out at free speed and nine trials of stair ascent were recorded. Between the two tests subjects were given a 5-min rest period. Then at the end of the second test, after a IO-min rest period, the MALs

A special vest was designed with four pouches (2 in the front and 2 in the back) to hold bags of lead shot. Added weight was thus carried on the front and back of the trunk just above the centre of gravity at the sacral level and the load was balanced. To prevent movement the load was stabilized by straps. The vest was worn throughout the experiment but the load (22 kg) was placed in the pouches prior to the load trials (EXP).

of the knee extensor muscles were recorded during maximal voluntary knee extensor contractions at 30 and 180 deg/s-I using a computer-controlled Kin-Corn (Chattecx Corp, Chattanooga, TN) dynamometer. The sequence of the stair ascent (CTL and EXP) and strength tests (30 and 180 deg s-l), was systematically alternated among subjects to attenuate possible effects of fatigue.

Records of movements and muscle acfivations during stair ascent tests

A TRIAX (Chattecx Corp, Chattanooga, TN) electrogoniometer (EGM) was used to measure the sagittal movements of the hip, knee, and ankle joints. This electrogoniometer has been shown to give reproducible and valid angle measures during gait 19. The electrogoniometer was applied while the subject stood in a standardized position. Each potentiometer for sagittal plane movements was aligned to the approximate centre of rotation of the corresponding joint. The zero reference position was recorded with the subject in an upright posture.

EMG activity was picked up with surface electrodes (Meditrace silver-silver chloride, 11 mm) placed over the muscle belly of six extensor muscles of the right leg: gluteus maximus (GM), medial hamstrings (MH), vastus medialis (VM), vastus lateralis (VL), medial gastrocnemius (MG), and soleus (SO). To reduce movement arte- facts, the electrodes were connected to miniature preamplifiers taped near the electrodes and then to a channelling box attached to the subject’s back. A 10-m shielded cable linked this box to the electrode selector unit of a GRASS polygraph (model 7D, Grass Instrument Co., Quincy, MA). The EMG signals were amplified and recorded as raw EMG on the polygraph paper to allow for visual inspection of the signals before being rectified and time-averaged with a time constant of 0.02 s (linear envelope) and fed to the computer.

Pressure-sensitive footswitches were taped to the heel, mid-foot and toe of both shoes. Signals from the footswitches were fed to DC preamplifiers (GRASS, type 7PI) by a shielded cable attached to the EMG cable. Tem- poral parameters of stair ascent, including cycle duration, percentage stance, percentage swing, percentage double leg support (DLS), and cadence were determined from the footswitch closures. Footswitch signals, EGM and EMG signals were recorded simultaneously with a sampling frequency of 100 Hz, processed by a computer (IBM PC) and stored for further analysis using software programs written in our laboratory.

Moffer er al.: Load-carrying during srair ascent 37

Staircase

Subjects were requested to ascend a staircase of three steps without holding the bannisters. Each step was 24 cm deep, 16 cm high, and 91 cm wide. The total slope of the stairs was 34 deg. The beginning of a stair ascent trial was determined by the initial contact of the right foot on the first step and ended when the same foot contacted the third step. Thus, only one cycle could be recorded for each trial of stair ascent, which began from a standardized starting position indicated by a landmark placed at a distance equal to 40% of the subject’s height from the first stair.

Records of muscle activation during maximal isokinetic contractions of the knee extensors

When the subjects had completed the stair ascent tests, the surface electrodes were left in place and EMG activity in the VM and VL was recorded while the subject performed maximal isokinetic knee extension movements at 30 and 180 deg s-l on a Kin-Corn dynamometer. The subject sat with the hip flexed approximately at 120 deg and stabilized by straps across the trunk, pelvis, and right thigh. The centre of rotation of the Kin- Corn rotational axis was aligned to the approximate centre of rotation of the knee joint and the dynamometer arm was attached on the leg above the ankle. Isokinetic tests began after a warm-up period consisting of 10 graded submaximal knee extension-flexion movements at 60 deg s-l. Torque, angle and EMG (linear envelope as previously described) signals were simultaneously recorded at a sampling frequency of 400 Hz. Each isokinetic test (30 and 180 deg s- I) consisted of three maximal contractions of the knee extensors from 90 deg of flexion to full extension of the knee preceded by a maximal static pre-loading contraction with a duration of 2 s to allow time for the contractile tension to rise before the movement was initiated*O. Contractions were interspersed by 1 -min rest periods while a 3-min rest period separated the two tests.

Data analvsis

Stair ascent tests For each test (CTL and EXP), five cycles with compar- able cycle durations among the nine recorded were retained for analysis. Choosing stair cycles with similar durations limited the effects of changes in velocity on muscle activations known to occur during gait*‘.**.

Temporal parameters For each subject, the mean (n = 5) cycle duration, percentage stance, percentage swing, percentage DLS, and cadence for stair ascent was calculated. The values for these temporal parameters for the two tests were firstly compared with the Wilcoxon test and then with the paired t-test after verifying that the values were nor- mally distributed. In the present study only the results obtained with the paired t-test are reported. Similar results were observed with the Wilcoxon test.

Muscle activation and movement profiles Muscle activation and movement profiles were norma- lized to a stair cycle of 100%. Mean values (n = 15 subjects) f 2 standard errors (SE) for muscle activations and movements were calculated at each 2% of the cycle and recorded in graphical form (EMG in uV on the Y axis and percentage of stair cycle on the Xaxis). This first descriptive analysis helped pinpoint the effects of load (EXP) on the CTL test values.

For further analysis, segments of the stair ascent cycle corresponding to intense activity for the different muscles were identified and then the area under the mean (n = 5 cycles) EMG profiles for these segments were calculated. The following areas were chosen as the representa- tive EMG parameters for statistical analysis: the area under the EMG activity curve from 0 to 20% of the cycle for GM, MH, VM, VL, and SO, the area from 0 to 60% (stance phase) for the GM, VM, VL and SO, the area from 40 to 70% for the MH, the area from 20 to 60% for the MG and SO, and the area from 0 to 10% and IO to 60% for the SO. Corresponding variables were defined for the movement profiles to examine the movement- muscle activation relationship. When statistical differences were found (paired t-test, one-tail, probability level P < 0.025) the extent of the change in muscle activation between tests was quantified by establishing a mean percentage change value (in comparison to the control value). The following formula was used:

EXP value - CTL value CTL value

x 100 = mean % change

Number of subjects

This calculation gives a positive mean percentage change when the EMG increased and inversely, a negative mean percentage change when the EMG decreased in the EXP situation.

Angular velocities Angular velocity was calculated at each 2% of the stair cycle from the EGM data by the method of finite differ- ence proposed by Winter23. Because of the relatively small number of subjects, the average of three consecu- tive mean angular velocity values was taken, yielding 10 measures per subject for each movement, during the stance phase. These values were submitted to an analysis of variance for repeated measures (MANOVA) to test the effect of load on the mean angular velocities.

Isokinetic tests For each subject the best strength trial at 30 and 180 deg s-l was chosen and the EMG activity recorded from the VM and the VL during the chosen contractions retained for analysis. The functional demand of stair ascent on the quadriceps (VM and VL) without and with a 22-kg load was determined by comparing muscle activation levels (MALs) during the maximal voluntary contractions and the activation levels during locomotor tasks to obtain the percentage maximal activation level (PMAL).

38 Gait & Posrure 1993; 1: No 1

Table 2. Comparison of temporal parameters of stair ascent unloaded (CTL) and when carrying 22 kg (EXP) in 15 normal men

Parameters

CTL

Tests

EXP

Paired t-test’ (p) Change (%)

Cycle durations (s) 1.5 (0.2) 1.5 (0.2) Cadence (steps/min) 83.4 (8.0) 82.1 (8.3) Stance (%) 60.7 (2.3) 62.1 (2.5) Swing (%) 39.3 (2.3) 37.9 (2.5) DLSc (%) 32.3 (6.0) 34.9 (5.4)

aPaired t-test, one-tail; Walues give mean (1 SD); cDouble leg support; ‘P < 0.025.

0.162 1.9 (6.6)b 0.174 -1.5 (6.2) 0.0009’ 2.2 (2.2) 0.0009’ -3.2 (3.4) 0.014’ 9.4 (14.6)

The PMAL for stair ascent without load and with 22 kg was calculated by taking into consideration the movement velocity and muscle length (knee angle) during the locomotor task as described by Richards et al.“. PMALs were calculated at each 2% of the stair cycle by means of a software program as illustrated in the following example. At 8% of the stair cycle, in a normal subject, an activation level of 937 pV in the VM was associated with a knee angle of 47 deg and an angular velocity of 67 deg s-1. The corresponding maximal EMG for the same knee angle and velocity (for velocities less than 100 deg s-l the EMG reference was obtained from the isokinetic contractions at 30 deg s-l; for greater velocities the isokinetic contractions at 180 deg s-1 were used) during the MVC at 30 deg was 1602 pV. Thus, the PMAL value at 8% of the stair cycle for this subject was 58% (937 pV/ 1602 pV x 100).

Results

Temporal parameters

Temporal parameters for the two tests are given in Table 2. In the loaded condition the percentage stance and the percentage DLS increased significantly (P c 0.025) while the percentage swing decreased (P < 0.025). In spite of these modifications with loading the mean cycle time and the cadence did not change significantly.

Movements during stair ascent tests

During stair ascent the hip, knee, and ankle joints inter- act to produce a smooth weight acceptance and transfer of the body over the support foot as the contralateral foot is lifted to the next step and weight is transferred to the forward leg. These movements in the sagittal plane are illustrated in Figure 1. As can be seen in the figure, the stair cycle begins with the three joints flexed at stair contact. The ankle position remains relatively dorsi- flexed as the knee and hip extend in the first 40-50% of the cycle while weight is transferred and the body rises over the support foot. Opposite foot contact occurs at about 45% of the cycle after which the ipsilateral ankle plantarflexes to assist the ‘pull-up’ of the body to the second step. The swing phase begins at 61% of the cycle and is characterized by flexion at the three joints until about 85% of the cycle. In the last 15% of the cycle the knee and then the hip extend until the foot meets the

C

60

50

40

-501 I I I* I I 0 20 40 60 60 100

Stair cycle ($1

Figure 1. Comparison of mean movement profiles of a, hip; b, knee; and c, ankle of normal men (n = 15) during stair ascent unloaded and when carrying a 22-kg load. Mean profiles for the 22-kg load condition (solid line) are superimposed on the mean (&2 SE) for the unloaded condition (shaded area). Significant changes with load (paired r-tests) are indicated by dotted lines and corresponding mean percentage change values. Triangles indicate the end of the stance phase.

third stair to initiate another cycle. The mean (n = 15) total angular displacement of the hip, knee, and ankle joints during the CTL test were 50,80, and 44 deg respectively. The movement curves during stair ascent with and without load for the hip, knee, and ankle are similar (Figure 1). Although a 22-kg load induces slightly more flexion than when ascending without a load, the move-

Moffet et al.: Load-carrying during stair ascent 39

Table 3. Comparison of movements of the leg during the stance phase of stair ascent unloaded (CTL) and when carrying 22 kg (EXP) in 15 normal men

Joints Segment of stair cycle (%) Mean area Paired t-test0 (p) Change (%)

Hip

Knee

Ankle

O-20 O-60

&20 O-60

O-20 O-60

20-60

CTL EXP

846 871 1377 1478

933 961 1405 1504

286 306 694 709 408 404

0.084 3.5 (7.8)b 0.017’ 9.2 (13.5)

0.067 3.0 (7.2) 0.024’ 7.8 (12.1)

0.199 14.9 (40.2) 0.385 8.6 (36.5) 0.451 -7.8 (39.6)

Statistical comparison between areas under segment of movement profile of the CTL and EXP tests (paired t-test, one-tail; P < 0.025). bVaiues give mean (1 so).

ment profiles fall within the f 2 SE of control values. The cycle they were 16% for the MG and 2 1% for the SO. hip and knee were more flexed (paired t-test: P < 0.025) Although inter-test differences (see Table 4) were with load during the stance phase (O-60% of the cycle). observed for the SO (O-IO and O-20% of the cycle) and The corresponding mean percentages of change of hip the MH (40-70% of the cycle), they were not significant and knee were 9% and 8% respectively (see Table 3). (P > 0.025).

Muscle activations during stair ascent tests Angular velocities during stair ascent tests

Figure 2 illustrates the muscle activation profiles of six lower extremity muscles (A-F). As can be seen in the figure, the hip (A and B), the knee (C and D) and one of the ankle extensors (E) are active during the weight acceptance phase (O-20%). Activations of these muscles are related to hip and knee extension (see Figure 1) and to the control of ankle dorsiflexion (eccentric contraction of the SO). The ankle plantarflexors (E and F) and knee flexors (B) are active preparatory to swing initiation (61% of the cycle) while only the knee flexors (B) remain active at the beginning of the swing phase. The contraction of the plantarflexors is related to ankle plantarflex- ion and most probably provides propulsive forces to assist the contralateral knee extensors in the elevation of the body over the next step. The MH activation burst between 50 and 70% of the cycle apparently assists swing phase knee flexion. The MH has a second burst of activation at the end of the swing phase which is sustained into the next stance phase. This burst, which occurs concomi- tantly with knee and hip extension (see Figure 1). could be related to the deceleration of knee extension (eccentric contraction of MH) at the end of the swing phase and the hip extension (concentric contraction of MH) at the beginning of the cycle.

The MANOVA analysis of the angular velocities (mean values between successive 6% of the stance period) revealed a non-significant main test effect but a significant test by velocity interaction for all three movements at the 5% level (Table 5). Post-hoc analyses indicated significant angular decreases with load (P < 0.005) in the 1424% segment of the cycle for the hip and in the 20-24% segment for the knee. These angular velocity changes were not apparently associated to a specific task during the stair cycle.

PMALs of the knee extensor muscles during stair ascent tests

Load-carrying induced significantly higher EMG activity during the stance phase in all extensor muscles (paired f-test: P < 0.025). More precisely, higher levels of EMG activity for the test with load occurred during the weight acceptance phase (O--20% of the cycle) for the hip and the knee extensors, and between 20 and 60% of the cycle for the ankle extensors. These EMG increments were respectively 28, 16, 17, and 18% in the GM, MH, VM. and the VL for the areas under the activation profiles from 0 to 20% of the cycle; from 20 to 60% of the

Figure 3 gives the PMAL profiles for the VM and the VL during stair ascent without and with a 22-kg load. Signi- ficant increments (P < 0.004, paired t-test) between PMAL levels (area under the mean PMAL profiles) were found during the stance phase (O-20 and O-60% of the cycle) for the loading condition. As shown in Table 6, the PMAL percentage changes ranged from 18% for the VL between 0 and 20% to 29% for the VM between 0 and 60% of the cycle. Otherwise the PMAL mean peak values for the VM and VL were 64 and 72% respectively for the CTL test. These values increased to 82% for each muscle in the EXP test, representing a peak value change of 18% (VM) and 10% (VL).

Discussion

In the present study the functional demand of a new test, stair ascent with a 22-kg load, was evaluated. The functional demand of this task was estimated by comparing the muscle activations and movements of the lower extremity during stair ascent with and without load. The main

40 Gait & Posture 1993; 1: No 1

200

28% P = 0.002

150 . . . . . . . . . . . . . . . . . . . . . . . . . .

27% P= 0.001

a 01 I I J

1200

17% PC 0.0001

. . . . . . . . . . . . . . . . . . . . . . . . .

F 800

3 19% P<0.0001

Y 400

e 0 20 40 60 80 100

b

d

300~.........,~ 16% P = 0.021

800 r

18% P = 0.0001 600 . . . . . . . . . . . . . . . . . , . . . . . . . . . . . . . I

19% PC 0.0001

16% P = 0.006 .

800

Stair cycle (%)

Figure 2. Comparison of mean muscle activation profiles of six leg extensor muscles: a, gluteus maximus (n = 11); b, medial hamstrings (n = 15); c, vastus medialis (n .= 15); d, vastus lateralis (n = 15); 8, soleus (n = 11); f, medial gastrocnemius (n = 15) in a group of normal men during the two stair ascent tests. Graphic representation as in Figure 1, Load condition, solid line; unloaded condition, shaded area.

finding was a significant increase in the activation of the six leg extensor muscles during the stance phase. This increase ranged from 16% for the MG and MH to 28% for the GM. In contrast, load-carrying resulted in only minor changes in the movement profiles and temporal parameters (less than 10%) of stair ascent. This specific increase in extensor muscle activations with load can be interpreted as an indicator of greater functional demand or the need for a greater force ouput to complete the task. Such an interpretation, made on the basis of changes in muscle activations, implies a near-linear relationship between EMG activity levels and force during a complex movement task such as stair ascent.

Many researchers have studied the EMG/force relationship during simple monoarticular tasks under static or dynamic conditions. A near-linear relationship has been reported between submaximal isometric force levels up to approximately 80% of maximal force and the mean rectified (or integrated) EMG at a given joint position,

for different large and small limb muscles2~32. Such a relationship has also been found between EMG and force during dynamic (concentric or eccentric) submaximal monoarticular contractions at constant angular velocity33pX. Of particular interest to the present study is the near-linear relationship observed between the EMG and graded concentric knee extensor isokinetic (15-150 deg s- I) contractions17v3s.

Furthermore, muscle contraction parameters such as muscle length or joint position*7J9*31, angular velocity of the movement (from which contraction speed is der- ived)33+M, muscle fatigue*sJ’J6*37, and the type of contraction” have been shown to modify the EMG/force relationship during simple tasks. For example, more EMG is needed for the same force increment, if the muscle is shorter or the velocity greater, in the presence of muscle fatigue, or during a concentric contraction instead of an eccentric contraction. Consequently, these factors should be considered when interpreting the intensity of func-


Table 4. Comparison of muscle activations of the leg during the stance phase of stair ascent unloaded (CTL) and when carrying 22 kg (EXP) in normal men

Muscles n Segment of stair cycle (%) Mean area Paired t-tests Change (%) (P)

CTL EXP

Gluteus maximus 11 O-20 1736 2271 0.002’ 28.3 (21.5)b O-60 3199 4135 0.001 l 26.5 (I 7.6)

Medial hamstrings 15 O-20 1372 1631 0.021 l I 6.0 (21.7) 40-70 3279 3654 0.036 12.9 (31 .O)

Vastus medialis 15 O-20 11381 13312 < 0.0001 l 17.3 (10.5) O-60 15783 18661 < 0.0001 l 19.5 (12.1)

Vastus lateralis 15 O-20 6216 7248 0.0001 l 17.7 (14.8) O-60 9371 11146 <O.OOOl’ 18.9 (13.3)

Soleus 11 O-10 1741 1869 0.107 11 .o (I 9.2) O-20 3423 3806 0.083 18.0 (22.9) O-60 11561 13332 0.008’ 19.5 (16.5)

1 O-60 9479 11055 0.010’ 21.1 (18.1) 20-60 8138 9526 0.003’ 20.6 (I 5.7)

Medial gastrocnemius 15 20-60 14464 17703 0.006’ 16.5 (24.1)

Statistical comparison between areas under segment of muscle activation profile of the CTL and EXP tests (paired t-test, one-tail) Qalues give mean (1 SD) l P < 0.025

Table 5. Summary of multivariate analysis of variance showing the significance of the main and interaction effects for the hip, knee, and ankle angular velocities during the stance phase

Joint Source F P

Hip Test 3.81 0.07 Test-velocity 6.73 0.02’

Knee Test 2.99 0.11 Test-velocity 5.16 0.01 a*

Ankle Test 0.95 0.35 Test-velocity 5.36 0.03’

aunivariate result with Greenhouse-Geisser adjustment. “P < 0.05.

tional demand from muscle activation level comparisons during complex locomotor tasks.

In contrast to a static or dynamic contraction made by relatively isolated muscle groups about a single joint (simple task), stair ascent is a complex locomotor task which involves a number of body segments and many muscles interacting to produce smooth coordinated movements. During such complex movements the length of the various muscles activated, the instantaneous angular velocity and speed of contractions, the type of contractions (eccentric or concentric) and the muscle demand vary as the movement is performed. It is not surprising that little is known about the direct EMG/ force relationship during locomotor movements in man, given the complexity of the (force-length-velocity) inter- actions and the limit imposed by the lack of direct force measures from the muscles as the task is performed. In cats, Walmsley et al.38 have shown that forces, measured

directly by special transducers implanted in the tendon of the medial gastrocnemius and the soleus, are linearly related to the EMG recorded over the corresponding muscles during graded locomotor tasks. This linear relation held for a wide range of speeds from slow walk to fast run but not during jumping, a task which required a much higher contraction speed and near-maximal calf muscle recruitment. In man, such transducers cannot be implanted in general practice and non-invasive external measures must be used to estimate the actual forces. For example, complex mathematical models which incorpor- ate kinematic and EMG measures have been developed to predict the forces controlling joint movements (individual muscle torques) during walking. In general, a good correlation has been obtained between the sum of individual muscle torques predicted by the model and the mechanical total torque measured by a force plate, sug- gesting that a close quantitative EMG/force relationship also exists during complex locomotor tasks3g-4i. In the light of these studies it seems reasonable to accept changes in the EMG activity level (linear envelope) as a rough estimate of the force contribution of muscles during submaximal complex tasks if parameters such as the muscle length (joint angle), instantaneous angular velocity, type of contraction, and muscle fatigue are taken into account. Finally, the speed of forward progression must also be considered in the interpretation of EMG levels during stair ascent because an increase in speed has been closely linked to higher EMG levels during walking*iJ-?.

In the present study we compared the EMG activity levels during a specific locomotor task, the stance phase of stair ascent, in two submaximal locomotor activities: stair ascent with and without load. Careful analysis of


100 . . . . . . . . . . . ..i 1 27% p = 0.0002

60

40

20

Za 0

A

80 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

60

bo 20 40 60 80 100

Stair cycle (%I

Figure 3. Comparison of the PMAL profiles of the knee extensor muscles (a, vastus medialis; b, vastus lateralis) of normal men (n = 11) during stair ascent unloaded and when carrying a 22-kg load. Mean profiles for loaded condition (solid line) are superimposed on the mean (i2 SE) for the unloaded condition (shaded area). Significant changes with load are indicated by dotted lines and corresponding mean percentage change values. Triangles indicate the end of the stance phase.

factors such as cadence, fatigue, angular velocity, and type of contraction indicates that they most probably only minimally influenced the EMG levels across the tests. Factors such as muscle length (angular position), on the other hand, may have contributed to the increase in EMG levels with load since the hip and knee are slightly more flexed with load (P c 0.025). This slight

increase in flexion can be expected to enhance the extensor demand at the knee4* and the total support moment3. The small changes in the leg movement profiles across tests is probably attributable to good load distribu- tion43.U.

Since cadence and stair cycle duration did not change with load carrying, it can be assumed that the average speed of forward progression was similar between the tests. The temporal structure of the cycle was, however, modified with load, more time being spent in stance (percentage stance and percentage DLS) and less in swing (percentage swing). This finding is consistent with the results of Martin and NelsoP, who showed that for the same speed of walking the DLS time increases and the swing time decreases when load carrying. Moreover, because a sufficient rest period was given between the two stair tests and the test sequence was sytematically alternated, fatigue should not have affected the results. Furthermore, it is not likely that a higher movement velocity affected the muscle activations because the subjects adopted a slightly slower movement velocity during stance, most probably to provide a stable base of support to the body, during this period. Finally, care was taken to compare the muscle activation profiles between tests in segments corresponding to either concentric or eccentric muscle contractions to control for the effect of contraction type. It is of interest that during the stance phase the only eccentric contraction occurs in the SO at the beginning of the stair cycle (O-10% approximately). All the other muscles (including the SO from 1040%) con- tract concentrically (see Figures 1 and 2).

The finding that the eccentric contraction phase of the SO was the only activation segment that was unchanged with load suggests that load carrying specifically affects EMG levels during concentric contractions which corre- late to the movement task of raising the body over the next step. Thus, after examining the various factors that can affect the EMG/force relationship during complex locomotor tasks, it appears justified within the context of the present controlled study conditions to assume that the increase in EMG extensor levels indicates that higher extensor forces are required to ascend stairs when carrying a load of 22 kg.

Ergonomic and biomechanical studies have previously

Table 6. Comparison of PMALa profiles of the knee extensor muscles during stair ascent unloaded (CTL) and when carrying 22 kg (EXP) in 11 normal men, and corresponding mean peak values

Muscles Segment of stair cycle (%) Mean area Paired r-testb (fl Change (%) Mean PMAL peak

CTL EXP CTL EXP

Vastus O-20 819 1024 0.0002’ 27.2 (19.9)c 64 82 medialis O-60 1244 1594 0.0004’ 28.9 (19.9)

Vastus O-20 867 992 0.0019’ 18.4 (16.9) 72 82 lateralis O-60 1406 1671, 0.0001 l 20.4 (13.1)

apercentage of maximal activation level. bstatistical comparison between areas under segment of PMAL profile of the CTL and EXP tests (paired t-test, one-tail). CValues give mean (1 SD). l P < 0.025.

shown that load-carrying increases the functional demand of locomotor tasks. The metabolic cost, measured by parameters such as the VO, max or the heart rate, increases when walking or ascending and descending stairs with a load4’-“. Similarly, during walking, the ground reaction force and the mechanical work are mag- nified as a function of load4’.5z.s?. In this study it was shown that EMG levels can reflect the functional demand on individual muscles during a complex task.

By comparing the MALs during the locomotor tasks and the MALs during a MVC while controlling for knee angle and movement velocity17, the PMAL thus obtained gives an indirect measure of the force demand. First introduced by Richard@’ the PMAL concept has been used to describe the muscle demand during different locomotor tasks such as gait, stair ascent and descent, and sitting or rising from a chair in patients recovering from conventional or arthroscopic meniscal surgery6,i5.‘7 or in patients with rheumatoid arthriti+16. In the present study PMALs were used to further quantify the functional demand placed on the knee extensor muscles during stair ascent tests. The mean peak PMAL was 64 and 72% for the VM and the VL respectively for the CTL test. and 82% (both VM and VL) for the EXP test. Thus, from comparison of the PMALs it is clear that the knee extensor muscle demand increased with load and had a greater impact on the VM than on the VL (18% of increment for the VM comparatively to 10% for the VL). It is also of interest to note that despite a PMAL increase up to 82% in response to loading, a muscle reserve of about 18% remains, confirming the submaximal char- acter of the task and the possibility of further increasing the task requirements. As pointed out earlier, because the EMG/force relationship is linear only up to 80% of the maximal force, the practical limit of this test may have been reached.

When the percentage change in the PMALs for the entire stance phase (area under the mean PMAL profile between 0 and 60% of the cycle) are compared instead of peak PMAL values, it is somewhat surprising to find that the amount of change with load is in proportion to the load carried. Indeed, the load represents 29% of the

mean body weight of the subjects (n = 15) and the percentage change in the PMAL for the VM is also 29% between 0 and 60% of the cycle. This similarity between the magnitude of the percentage change in the PMAL and the load carried (in percentage of the body weight), however, appears to be a coincidence since this relationship was not significantly correlated.

The lack of a significant association between the added load as expressed in percentage of body weight and the percentage change in the PMAL suggests that factors other than body weight are key determinants in the load compensation synergy. Moreover, in a preliminary study we were able to detect the effects of 6 kg (16 kg versus 22 kg) on the load compensation strategies. We thus chose 22 kg (50 lb) as the test load because it corresponded to requirements of a frequent task in daily activities that significantly increases the metabolic cost48 without elicit- ing large postural adaptations. A fixed load, instead of a


load adjusted to body weight, also simplifies the apph- cation of the test. Moreover, a fixed load of 22 kg was shown to elicit significant changes in the PMALs of the knee extensors for the majority of the subjects and, as previously mentioned, the percentage change in the PMALs were not related to body weight.

In conclusion. stair ascent with a 22-kg load is a pro- mising test for the evaluation of near-maximal functional capacity of physically impaired subjects. This new functional test is currently being applied to the evaluation of patients recovering from an arthroscopic meniscectomy who still have knee extensor strength deficits of j-30%. The resuls of these evaluations should help to validate this test as a sensitive measure capable of depicting small strength deficits in the knee extensors. Furthermore, if the demand placed upon the knee extensor muscles during this test is not met efficiently, the test should reveal adaptative strategies (in the EMG, movement profiles, or in the temporal parameters) which characterize the functional capacity or the stage of recovery of these patients.

Acknowledgements

This work was supported by a grant from Fonds de la Re-cherche en Sante du Quebec. The authors thank F. Comeau, L. Gagnon, L. Laroche, C. Potvin and D. Tardif for technical assistance.

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Load-carrying during stair ascent: a demanding functional test

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